ics notes by polarao sir

7/29/2019 Ics Notes by Polarao Sir

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ICS NOTES-1 UNIT-1

R.Pola Rao, Asso.Prof, Dept of ME,GMRIT-Rajam

Instrumentation and objective of instrumentation:

Instrumentation:

Instrumentation is a part of engineering science that involves continuous monitoring and

controlling of physical parameters to increase the safety in working areas. It also deals with' the

various methods used for measurement, measuring instruments employed and the problems

associated with the methods used for measurement.

Instrumentation plays a very vital role in both measuring and collecting information form

working areas (field) and changing the field parameters as required by the process.

Objectives

1. The major objective of instrumentation is to measure and control the field parameters to

increase safety and efficiency of the process.

2. To maintain the operation of the plant within the design expectations and to achieve a

good quality product.

3. To achieve automation or automatic control of process there by reducing human fatigue.

4. To perform manipulations on the collected data automatically.

5. To achieve, good quality control.

REFERENCE BOOKS:

1.A course on mechanical measurements and instrumentation- A.K Sawhney

2.Instrumentation and Control systems- S.Sudhakarreddy&P.Divakararao 3.Instrumentation Measurement & Analysis- B.C Nakra and K.K Choudhry

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Generalized configuration of measurement system with a neat block diagram:

General Input-output Configuration of Measuring Instruments and Measurement Systems

The general configuration of measuring instruments as three types of inputs.

They are as follows,1. Desired Inputs

2. Interfering Inputs

3. Modifying Inputs.

Figure: General Input output Configuration of Measurement Systems

Desired Inputs

The input quantity for which the measurement system is designed to measure and produce output

is known as desired input. The desired input is represented as rD According to the input-output

relationship of mathematical model, the output (CD) produced due to the desired input rD is given

by,

D D Dr GC =

Where

GD = Transfer function

i.e., it is a mathematical operation required to get the output from desired input. If the transfer

function of the system is G and the applied input is, 'r' then the output will be

C =Gr.

From this we can understand that the transfer function is a constant K, and it will be 'multiplied

with input rD to produce output CD =K rD. It is because to get an attenuated or amplified output in

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linear systems. Whereas in non linear systems, the transfer function will be represented by using

algebraic or transcendental function. If the inputs applied to the system are dynamic in nature,

then its input-output relationship will be represented by differential equations.

2. Interfering InputsInterfering input quantities are those which make the measurement system or instrument

unintentionally sensitive. The measurement systems or instruments respond to -the interfering

inputs and produces an output though they are not desired to respond. This occurs because of its

design, operating principle and some other factors like environments in which they are placed.

The interfering input is represented by r1 The method of producing output using referring input,

r1 and transfer function,. GI is similar to producing output using desired input, rD and transfer

function GD'

3. Modifying Inputs

The inputs which causes a change in the input output relationship of a measurement system for

both desired and interfering inputs or any of the inputs alone. Modifying input is represented by

rM which modifies both GD GI or anyone of these (i.e., GD or GI). The manner in w4ich rM affects

GD is represented by GMD and the manner in which rM affects GI is represented by GMI. The

interpretation of these GMD and GMI is same as

GD and GI

The instantaneous outputs due to desired, modifying and interfering inputs are given to the

summer or summing point which produces the sum of the instantaneous values. The

measurement system or instrument produces several outputs if it is subjected to several: inputs of

each of these three types. But in this case the: required block diagram will be more complex.

REFERENCE BOOKS:

1.A course on mechanical measurements and instrumentation- A.K Sawhney2.Instrumentation and Control systems- S.Sudhakarreddy&P.Divakararao 3.Instrumentation Measurement & Analysis- B.C Nakra and K.K Choudhry

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ICS NOTES-3 UNIT-1


Method of measurements:

Depending on the requirement application, and the standards used the methods of measurement

are classified as,

1. Direct and indirect measurements.

2. Primary, secondary and tertiary measurements.

3. Mechanical, electrical and electronic instruments.

4. Absolute and secondary instruments.

5. Analog and digital instruments.

Among these methods the generally used methods of measurements are,

1. Direct and indirect measurement.

2. Primary, secondary and Tertiary measurements.

Direct Comparison Method

In direct comparison method of measurement, the physical quantity 'to be measured (measurand)

is directly compared with either primary or secondary standards and the result is obtained as a

number and a, unit. This method is employed for the measurement of physical parameters, like

length, time, mass etc. If the measurand is too small it is not possible to make direct comparison

accurately. Also it is not possible to distinguish wide margins of the quantity being measured.

Thus in some applications direct comparison method is not feasible and practicable.

Indirect Comparison Method

In indirect comparison method of measurement the measurand is indirectly compared with

secondary standards through calibration. This method employs a transducer.

The transducer converts the measurand into its equivalent electrical signal. This signal is then

processed in the remaining stages of the measurement system and produces an, output. Thus the

unknown quantity is first converted into some other form and then compared with' the standard.

2. Primary Measurements

In this method, the measurement of an unknown quantity is made by direct observation. i.e., a

physical Principle of Production of Eddy Currents If a conducting material is placed near a coil

carrying alternating currents, eddy currents are induced in the conducting material. This is the

basic principle of operation of these type of inductive transducer material produces its own

magnetic field which acts against the main magnetic field created by the coil resulting in the,

reduction of net flux linking with coil and so the inductance of the coil is reduced. As the

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distance b6tween the conducting material and the coil decreases more eddy current are induced

and thus higher is the reduction in the inductance of the coil. Thus, inductance of the coil

changes with the movement of the conducting material quantity is measured by comparing

directly with reference standard.

Example

Measuring the length of a cable by comparing with other cable (whose length is already known)

Secondary Measurements

In this method of measurement the output (final result) is obtained by one conversion of the

measurand.

Example:

Conversion for force or pressure into displacement. Tertiary Measurements

In this method of measurement the output is obtained by two conversions of the measurand.

Example

Measurement of temperature using thermocouple (Here to measure the temperature the unknown

temperature is first converted into voltage. This voltage is then converted into length).

REFERENCE BOOKS:


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Applications of measuring instruments:

The various applications of a measuring instrument are,

1. Measurement of Different Parameters of a Process

Measuring instruments are used to measure different unknown parameters related to a system ora process. A measuring instruments also provides the measured information in the form of

display, recording, registering, monitoring depending on the requirement.

2. Automatic Control Systems

A measuring instrument is an integral part of an automatic control system. Here a measuring

instrument not only used to measures a particular parameter but also controls and manipulates in

order to make the process to run at a predefined set point. Therefore it is used in all types of

automatic control systems such as chemical plants, oil refineries, textile mills to control

humidity, pressure, viscosity, flow rate, temperature and other relevant parameters.

3. Quality Control

Measuring instruments are used to test the quality of the material, to maintain the standards and

specifications of the products in industries.

A good quality product is made by continuously performing quality control tests on mass

produced industrial products. This act is accomplished by measuring instruments.

4. Experimental Studies

To gather information and to form certain empirical relations where sufficient theory is not

available, and to develop new theories, to innovate new phenomena and new products measuring

instruments are necessary. They can be used to verify the already existing physical phenomena

and scientific theories etc.

5. Simulation of Conditions of a Process

In order to reveal the actual behaviour of the process under different working conditions, it is

necessary to simulate true conditions of complex situations in a process experimentally. To

convert this experimental results into prototype analytical tools are required.

6. Measuring instrument is an essential tool of any measurement hence no measurement is

carried out without it. Thus it is used in all measurement processes 'and industries such as

process industries, power plants, automatic production machines automatic landing of aircraft,

radar tracking systems, missile guidance, autopilots etc.

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Types of performance characteristics of an instrument:

Performance characteristics

The performance characteristics of an instrumentation system is concluded by how accurately the

system measures the required input and how absolutely it rejects the undesirable inputs. Forchoosing the most suitable instrument for specific measuring task the performance characteristics

of an instruments are necessary. It can be generally divided into two distinct categories.

(a) Static characteristics

(b) Dynamic characteristics.

(a) Static Characteristics

When the measurand does not vary with time, static characteristics are determined. The static

characteristics of an instrument are determined by a process called static calibration in which the

relationship between the output signal and the quantity under study is experimentally

determined.

The important term which specifies the static characteristics are as follows.

(i) Accuracy

(ii) Precision

(iii) Sensitivity

(iv) Linearity

(v) Stability

(vi) Error

(vii) Threshold

(viii) Resolution

(ix) Hysteresis

(x) Dead space

(xi) Range and span

(xii) Reproducibility.

(xiii) Drift

(i)Accuracy :

The closeness of the measured value with respect to the true value is called as accuracy.

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Accuracy is influenced by the affects of disturbing inputs such as temperature, humidity and also

by the limits of intrinsic errors and instability of natural zero. Under certain operating conditions

accuracy can also be determined by calibrating.

The accuracy of the whole system depends upon the accuracies of each individual instruments inthat system. Generally, the accuracy of the instrument depends upon the natural limitations of the

instrument as well as on the shortcomings in the measurement process.

(ii) Precision

The instrument ability to reproduce a certain group of readings within a given accuracy is known

as precision i.e., if a number of measurements are made on the same true value, then the degree-

of closeness of these measurements is called precision.

Precision of an instrument depends upon the random errors. It refers to the ability of an

instrument to give its readings again and again in same manner for constant input signals.

Instruments having high accuracy should also be highly precise.

(iii) Sensitivity

It is defined as the ratio of change in output to that of change in the quantity being measured.

Sensitivity is also known as incremental sensitivity or linear sensitivity.

Inputof

Outputof

Change

change ySensitivit =

iC C ∆

∆=0

The sensitivity differs for different values of input s shown in the figure (a) but when the

calibration curve is straight line, then the-sensitivity remains constant over the entire range and is

given as the slope of calibration curve as shown in the figure (b). For better performance of the

system the sensitivity of an instrument should be high.

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Figure(a):Incremental Sensitivity

Figure(b):Linear Sensitivity

Linearity

It is expressed as a percentage of deviation from the linear value. .

It defines the proportionality between the measurand and output signal. The calibration curve is

said to be linear, when the sensitivity is constant for all values from lower scale to upper scale of

the measuring system.

The linear characteristics of the, calibration curve are shown in the figure (c).

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Figure(c): linear and Nonlinear Calibration Characteristic

(v) Stability

It is the ability of the instrument to have the same standard of performance over a prolonged

period of time. The need for calibrating the instrument frequently is less for a instrument having

high stability.

(vi) Error

In any device, when the measured value or indicated value of measurand differs from its true

value then this difference between the measured value and true value is referred as error.

In an ideal device the error is zero i.e., the output is always equal to the true value of the

measurand. When the measured value exceeds the true value of measurand the error is said to be

positive. By minimizing the error of the device, its accuracy can be increased.

(vii)Threshold

It is defined as the minimum input quantity required for a detectable change in the output signal

from the zero indication i.e., when the input to an instrument is gradually increased from zero,

then the input must reach to a certain minimum value, so that the change in the output can be

detected. This minimum value of the input refers to threshold.

(viii) Resolution

It is defined as the increment in the input of the instrument for which the output remains constant

i.e., when the input given to the instrument is slowly increased for which the output remains

same until the increment exceeds a definite value.

(ix) Hysteresis

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Hysterisis can be defined as the maximum variation of the output due to the change in applied

input. The hysterisis characteristics are shown in the figure, where it can be observed that output

for increasing and decreasing value of input is not same.

(x) Dead Space

The change of input quantity upto the maximum extent for which there is no output of instrument

is known as dead space. The change of input quantity upto the maximum extent to which the

measuring system does not respond 0 is known as dead space or dead zone or dead band.

(xi) Range arid Span

The region between the limits within which an instrument is designed to operate for measuring a

quantity is called the range of the instrument.

(xii) Reproducibility

Ability to reproduce the output signal exactly when the same input is measured under different

conditions is called reproducibility.

(xiii) Drift

The slow variation of the output signal of a measuring instrument is known as drift. The

variation in the output signal is not due to any changes in the input quantity but it is

due to the changes in the working conditions of components insides the measuring instrument.

REFERENCE BOOKS: 1.A course on mechanical measurements and instrumentation- A.K Sawhney2.Instrumentation and Control systems- S.Sudhakarreddy&P.Divakararao 3.Instrumentation Measurement & Analysis- B.C Nakra and K.K Choudhry

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ICS NOTES-6 UNIT-1


Dynamic characteristics:

(i) Bandwidth

(ii) Dynamic range

(iii) Setting time(iv) Measurement lag

(v) Fidelity.

(i) Bandwidth

The satisfactory ratio of peak amplitude of output signal to that of input signal is known a

dynamic sensitivity. The dynamic sensitivity is obtained for certain range of frequencies and this

range of frequencies are known as bandwidth.

(ii) Dynamic Range

It is the range of signals for which the measuring system is possible to respond constantly under

dynamic conditions. It is generally demonstrated as the ratio of the amplitudes and the ratio is

generally expressed in dB.

(iii) Setting Time

Settling time is the time taken by a system after the application of a step input for the instrument

to reach and stay within a close range of steady state value.

(iv) Measurement Lag

It is the time delay in response of the output signal to the changes in the input signal.

Measurement lag is dependent on the characteristics of the system only. For different types of

input signal, measurement lag can be specified in different ways.

(v)Fidelity

For a time-varying input the quality of indication by the instrument is referred as fidelity. The

perfect fidelity is obtained only by a zero order system i.e., fidelity is the degree of nearness with

which the output reproduces the time varying input within a conversion factor. But for higher

order systems the output cannot reproduce the input signals constantly at all instant and for all

types of input time varying function. Under steady state conditions for sinusoidal input functions

the perfect fidelity of an instrument would indicate that the waveforms of the output and input

signals occur simultaneously with each other at all instants. Hence, there will be neither

amplitude error nor phase error. Fidelity requirements is generally associated to cover deficiency

In amplitude frequency characteristics and with the applications the fidelity requirements differ.

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It's specification is reduced, while describing. The working of electronic amplifiers and such

other electronic apparatus meant for entertainment. Thus, fidelity is maintained or a wider range

of frequencies.

REFERENCE BOOKS:


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ICS NOTES-7 UNIT-1


Calibration:

It can be stated as a process of making a set of operations to establish a relationship between the

values indicated by the measuring system and the corresponding known value of the physical

quantity (or measurand).Procedure of Calibration

To calibrate an instrument initially adjust the instrument such that it produces null output hen on

input is applied. Then apply on accurately known value of measurand and adjust the instrument

again until its scale exactly indicates the value of measurand. This process of adjusting the

instrument is called calibration.

After calibrating an instrument, it should be used under those environmental conditions which

are identical to the conditions prevailing during calibration. Otherwise the instrument itself

becomes a source of error and can not provides an accurate value of the measurand. i.e., the

changes in environmental conditions affect the characteristics of instrument.

Calibration is very important especially in those cases where the sensing system and measuring

system of the instrument are different. In this case the calibration is done by considering the two

different systems as a whole.

REFERENCE BOOKS:


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ICS NOTES-8 UNIT-1


Block diagram of measurement system and the functions of various blocks:

Measurement:

Measurement is the conversion of physical parameters in their corresponding numerical values.

In the measuring process, under certain conditions the object's property will be compared to astandard unit i.e., a unit which is already defined for that particular property. There are two

requirements which are to be satisfied to get good result form the measurement.

1. An accurately defined and accepted standard should be used for the comparison purposes.

2. The components or apparatus which are used for measurement and also the measuring

method should be provable.

The main objective of a measuring instrument or measurement system is to provide a numerical

value which is proportional to the quantity of the variable being measured.

The instrumentation system contains following functional elements. They are as follows,

1. Primary sensing elements2. Variables conversion element

3. Variables manipulation element

4. Data transmission element

5. Data storage and presentation element.

1. Primary Sensing Elements

The primary sensing element is that which first receives energy from measured medium and

produces an output depending on the measured quantity. The primary sensing element is

sensitive to the measured variable.

The measured is first detected by primary sensor, which converts the measurand into an,

analogous electrical sensor. This is done by a transducer. A transducer is a device which converts

one form of energy into another form. The first stage of a measurement System is known as a

detector transducer stage. An instrument always extracts some energy from the measured

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.medium. Thus the measured quantity is always disturbed by the act of measurement which

makes a perfect measurement impossible. Good instruments are designed to minimize this effect,

but it is always present to some degree.

2 Variables Conversion Element :The output of the primary sensing element may be physical variable such as displacement or

voltage. The output from the primary sensing element may not be suited for the further stages, in

such cases there is need for variable conversion element. The variable conversion element makes

the instrument to perform the desired function, by converting the variable to another more

suitable variable while preserving the information content of the original signal.

For example, suppose the output is in digital form and th~ 'next stage of the system accepts input

signals any in analog form and therefore, D/A converter is be used for converting signals from

digital to analog.

3. Variables Manipulation Element

To perform the intended task, an instrument may require that a signal represented by some

physical variable be manipulated in some way. Manipulation here mean specifically a change in

numerical value according to some definite rule but a preservation of the physical nature of the

variable. For example an electronic amplifier accepts a small voltage signal as input and

produces an output signal that is also a voltage but is some constant times the input. It is not

necessary that a variable manipulation element should follow the variable conversion element. A

fundamental process is to prevent signal being contaminated by unwanted, signals like noise due

to an extraneous source which may interfere with the original output signal. Another process is

that a weak signal may be distorted by processing equipment. The signal after being sensed

cannot be directly transmitted to \the next stage without removing the interfering sources as

otherwise highly distorted results may be obtained which are far from true. It becomes necessary

to perform certain operations on the signal before it is transmitted. These operations may be

linear or nonlinear. The linear operations performed on the signals may be amplification,

attenuation, integration, differentiation hopping etc. are also performed on the signal to bring it

to the desired form to be accepted by the next stag of measurement system.

This process of conversion is called signal conditioning. The signal conditioning includes

Amplification, Signal filtration, Signal compensation / Signal linearization, Signal averaging,

Signal sampling.

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Amplifier

The term amplification means increasing the amplitude of the signal without, affecting its

waveform. The reverse phenomenon attenuation i.e., reduction of the signal amplitude were

retaining its original waveform. Mechanic al amplifying elements such as levers, gears, or acombination of the two designed to have multiplying effect on the input transducer signal

hydraulic pneumatic amplifying elements employing various types of values or constructions,

such as venturimeter / orificemeter.

Signal Filtration

The term signal filtration means, the removal of unwanted noise signals that tend to obscure the

transducer signal. The signal filtration element could be mechanical filters, pneumatic filters,

electrical filters.

Data Transmission Element

When the elements of an instruments are actually physically separated, it becomes necessary to

transmit data from one to another. The element that performs this functions is called a Data

Transmission Element. The transmission element can be as simple as a shaft and bearing

assembly or as complicated as a telemetry system for transmitting signals from satellites to

ground equipment by radio.

Data Storage and Presentation Element

The storage in the form of pen/ink recording is often employed. Some applications requires a

distinct data storage/play back function which can easily recreate the stored data upon command.

The magnetic tape recorder / reproducer is the classical example. Modern instruments digitize

the electric signals and store them in a computer like digital memory.

The information about the quantity under measurement has to be conveyed to the personnel

handling the instrument or the system monitoring, control, or analysis purpose. The information

conveyed must be in a form intelligible to the 'personal or to the intelligent instrumentation

system. This function is done by data presentation element. In case data is to be monitored,

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visual display devices are needed. These devices may be analog or digital indicating instruments,

like ammeters, voltmeters etc. In case the data is to be recorded, recorders like magnetic tapes,

high speed camera and T.V. equipment storage type CRT, printers, analog and digital computers

or microprocessors may be used. For control and analysis purpose microprocessors or computersmay be used.

REFERENCE BOOKS:


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LVDT- Linear Variable Differential Transformer:

Linear Variable Differential Transformer (LVDT) consists of one primary winding (P) and two

secondary windings (S1 & S2) with equal number of turns wound on a cylindrical former. The

two secondary windings are connected in series opposition and are placed identically on eitherside of primary winding to which an AC excitation voltage is connected. A movable soft iron

core is placed within the cylindrical former. When the displacement to be measured is applied to

the arm of the core, the LVDT converts this displacement into in electrical signal.

The construction of LVDT is illustrated in figure (1)

Figure 1: Construction of LVDT

Figure :2- Circuit Diagram of a LVDT

The operating principle of LVDT depends on mutual inductance. When the primary winding is

supplied with A.C. supply voltage, it generates alternating magnetic field. Due to this magnetic

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field an alternating voltage will be induced in the two secondary windings. In the figure 2 eS1 is

the output voltage of secondary winding S1 and eS2 is the output voltage of secondary winding

S2, In order to get single differential output voltage two secondary windings are connected iQ

series opposition. Thus the differential output voltage is given by,eo= eS1 – eS2.

From figure (2), when the core is placed symmetrically with respect to two secondary windings

an equal amount of voltage will 1 & induced in both windings. Therefore

eS1 = eS2 and the output voltage is '0'. Hence, this position is known as null position.

Now if the core is moved towards up from null position, more magnetic field links with

secondary winding S1 and small field links with secondary winding S2, Therefore more voltage

will be induced in S1 and less in S2 i.e., eSl will be larger than eS2 Hence, the differential output

voltage is eo=eS1 - eS2 and is inphase with primary voltage.

But when the core is moved towards down from mill position more magnetic field links with

secondary winding S2 and small field links with secondary winding S1. Therefore more voltage

will be induced in S2 and less in S1 i.e., eS2 will be larger than eS1. Hence, the differential output

voltage is eo = eS2 – eS1 and is 1800 out of phase with primary voltage.

Thus the output voltage eo position of the core and hence the displacement applied to the arm of

the core.

Merits

(i) LVDT has good linearity i.e., it produces linear output voltages.

(ii) It can measure displacements of very high range usually from 1.25 mm to

250 mm.

(iii) It has high sensitivity.

(iv) Since it produces high output, it does not require amplification devices.

(v) It is simple and rugged in construction. Therefore it can withstand high degree of

shocks and vibrations.

(vi) It has no sliding contacts. Therefore there is no problem of friction.

(vii) It has low hysteresis.

(viii) It consumes less power (about < 1W).

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Demerits

(i) It is sensitive to stray magnetic fields.

(ii) The performance of LVDT is affected by variations in temperature.

(iii) It has limited dynamic response.(iv) To provide high differential output, it requires large displacements.

(v) It provides A.C. output. Therefore it requires a demodulator circuit if the receiving

device operates only on DC.

REFERENCE BOOKS:


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Variable Reluctance Displacement Transducer:

Transducers of variable reluctance type consists of a coil wound on a ferromagnetic core. The

displacement which is to be measured is applied to a ferromagnetic target. The core and the

target are separated by an air gap.

The self inductance of coil is given by,

gi R R

N L

+=

2

Ri = Reluctance of iron parts

Rg = Reluctance of air gap

As Ri <<Rg

g R

N L

2

=

But the reluctance of air gap is given by,

0. µ g

g

g A

l R =

lg = Length of air gap

Ag = Area of flux path

As µ0 and Ag are constant

g R∴ ∝ lg

L ∝ 1/lg

i.e. the self inductance of a coil is inversely proportional to the length of the area.

When the target is near the core, lg is small and hence 'L' is large.

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Since, it is the displacement which changes the length of area, the variation in the self

inductance is a function of displacement as lg ∝ x1

Therefore the input, output relation is non-linear.

REFERENCE BOOKS:


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RVDT- Rotary Variable Differential Transformer:

An RVDT is used to convert the angular displacement into electrical signal. The construction

and working of RVDT is same as that of LVDT except that it employs a cam shaped core as

shown below.

Figure: Circuit Diagram of an RVDT

The cam shaped core rotates between primary and two secondary windings when connected to

shaft whose angular displacement has to be measured. At the null position equal amount of

voltages will be induced in both secondary windings. Therefore

eS1=eS2, and the net output voltage is zero. When angular displacement is applied, a differential

voltage will be generated at the output. This differential output voltage increases with increase of

angular disp1acement. Thus the relation between angular displacement and output will be linear.

When the shaft rotates in clockwise direction, the output voltage increases in one phase, and if

the shaft rotates in anti-clockwise direction. The output voltage increases with an opposite phase.

Therefore the amount of applied angular displacement is known by the magnitude of the output

voltage where as the direction is known by the phase of the output voltage.

REFERENCE BOOKS:


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Principle of Capacitive transducer:

Capacitive transducer operates on the principle of capacitance of a parallel plate capacitor which

is given by,

d AC ε = (or)

d

AC

r 0ε ε

= (1)

Where,

C = Capacitance of a capacitor (Farads)

ε = εr ε0 = Permittivity of medium (F/m)

εr = Relative permittivity (dielectric constant)

ε0 =Permittivity of free space (8.5 x 10-12

F/m)

d = Distance between two plates (m)

A = Overlapping area of two plates (m2) .

The capacitance of a capacitor varies when,

(a) The overlapping area (A) of the plates changes.

(b) The distance (d) between the two plates changes.

(c) The dielectric constant εr changes.

REFERENCE BOOKS:


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Measurement of displacement- Change in Capacitance Due to Change in

Overlapping Area of Plates.

Figure: Capacitive Transducer using the Principle of Change in Capacitance Due to

Change in Overlapping Area of Plates.

From equation (1) it is clear that the capacitance of the capacitor is directly proportional to the

area of plates. Hence, the capacitance varies linearly with the variation in the area of plates. The

area linearly varies with the applied displacement. Therefore the capacitive transducer using this

principle is used to measure linear displacements of about 1mm to 10 mm.

From Figure (1), the capacitance of parallel plate capacitor is,

Where,

d

AC

ε =

d

lbC

ε =

Where,

l = Length of overlapping area of plates

b =Width of overlapping area of plates.

REFERENCE BOOKS:


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Measurement of displacement -Capacitive Transducer using the Effect of

Variation of Distance between the Plates

Figure: Capacitive Transducer -Change in Capacitance Due to Change in Distance

Between Plates

The capacitor operates on the principle of variation of capacitance due to variation in distance

between plates use by two plates, one is fixed and the other is movable. From equation (1) it is

clear that the capacitance of the two plate capacitor is inversely proportional to the distancebetween the plates. When the movable plate moves towards the fixed plate or moves away from

the fixed plate w.r.t. applied displacement, the distance between the plates and hence capacitance

changes. In this case the response is non-linear. Hence, it is used to measure only small

displacements.

Capacitive Transducer using the Effect of Variation of Dielectric Constant

Figure: Capacitive Transducer using the Principle of Change in Capacitance Due to

Change in Dielectric Constant

The capacitive transducer working on the principle of change in capacitance due to variation of

dielectric constant w.r.t. linear displacement is shown in figure. It contains two fixed plates. A

dielectric material with relative permittivity εr moves between these two plates w.r.t applied

displacement.

At initial condition the capacitance of the transducer is,

SANYASIRAO



ICS NOTES-14 UNIT-2


t

bl

t

blC

r

20

10 ε ε ε +=

( )210 lr

lt

bε ε +=

Where the dielectric material moves towards left by distance x, the capacitance varies from C to

∆C.

( ) ( ) xlt

b xl

t

bC C

r ++−=∆+∴ 2010 ε ε ε

[ ])( 210 xl xlt

b

r ++−= ε ε

−++= )1() 0210 r r

t

bxll

t

bε ε ε ε

)1(0 −+r

t

bxC ε ε

)1(0 −=∆r

t

bxC ε ε

Variation in capacitance is

)1(0 −=∆r t

t

bxC ε

Thus the variation of capacitance is directly proportional to applied displacement.

REFERENCE BOOKS:


SANYASIRAO



ICS NOTES-15 UNIT-2

[Type text]


Temperature measuring instruments:

The temperature measuring instruments are classified into four different types based on the nature of

change, produced in the temperature sensing element. They are,

1. Expansion thermometers

2. Change of state thermometers

3. Measurement of temperature using electrical methods

4. Optical and radiation pyrometry.

Classification of Expansion Thermometers:

The expansion thermometers are classified into different types. They are,

Bimetallic thermometer

(i) Expansion of solids Solid-rod thermometers

Liquid-in-glass thermometers

(ii) Expansion of liquids . Liquid-in-metal thermometers

(iii) Expansion of gases - Gas thermometer

REFERENCE BOOKS:


2.Instrumentation and Control systems- S.Sudhakarreddy&P.Divakararao

3.Instrumentation Measurement & Analysis- B.C Nakra and K.K Choudhry

SANYASIRAO



ICS NOTES-15 UNIT-2

[Type text]


SANYASIRAO



ICS NOTES-16 UNIT-2

R.Pola Rao, Asso.Prof, Dept of ME,GMRIT-Rajam Page 30

Bimetallic Thermometers:

Construction and Working Principle

Bimetallic thermometers are of solid expansion type of thermometers. When two different materials

which have different thermal expansion coefficient are joined together, then a bimetallic strip or

bimetallic thermometer (or) bimetallic sensor is formed. The two types of materials used are Brass and

Invar of these two, brass has high thermal expansion coefficient, and Invar has low thermal expansion

coefficient. The bimetallic strip can be available in helical, cantilever flat types and also in spiral shape.

(However the working principle of all these types are same)

A spiral shaped bimetallic thermometer is shown in the below figure..

Figure: Bimetalli Thermometer

One end of the spiral shaped bimetallic strip is fixed and the other end is left out free. A pointer is

attached at the free end of the bimetallic strip. Tile temperature which is to be measured is applied to

the bimetallic strip. As soon as the composite strip senses the temperature it will expand or contract

(depending on the range of temperature). When this happens the pointer attached to the free end of

the strip moves over the calibrated scale which indicates the value proportional to the applied

temperature.

SANYASIRAO



ICS NOTES-16 UNIT-2


Applications of Bimetallic Thermometers

1. These are used in control devices in a process.

2. Bimetallic shaped strips (helical type) are widely used in oil burners, refineries, type vulcanizers

etc.

3. A spiral shaped strip find its applications in air conditioning thermostats.

Merits

1. Since the scale is calibrated in terms of temperature we can take the readings easily and

directly.

2. These are available in various types. Therefore we can choose anyone type depending upon the

requirement.

3. Simple construction

4. Simple operation

5. Fast response

6. Light weight

7. Cost is less.

Demerits

1. Cannot be used for measurement of high range of temperature.

2. Measurement of temperature at remote areas is not possible.

3. Low accuracy.

SANYASIRAO



ICS NOTES-16 UNIT-2


Liquid-in-glass Thermometer

The basic schematic of liquid-in-glass thermometer is shown in figure.

It is the most frequently used temperature measuring instrument in different applications. It consist of a

temperature sensing bulb, a responsive fluid and a scale. One end of the capillary tube is connected to

safety bulb and other end is connected to temperature sensing bulb. The most widely used fluids in

liquid-in-glass thermometer is either mercury or alcohol. This fluid is obtained with in a temperature

sensing bulb and a capillary tube.

The quick transfer of heat is possible with the liquid-in-glass, when the temperature sensing bulb walls

should be thin. One end of the capillary is connected to safety bulb since the volume of the capillary

tube is less than the capacity of bulb. Alcohol, has higher coefficient of expansion than mercury hence it

is widely used than mercury. The higher temperature range of this instrument is 340DC.If the space

present above mercury is filled with corbondioxide or N2 at high pressure then the boiling point of

SANYASIRAO



ICS NOTES-16 UNIT-2


mercury is increased hence the temperature range of this instrument is extended to 5600C. If we use

this type of thermometers under some optimum conditions, its accuracy cannot exceed

0.10C.When there is a necessity to increase the accuracy then the Beekman thermometer is used

through which we can obtain an accuracy of 0.0050C. Here the thermometer measurement range is

limited to 5 - 60C.

Gas Thermometer

When the pressure of a gas is maintained constant, as temperature increases the volume of the gas also

increases. Therefore, in case of constant pressure thermometer as temperature increases the volume of

the gas also increases. Here the pressure and mass of the gas are kept constant. When the volume of

the gas is maintained constant, as temperature increases the pressure of the gas also increases.

Therefore, in case of constant volume the thermometer as temperature increases the volume of the gas

also increases. Here the volume and mass of the gas are kept constant.

We know that at constant volume,

PT = Po (l+βl T) (.: as temperature increases the pressure of the gas also increases)

Where,

PT = pressure at Toe

Po = pressure at 00C

β1 = thermal coefficient of pressure

The pressure change in the gas pressure is given by

)( 1210 T T PP −=∆ β

T PP ∆=∆ 10 β

Where,

∆P = Pressure change

∆T = (T2-T1) = Change in temperature

SANYASIRAO



ICS NOTES-16 UNIT-2


In equation (1), ∆P is proportional to ∆T

The basic schematic diagram of a gas thermometer is shown in figure. It consists of a sensing bulb, a

bourdon tube and a capillary tube. A bourdon tube is a pressure transducer which is used to measure

the change in the pressure of a gas.

Figure: Gas Thermometer

The bourdon tube calibrated directly on the basis of change in pressure corresponding to the

temperature of the bulb. The actuating power of the bourdon tube is limited due to the pressure change

in temperature is very small.

The volume of gas in the capillary is very small compared to that of volume of gas in the bulb since this

thermometer bulbs are made large. Therefore, the effect of ambient temperature is reduced, due to this

the dynamic response of the gas -thermometer for transient changes is also reduced.

REFERENCE BOOKS:




SANYASIRAO



ICS NOTES-16 UNIT-2


SANYASIRAO



ICS

R.Pola Rao, Asso.Prof, Dept of

Pressure- force exerted by a flui

Units of Pressure

1 Pascal =1

1 Bar =1

1 atm =1

=1

=7

=1

1 torr = 1

Absolute Pressure = Atmospheri

Vacuum Pressure = Atmospheri

NOTES-17

E,GMRIT-Rajam

on unit area

N/m2

05

N/m2

.013x 105Pa

.013 bars

60 mm 0f mercury

0.3 m of water

mm of Hg(1 mm of mercury)

pressure + Gauge pressure

pressure - Absolute pressure.

UNIT-3

Page 35

SANYASIRAO



ICS


Static Pressure: It is defined as t

parallel to the wall in a pipe.

Stagnation or Total Pressure: It

stream were brought to rest isent

REFERENCE BOOKS:

1.A course on mechanical me

2.Instrumentation and Control

3.Instrumentation Measurem

NOTES-17

E,GMRIT-Rajam

he pressure acting on the wall by a fluid at rest

is defined as the pressure that would be obtaine

ropically

asurements and instrumentation- A.K Saw

systems- S.Sudhakarreddy&P.Divakarara

nt & Analysis- B.C Nakra and K.K Choudh

UNIT-3

Page 36

r flowing

if the flu id

ney

y

SANYASIRAO



ICS NOTES-18 UNIT-3


Classification of Pressure Measuring Devices

According to the Range of Pressure to be Measured:

(a) High pressure measuring instruments (above 700 atm).

(b) Moderate pressure measuring instruments.

(c) Low pressure measuring instruments (1 mm Hg or below).

According to the Principle of Operation:

I. Balancing the pressure by a column of liquid (or) dead weight

(a) Manometers

(b) Dead weight tester

(c) Mcleod gauge

2. By measuring elastic deformation caused by the pressure

(a) Bourdon tube pressure gauge

(b) Diaphragm pressure gauge

(c) Bellows pressure gauge

3. Special methods (By measuring change in electrical quantities that vary with pressure) .

(a) Pirani thermal conductivity gauge

(b) Ionization gauge

(c) Knudsen gauge

(d) Bridgman gauge

Pressure gauges and their applications:

SANYASIRAO



ICS NOTES-18 UNIT-3


REFERENCE BOOKS:




SANYASIRAO



ICS


Manometers

Devices used for measuring pres

fluid by balancing the column of

A simple manometer consists of end is connected to a point wher

1.Simple manometers.

• Piezomete

• U - Tube

• Single col

2. Differential manometers

• U - Tube

Operation of Pressure Measurin

• Manometers

• Bourdon tube

• Bellows gage

Manometers

1. Piezometer:

Used to measure the gauge p

One end of the manometer i

the other end is open to the a

NOTES-19

E,GMRIT-Rajam

sure at a point or difference of pressure betwee

fluid by the same or another column of the flu

a glass tube having one of its ends open to atmthe pressure is to be measured

r

anometer

mn manometer

ifferential Manometer:

Devices

Bourdon tube Bello

ressure at a point in the fluid.

s connected to the point where the pressure is t

mosphere

UNIT-3

Page 39

two points in a

id.

sphere and other

ws gage

be measure and

SANYASIRAO



ICS


If 'h' is the rise in liquid level

P = wh= ρ

where, w = Specific weight

ρ = density of the liq

2. U- Tube Manometer:

One end of the V-tube is open

the pressure is to be measured

The U-tube is generally fill

greater.

Than the specific gravity of th

3.Single Column Manometer:

It is a modified form of U-T

In which a reservoir of large

manometer

When the manometer is con

height 'h2

Let '∆h' be the fall in liqu

Rise of heavy liquid in right

NOTES-19

E,GMRIT-Rajam

gh

f liquid= ρg.

uid

It consists of a glass tube bent into U-shape.

to the atmosphere and other end is connected t

as shown.

d with mercury or any other liquid whose s

e liquid whose pressure is being measured.

h=h2s2-h1s1

P = wh= ρgh

be manometer

cross sectional area is connected to one of the li

ected to the pressure point, liquid in the right li

id level in the reservoir.

limb is equal to the fall of heavy liquid in the re

UNIT-3

Page 40

the point where

ecific gravity is

mbs of the

b is raised to a

servoir.

SANYASIRAO



ICS


REFERENCE BOOKS:




NOTES-19

E,GMRIT-Rajam




UNIT-3

Page 41

ney

y

SANYASIRAO



ICS


U - Tube Differential Manomet

Two ends of which are c

measured.

The U-Tube contains he

P A+1000

Advantages of the Manometers:

I. High accuracy and good sensit

2. Relatively inexpensive.

3. Easy to fabricate.

4. Suitable to measure low press

5. Simply by changing the mano

Limitations of the Manometers:

1. Fragile construction.

2. Readings are effected by chan

3. Surface tension of manometri

4. Recording is not possible.

5. High pressures cannot be mea

NOTES-20

E,GMRIT-Rajam

r:

nnected to two points whose pressure differenc

vy manometric liquid.

1g(x+y)-1000S 2gx-1000S 1gy=P B

ivity.

res.

metric liquid the sensitivity of the instrument ca

ges in gravity and temperature.

liquid creates capillary affects.

sured. (i.e., more than 2 atms).

UNIT-3

Page 42

e is to be

n be altered.

SANYASIRAO



ICS


Dead-weight Pressure Gauge

It is generally used for th

It consists of a piston wh

The chamber and the cyli

A pressure gauge which i

Operation-The oil is pres

are placed on the platfor

The pressure acting on th

PA=W

PA = W + Frictional force.

Applications:

I. It is used to calibrate all

engine indicators etc.

Advantages:

I. Simple in construction and eas

2. Wide range of instruments ca

3. Fluid pressure can be varied e

Disadvantages:

I. The accuracy of the instrumen

reduced by applying proper lubri

NOTES-21

E,GMRIT-Rajam

calibration purposes.

ch moves in a cylinder as shown in the figure.

nder are filled with oil.

s to be calibrated is fitted to the chamber.

surised with the help of a plunger and known st

of the piston until equilibrium is achieved

e piston

inds of pressure gauges such as industrial press

y to operate.

be calibrated

sily.

is limited due to friction. However frictional r

cation.

UNIT-3

Page 43

ndard weights

ure gauges,

sistance can be

SANYASIRAO



ICS


Elastic Transducers

Elastic elements get defo

The deflection / deforma

Bourdon Tube Pressure

It consists of an elastic t

a circular arc.

This tube is called bourd

One end of the tube is cl

pressure is to be measure

Systems of gears are pr

pointer on a graduated sc

Operation- When the op

point,

Fluid under pressure ente

to circular shape.

This change in sh

The change in cu

pointer which rotates on

Advantages:

1. Low cost and simple c

2. High accuracy

3. Rugged construction

NOTES-22

E,GMRIT-Rajam

rmed when subjected to pressure.

ion of the elastic element is measured by a con

Gauge:

be of steel or bronze which is of elliptical shap

on tube which acts as a pressure sensing eleme

sed and other end is open to allow the fluid int

d.

vided to magnify the deflection of the tube

ale.

n and the end of the bourdon tube is connecte

rs the tube and the elliptical shape of the tube g

ape causes the tube to straighten out slightly.

vature of the tube is transmitted through a syste

he graduated dial.

onstruction

UNIT-3

Page 44

enient method.

and is bent into

t.

o the tube whose

nd to rotate the

d to the pressure

radually changes

m of gears to the

SANYASIRAO



ICS


4. Wide range of measur

5. Portable size

6. Direct reading is obtai

Disadvantages:

1. It responds slowly to t

2. It exhibits hysteresis.

3. It is sensitive to shock

4. It requires amplificatio

Diaphragm Pressure Ga

Diaphragm is a thin plate

Diaphragms are made o

nickel alloys.

The diaphragm gets def

sides.

It always deflects towar

The deflection is then se

NOTES-22

E,GMRIT-Rajam

ment are possible

ed

e pressure changes

and vibration

n.

uges

of circular shape clamped firmly around its ed

elastic metal alloys such as bronze, stainless

lected in accordance with the pressure differe

s the low pressure side.

sed by an appropriate displacement transducer.

UNIT-3

Page 45

es.

teel and ferrous-

ntials across the

SANYASIRAO



ICS


An electrical-resistance s

deflection.

Other diaphragm defle

transducers.

Amplification of deflectilinkages.

Advantages:

I. They are available in s

2. They exhibit linearity

3. They can withstand ov

4. Absolute and different

5. Minimum hysteresis a

Limitations:

1. They need protection f 2. They cannot be used t

3. Difficult to repair thes

Applications:

]. They are used as low p

2. Used as draft gauges.

REFERENCE BOOKS:




NOTES-22

E,GMRIT-Rajam

train gauge may also be installed on the diaph

ction measuring transducers are capacitiv

on of the diaphragm can also be done by mea

all sizes and at low cost.

ver wide range.

er pressures.

ial pressures can be measured.

d no permanent zero shift.

rom vibration and shock.measure high pressures.

gauges.

ressure absolute gauges.




UNIT-3

Page 46

agm to sense the

and inductive

s of mechanical

ney

y

SANYASIRAO



ICS

R.Pola Rao,Asso.Prof,Dept of M

Bellows Pressure Gauge

Bellows gauge is made of a thin

Common bellows materials are t

copper.

When the pressure is applied at f

movable end is the measure of a

Bellows Differential pressure g

Advantages:

I. Simple and rugged constructio

NOTES-23 UNI

E

s

metallic tube having deep circumferential corru

rumpet brass, stainless steel, phosphor bronze a

ixed end of the bellows, it gets elongated. The

plied pressure

uge

n.

-3

Page 47

gations.

d beryllium

eflection of the

SANYASIRAO



ICS NOTES-23 UNIT-3

R.Pola Rao,Asso.Prof,Dept of ME Page 48

2. They can be used for the measurement of both gauge and differential pressures.

3. They can be used for low and moderate pressures.

4. Moderate cost.

Limitations:

I. Problems of hysteresis and zero shift.

2. Not suitable for dynamic measurements due to their large mass.

3. Temperature compensation is required.

4: Cannot be used for high pressure measurement.

REFERENCE BOOKS:




SANYASIRAO



ICS


Electrical Resistance Pressure

Principle-that when an electrical

the conductor occurs due to the

The change in resistance may be

Resistance of an electrical condu

Where, ρ =specific resistance o

L =length of the conduct

The conductor wire will be subje

From eqn ------(1)

NOTES-24 UNI

E

auge

conductor is subjected to high pressure. change

ulk-compression effect.

calibrated in terms of the applied pressure

ctor of diameter D is given by

the conductor

r

eqn ------(1)

cted to a biaxial stress

-3

Page 49

in resistance of

SANYASIRAO



ICS


If ρis independent of pressure, th

REFERENCE BOOKS:




NOTES-24 UNI

E

en




-3

Page 50

ney

y

SANYASIRAO



ICS


Bridgman or Bulk mod

It consists of pressure se

Bellow is filled with kero

element.

Sensing element are mad

Thermal conductivity g

It measures the pressure throug

At low pressure there is a relatio

i.e., the heat conductivity decrea

The temperature of an elcurrent and the rate of he

If the current is-kept con

temperature is governed

The lower the gas pressu

higher the filament temp

NOTES-25 UNI

E

lus gauge:

sing wire which is enclosed in flexible bellows.

sene for transmitting the pressure to be measur

with magnanin and an alloy of gold and 2.1 %

uges:

a change in the thermal conductivity of the gas

nship between pressure and thermal conductivit

es with decreasing pressure

ctrically heated filament depends upon the magat dissipation from the filament.

tant, then the heat loss from the filament and h

y the conductivity of the surrounding gas medi

e, the lower the thermal conductivity and cons

rature for a given electric energy input.

-3

Page 51

d to the sensing

chromium.

.

y,

nitude of the

nce its

m.

quently the

SANYASIRAO



ICS NOTES-25 UNIT-3


Estimation of the gas pressure is made by measuring either temperature or resistance

variation of the filament.

REFERENCE BOOKS:




SANYASIRAO



ICS


Cup and vane anemometers

Which measure the speed of

The devices essentially cons

velocity of flow

Which measure the speed of

The devices essentially cons

velocity of flow

The drag on the cup A is gre

The resultant torque rotates t

The number of revolutions i

of rotation gives a measure o

In a vane anemometer, vane

flow is parallel to the axis of

The rotor drives a low fric

wind speed

The cup type unit is best f

speeds more accurately.

Experiments indicate that pr

speed and angular velocity o

NOTES-26 UNI

E,GMRIT-Rajam

air movement.

ist of a rotating element whose speed of rotatio

air movement.

ist of a rotating element whose speed of rotatio

ter than that on cup B .

he assembly in the anticlockwise direction.

read from a dial for a given period of time, a

f the average speed of air in the region traverse

s of the wind mill type are mounted in a sup

rotation.

ion gear train which in turn drives a pointer

r relatively low speed whereas the vane typ

vided the wind speed is not too large, the relati

cup/vanes is linear.

-3

Page 53

n varies with the

n varies with the

nd the frequency

by the air.

ort so that fluid

hat indicates the

measures large

on between wind

SANYASIRAO



ICS


Current meters:

A current meter consists

vanes are fixed.

The unit is suspended i

tension) by a streamline

NOTES-27 UNI

E,GMRIT-Rajam

of a horizontal wheel on which conical buc

to the flow stream by suspension cable whic

weight.

-4

Page 54

ets or V-shaped

is held taut (in

SANYASIRAO



ICS


Horizontal positioning (

streamlined tail vane.

When the meter is held i

wheel in rotation.

At every revolution, sig

through electrical contact

Turbine meters:

The turbine flow meter c

The rotor is supported by

along which the flow occ

NOTES-27 UNI

E,GMRIT-Rajam

placement of unit along the flow direction)

a flowing stream, the liquid strikes the bucket

als are transmitted to the observer or to a re

s.

nsists of a freely rotating wheel with multiple

ball or sleeve bearings, and is located centrally

urs.

-4

Page 55

is ensured by a

and that sets the

volution counter

lades.

in the pipe

SANYASIRAO



ICS


The flowing fluid imping

and sets the rotor in moti

The rotor speed is measu

up and associated counte

The electro-magnetic, pi

one of the rotor blades w

As the magnet moves pa

The voltage pulse is pick

Faster the fluid flow, the

Each pulse represents a d

taken as an indication of

Where

f is the pulse freq

Q is the volume fl

K is the flow coef

Which depends on the fl

By minimizing the beari

linear output.

Advantages and limitati

Good accuracy, excellent

Low pressuredrop; good

Easy to install and maint

Good transient response

Relatively high cost and

Requires in-line mounti

NOTES-27 UNI

E,GMRIT-Rajam

ing on the turbine blades imparts a force on the

n with an angular speed proportional to the flu

red with a mechanical counter or with an electr

.

k up could be a small permanent magnet moun

th a coil being placed just outside the tube.

st the coil, an e.mf. is induced.

d up for each rotation of the wheel.

greater the count per second.

efinite flow quantity, and the total number of p

otal flow

ency,

ow rate and

ficient

w rate and the viscosity.

ng friction and other losses ,the device can be d

ons :

repeatability

emperature and pressure ratings

in adaptability to flow totalising and to digital

limited use for slurry applications

g and a straight run of pipe (15 diameters) ahea

-4

Page 56

blade surfaces

d velocity.

-magnetic pick

ed at the tip of

lses may be

signed to give a

lending systems

d of the meter.

SANYASIRAO



ICS


Rotameter (or)Variable Ar

The Rota meter consists

or active element (float)

This tapering tube is pro

The float or bob material

With increase in the flo

the annular area between

The float adjusts its posi

i.e., the float rises higher

The discharge equation f

Advantages and limitati

Simplicity of operation, e

Relatively low cost

NOTES-28 UNI

E,GMRIT-Rajam

a Meter (or) Area Meter

f a tapered metering glass tube, inside which is

f the rotor.

ided with suitable inlet and outlet connections.

has specific gravity higher than that of the fluid

rate, the float rises in the tube and there occuthe float and the tube.

ion in relation to discharge through the passage

or lower depending on the flow rate.

r flow through a Rota meter is given by

ns of a Rota meter:

ase of reading and installation

-4

Page 57

located the rotor

to be metered.

rs an increase in

SANYASIRAO



ICS


Handles wide variety of

Easily equipped with dat

Possibility of convenient

side by side

Glass tube subject to bre

Limited to small pipe siz

Less accurate compare to

Must be mounted vertica

Subject to oscillations in

Hot wire anemometer:

The sensor is a 5 micron

prongs of the probe and

When the probe is introd

instantaneous velocity an

diminish.

The rate of cooling of wi

wire, (ii) difference of th

properties of the fluid, an

NOTES-28 UNI

E,GMRIT-Rajam

corrosive fluids

a transmission, indicating and recording device

and visible flow comparisons by mounting se

akage

es and capacities

venturi and orifice meters

lly

pulsating flows

diameter platinum-tungston wire welded betwe

eated electrically as part of a wheat-stone bridg

ced into the flowing fluid, it tends to be cooled

d consequently there is a tendency for the elect

e depends upon the (i) dimensions and physical

temperature between the wire and the fluid, (ii

d (iv) stream velocity under measurement.

-4

Page 58

eral Rota meters

n the two

e circuit,

by the

ical resistance to

properties of

i) physical

SANYASIRAO



ICS NOTES-28 UNIT-4


For a simple hot wire anemometer the first three conditions are effectively constant and

the instrument response is then a direct measurement of the velocity.

REFERENCE BOOKS:

1.A course on mechanical measurements and instrumentation- A.K Sawhney 2.Instrumentation and Control systems- S.Sudhakarreddy&P.Divakararao


SANYASIRAO



ICS


Electro-Magnetic flow

Principle:

Voltages generated when

From these measurement

The measurement of flo

law of electromagnetic in

When a pipe or tube carr

magnetic field an e.m.f.

This voltage gives the m

An electromagnetic flow

carry the flow whose vel

To the opposite sides of t

the fluid flow inside the

Now this pipe is placed b

that produces magnetic fi

Working :

When the conducting flui

pipe, it cuts the magnetic

NOTES-29 UNI

E,GMRIT-Rajam

eters

a conducting fluid flows through an applied m

s, the flow rate may be deduced.

rate using an electromagnetic flow meter depe

duction.

ing electrically conducting fluid is placed in a

ill be induced across the electrodes connected

asure of the velocity of the fluid or flow rate of

meter consists of a nonmagnetic and non condu

city or flow rate is to be determined.

his pipe a pair of insulated electrodes which are

ipe are connected.

etween the two poles of an electromagnet or pe

eld.

d whose flow rate is to be measured is made to

field causing some e.m.f. to be induced across

-4

Page 60

gnetic field.

ds on Faraday's

ransverse

o it.

the fluid.

cting pipe to

in contact with

manent magnet

flow through the

he electrodes.

SANYASIRAO



ICS


This induced voltage is g

The above equation impl

induced e.m.f. as long as

NOTES-29 UNI

E,GMRIT-Rajam

iven by,

es that the volume flow rate is directly proporti

the flux density remains constant.

-4

Page 61

onal to the

SANYASIRAO



ICS NOTES-29 UNIT-4

Page 62


Magnetic Type Level Indicator :

Magnetic type level indicator is used to measure the level of liquids which contain

corrosive and toxic materials.

It contains a float in which a bar magnet is arranged and is placed in the chamber whose

liquid level is to be determined.

The float moves up and down with the increase and decrease in the level of liquid

respectively.

A magnetic shielding device and an indicator containing small wafers arranged in series

are attached to the sealed chamber.

These wafers are coated with luminous paint and rotates about 1800

to expose a

differently coloured surface.

SANYASIRAO



ICS NOTES-29 UNIT-4

Page 63


As the level changes, the float moves (along with magnet) up and down.

Due to this movement of magnet ,the wafers rotate and present a black coloured surface

for the movement of float in one direction and an yellow coloured surface for themovement of float in opposite direction.

REFERENCE BOOKS:




SANYASIRAO



ICS NOTES-30 UNIT-4


Laser Doppler Anemometer (LDA)

Basic Principle :

Laser Doppler Anemometer (LDA) works on the principle of Doppler effect.

It states that whenever a laser beam passes through the moving fluid, frequency shift

takes place in the light scattered by the small particles present in that fluid .This shift in

the frequency of the beam is directly proportional to the velocity of the fluid flow.

Working: It consists a

He-Ne laser source,

Beam splitter,

Focusing and receiving lenses,

Photo detector,

Signal processing and Recording circuit.

SANYASIRAO



ICS NOTES-30 UNIT-4


The beam splitter can be optical prism or rotating optical gating or half silvered mirror.

The beam splitter is placed at 45° to the laser beam.

The laser beam from laser source passes through the beam splitter which split the beam

into two parts.

These two parts of beam then passes to focusing lens L1 which focus the beam at a point

in the flow stream where the velocity is to be determined.

If the flow stream containing tiny tracer particles such as microscopic dust or dust

particles passes through the high intensity area, the particles scatter the light. Due to this

frequency shift occurs in the scattered light.

When this scattered light falls on the photo detector circuit which shows the varying

electrical signal.

The frequency of this signal is proportional to the rate at which the dust particles crosses

the interference fringes.

The spacing between the fringes can be expressed as,

Where,

λ - is wavelength of laser beam

θ- is angle between two converging beams.

Advantages :

1. This method does not disturbs the fluid flow.

2. It does not require any calibration to determine the flow. .

3. It does not require any physical contact with the fluid whose flow rate is to be

determined i.e., it is a non-contact type instrument

4. Accuracy is very high (approximately ± 0.2%).

5. It can be used to measure both liquid and gaseous flows.

Disadvantages :

1. It is suited for the measurement of flow passing through transparent channel only:

SANYASIRAO



ICS


2. It is not suited for the

3. A skilled operator is re

4. Cost is very high.

Rotating Vane Type Fl

It consist of an eccentric

The vanes move when th

rotor.

The inlet flow (flow com

Therefore a fixed volum

Thus the total number of

Advantages :

1. It can be used for liqui

2. Good accuracy of the

3. Permits low pressure d

Disadvantages :

1. It is expensive.

2. Bulky an heavy.

3. Accuracy tends to dec

NOTES-30 UNI

E,GMRIT-Rajam

lows of clean fluids.

quired to use this instrument.

w Meter:

rum as a rotor and spring loaded vanes.

rotor rotates radially in and out of the slots in

ing inside) rotates the rotor.

of fluid is trapped and delivered outside for ev

rotations of the rotor gives the volumetric flow.

s containing viscous materials.

rder of 0.5%.

rop.

ease for low flow rates.

-4

Page 66

he eccentric

ery rotation.

SANYASIRAO



ICS


Lobed-Impeller Flow M

The unit consists of two

The rotors are lobed, and

The rotor lobes are of cy

The swept volume betwe

meter four times per revo

The Impeller rotating spe

This meter is used prima

drop and good accuracy

Reciprocating Piston M

NOTES-31 UNI

E,GMRIT-Rajam

ter

otors which are mounted on separate parallel s

revolve in opposite directions in a close-fitting

loidal or involute form and this ensures their c

n each impeller and the chamber wall is passe

lution.

ed is proportional to the volume of fluid flow.

rily for metering gases, and has the advantages

articularly at high flow rates.

ter:`

-4

Page 67

afts.

chamber.

rrect mating.

through the

of small pressure

SANYASIRAO



ICS NOTES-31 UNIT-4


The meter essentially consists of a piston reciprocating inside a cylinder.

Inlet and outlet valves with their opening and closing being controlled by a slide valve;

and slide valve actuated by the piston movement.

With the motion of the piston, the fluid is passed from the inlet to outlet alternately

through each end of the cylinder.

The volume of the fluid flow is directly proportional to both the stroke length and the

piston speed.

The meter gives quite accurate measures of discharge in the range 10to1000 gpm. The

use is, however ,limited. to non-corrosive and low viscosity liquids

REFERENCE BOOKS:




SANYASIRAO



ICS


Nutating Disk Meter:

It consists of an eccentric

The peculiar wobbling o

only a rotation but a swe

The top and bottom of th

chamber.

The chamber is thus seal

and emptied; each compa

The viscosity of the liqui

The compartments progr

liquid pressure; and thusoutlet.

For a known capacity of

quantity of fluid flow can

The motion of the disk c

can be directly calibrated

NOTES-32 UNI

E,GMRIT-Rajam

ally mounted disk which nutates in a hemisphe

r nutating motion of the disk implies that the di

ping motion also.

disk maintain tangential contact with the top a

d off into separate compartments which are suc

rtment holding a definite volume.

takes care both of sealing and lubrication.

ss from suction to discharge side as the disk w

cause a definite volume of liquid to be passed f

he chamber and the number of disk oscillations

be metered.

n be made to drive a recording and indicating

in terms of the liquid discharge.

-4

Page 69

rical chamber.

k provides not

nd bottom of the

cessively filled

bbles under

om inlet to

, the total

echanism which

SANYASIRAO



ICS


Classification:

1. Direct method

a. Liquid lev

b) Float type level-Ic) Float and shaft li

d) Float operated po

e) Hook type level i

2. Indirect method.

a) Hydrostati

b) Bubbler o

c) Float Ope

d) Capacitan

e) Ultrasonic

f) Radio Act

Liquid level sight glass :

It is frequently used visu

It is a tube made up of gl

NOTES-33 UNI

E,GMRIT-Rajam

el sight glass

dicatoruid level gauge

entiometer,

dicator.

c Pressure Level Measuring Devices

Purge System

ated Rheostat

e Liquid - Level Sensor

Level Indicator

ive or Gamma - Ray Liquid Level Indicator or

l type of level indicator is gauge glass .

ss and is fixed to one side of the tank or vessel.

-4

Page 70

ucleonic Gauge

SANYASIRAO



ICS NOTES-33 UNIT-4


The tank is filled with liquid and the variation in the level of the liquid can be measured.

The liquid level inside the tank and the level inside the gauge glass are maintained at an

equal level or same level.

A scale is fixed to the gauge glass or some markings will be made on the gauge glass.

The calibrated scale shows the rising and falling level of the liquid inside the gauge glass

which in turn gives the level of the liquid inside the tank.

Application:

Can be used in the measurement of liquid level in a closed tank

Merits:

1. We can read the reading directly on the calibrated scale

2. This type of construction is simple

3. Inexpensive

Demerits:

1. Accuracy in measurement is achieved provided the liquid is clean.

2. It is not used to measure level of hot liquids because the glass will break.

3. Cannot be used with viscous fluids and slurry fluids

Float type level-Indicator :

SANYASIRAO



ICS


The float is dipped in the

Any corrosion resisting

The float is connected to

A pointer scale arrangem

As the level of the liquid

When the level increases

the pulley makes the pull

With this the pointer atta

the present level of the ta

Application :

Can be used to know the

Merits :

1. This arrangement is av

NOTES-33 UNI

E,GMRIT-Rajam

water tank whose level is to be measured.

aterial is used to make the float.

the pulley through a stainless steel cable.

ent is also attached to the pulley.

varies the position of the float varies

the float will be lifted up and the cable which is

ey to rotate.

ched to the pulley moves over a calibrated scal

nk.

level of liquids in sumps, reservoirs and in ope

ailable in different number of designs.

-4

Page 72

wound around

and indicates

tanks

SANYASIRAO



ICS NOTES-33 UNIT-4


2. Can be used for liquids of high temperatures.

Demerits :

1. Wear-tear problems due to movable parts.

2. These are used for liquids only with moderate pressure

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ICS


Hook type level indicato

A-A -is the initial liquid l

B-B-Final liquid level.

The hook is adjusted in

A and B-B and the readi

The difference of the two

Bubbler or Purge System

NOTES-34 UNI

E,GMRIT-Rajam

evel.

uch a way that the tip of the hook is coincides

g is taken.

readings is the increase in the level of the liqui

:

-4

Page 74

ith the level A-

SANYASIRAO



ICS


The bubbler tube

reference level, a

gauge.

Now the supply o

is slightly highertank.

When there is a small flo

by the pressure gauge is

provided the gauge is cal

REFERENCE BOOKS:

1.A course on mechanical

2.Instrumentation and Co

3.Instrumentation Measur

NOTES-34 UNI

E,GMRIT-Rajam

is dipped in the tank such that its lower end is a

d the other end is attached to a pressure regulat

f air through the bubbler tube is adjusted so tha

han the pressure exerted by the liquid column i

w of air and the fluid has uniform density, the p

irectly proportional to the height of the level in

ibrated properly in units of liquid level.

measurements and instrumentation- A.K S

trol systems- S.Sudhakarreddy&P.Divakar

ment & Analysis- B.C Nakra and K.K Cho

-4

Page 75

zero level i.e.,

or and a pressure

the air pressure

the vessel or

ressure indicated

the tank

awhney

rao

dhry

SANYASIRAO



ICS


Capacitance Liquid - Lev

Principle-Capacitance of

dielectric constant of it c

In this case the capacitan

change in area of plates i

Therefore when the heig

and output capacitance in

Similarly when the heigh

Merits :

1. This method of level

2. This method can be us

3. No problem of wear-te

NOTES-35 UNI

E,GMRIT-Rajam

el Sensor

the parallel plate capacitor varies or changes if

anges.

e varies as the height of the liquid changes i.e.,

used.

t of the liquid increases the area between the pl

creases.

t decreases the capacitance also decreases.

easurement is very sensitive.

d for small systems.

ar since it does not contain any movable parts.

-4

Page 76

he area or

the principle of

ates decreases

SANYASIRAO



ICS


4. It can be used with slu

Demerits :

1. The performance will

2. The connection and m

some errors may occur.

Ultrasonic Level Indicator :

Principle - Reflection of

The transmitter (T) sends

The waves get reflected f

receiver (R).

The time taken by the tra

back to the receiver gives

As the level of the liquid

back to receiver also cha

Thus the changes in the l

NOTES-35 UNI

E,GMRIT-Rajam

ry fluids.

e affected by the change in temperature.

unting of metal tank with the meter should be

sound wave from the surface of the liquid.

the ultrasonic waves towards the free surface o

rom the surface. These reflected waves are rece

nsmitted, wave to travel to the surface of the liq

the level of the liquid.

changes, the time taken to reach the surface of

ges.

vel of the liquid are determined accurately.

-4

Page 77

roper, otherwise

f the liquid.

ived by the

uid and then

liquid and then

SANYASIRAO



ICS NOTES-35 UNIT-4


Advantages :

1. Operating principle is very simple.

2. It can be used for various types of liquids and solid substances.

Disadvantages :

1. Level measurement of this type requires very good experienced and skilled operator.

2. It is very expensive.

REFERENCE BOOKS:




SANYASIRAO



ICS NOTES-36 UNIT-4


Magnetic Type Level Indicator

Magnetic type level indicator is used to measure the level of liquids which contain

corrosive and toxic materials.

It contains a float in which a bar magnet is arranged and is placed in the chamber whose

liquid level is to be determined.

The float moves up and down with the increase and decrease in the level of liquid

respectively.

A magnetic shielding device and an indicator containing small wafers arranged in series

are attached to the sealed chamber.

These wafers are coated with luminous paint and rotates about 1800 to expose a

differently coloured surface.

As the level changes, the float moves (along with magnet) up and down.

Due to this movement of magnet ,the wafers rotate and present a black coloured surface

for the movement of float in one direction and an yellow coloured surface for the

movement of float in opposite direction.

Advantages :

1. Direct method of level measurement shows actual level of the fluid directly.

2. It is a very simple method of liquid level measurement.

3. It does not require any compensation for variations in level due to temperature.

SANYASIRAO



ICS NOTES-36 UNIT-4


4. Liquid level measurement 'Using direct method does not require any calibration.

Limitations :

1. This method is not suitable for corrosive and viscous liquids.

2. It is not possible to determine liquid levels of tanks located at remote areas.

3. It is not possible to record/the output for future use.

4. The glass gauges are fragile and liable to break.

5. Is not suited for the level measurement of very high tanks.

6. Cannot be used for high pressure ranges.

REFERENCE BOOKS:




SANYASIRAO



ICS NOTES-37 UNIT-5


Tachometers:

Tachometers are the devices which are commonly used for measuring the angular speed.

Tachometer may be defined as "an instrument which either continuously indicates the value of

rotary speed or continuously displays a reading of average speed over short intervals of time“.

Mechanical tachometers :

Vibrating reed tachometer

Centrifugal force tachometer

Slipping clutch tachometer

Hand speed indicator.

Electrical Tachometer :

Electrical tachometers

Drag cup tachometer

Commutated capacitor tachometer

Tachogenerators

Contactless tachometers.

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ICS NOTES-38 UNIT-5


Vibrating reed tachometer

When the tachometer base plate is kept in contact with any non moving part or the

machine reed turned to resonance with the machine vibration frequency.

The indicated reed vibration frequency is the direct measure of speed of the machine, if

the reed frequency is-calibrated in terms of speed.

It operates on the principle that, 'Centrifugal force is proportional to the speed of

rotation‘.

The main advantage of this tachometer is, it can be used to measure speeds of different

ranges simply by changing the gear train & linkages.

Centrifugal tachometers are used to ,measure speeds up to 40,000 rpm with an Accuracy

of I%.

Slipping Clutch Tachometer

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ICS NOTES-38 UNIT-5


Friction material is placed between slipping clutch and input shaft.

The spiral spring is placed between slipping clutch and indicator shaft.

The indicating shaft is driven by the rotating shaft through a slip clutch and hence,

named is called as slipping clutch tachometer.

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ICS


Hand speed indicator.

It consists of an integral

The spindle operates whe

But the counter does not

watch. .

The instrument indicates

designed to indicate the s

Electrical Tachometer

Contact type tachometers:

Eddy current (or)

Tachometer Gene

(b) A.C. T

Non-Contact type tachometers:

Inductive pick -u

Capacitive pick -

NOTES-39 UNI

E,GMRIT-Rajam

top watch and counter with automatic disconne

n meshed with shaft.

unction until the start and wind button is press

the average speed over a short interval of time

peed directly in rpm.

drag cup tachometer

rators:

(a) D.C. Tachometer generator.

achometer generator.

tachometer (Toothed rotor tachometer).

up tachometer.

-5

Page 84

ct.

d to start the

nd the dial is

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ICS


Eddy current (or) drag cup tac

When the permanent ma

eddy current is induced i

This eddy current produc

spring.

When the permanent ma

eddy current is induced i

This eddy current produc

spring.

The disc rotates in the di

spring .

D.C. Tachometer generator.

It consists of a small arm

horse type magnet.

NOTES-40 UNI

E,GMRIT-Rajam

hometer :

net along with the steel cup is coupled to the ro

the aluminum cup.

es a torque which rotates the cup against the tor

net along with the steel cup is coupled to the r

the aluminum cup.

es a torque which rotates the cup against the tor

ection of the which rotates the cup against the t

ature which is coupled to the rotating shaft and

-5

Page 85

tating shaft an

ue of the

otating shaft an

ue of the

rque of the

permanent

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ICS NOTES-40 UNIT-5


When the armature rotates along with the shaft, a pulsating D.C. voltage which is

roportional to the speed of the shaft is generated.

A.C. Tachometer Generator:

Consists of a rotating permanent magnet which is coupled to the shaft whose speed is

to be measured

An a.c voltage is induced in the stator coil when the shaft rotates .

Frequency and amplitude of the induces voltage is proportional to the speed of the shaft.

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ICS


Inductive pick -up tachometer

When the toothed gear

which will produce a cha

Due to this, the magneti

The frequency of the vol

speed of rotation.

Capacitive pick - up tachomete

NOTES-41 UNI

E,GMRIT-Rajam

(Toothed rotor tachometer).

otates, the air gap between the magnet and t

nge in the reluctance of the magnetic circuit.

field expands and collapses and a voltage is in

age pulses depends upon the number of teeth o

r.

-5

Page 87

he rotor changes

uced in the coil.

the rotor and its

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ICS NOTES-41 UNIT-5


When the vane comes in between the two capacitor plates, the capacitance of the circuit

changes and its pulses differs when vane is not present between the plates.

The pulses thus produced are amplified, squared and finally fed to the pulse counter.

REFERENCE BOOKS:




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ICS


Optical Tachometers

Optical Shaft Encoders:

Photoelec

Rotating

Photoelectric Tachometer :

When a light beam hits t

obtained .

And the reflected light is

The frequency of electric

speed of the rotating shaf

Rotating Disc Photoelectric Ta

NOTES-42 UNI

E,GMRIT-Rajam

tric Tachorneter :

isc Photoelectric Tachometer:

e reflecting surface on the rotating shaft, light

focussed on to the photoelectric cell.

al output pulses from photoelectric cell is propo

t.

chometer

-5

Page 89

ulses are

rtional to the

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ICS


Whenever a hole comes

is produced.

The frequency of pulse g

its speed of rotation.

Since the number of ho

transducer output is direc

Stroboscopic Method

The principle involved in measu

visible only at specific intervals An identification mark is

The flashing light from t

flashing is adjusted so th

speed of rotation is equal

REFERENCE BOOKS

1.A course on mechan 2.Instrumentation and 3.Instrumentation Mea

NOTES-42 UNI

E,GMRIT-Rajam

between the light source and the light sensor, a

eneration is determined by the number of holes

les are fixed for a given disc, the frequency

tly proportional to the speed of the rotating sha

ement of speed –stroboscope is to make the m

of time by adjusting the flashing frequencymade directly on the shaft or on a disc mounte

e stroboscope is made to fall on the mark and

t the mark appears to be stationary. Under such

to the flashing frequency.

:

ical measurements and instrumentation- A.ontrol systems- S.Sudhakarreddy&P.Divaurement & Analysis- B.C Nakra and K.K C

-5

Page 90

pulse of voltage

on‘ the disc and

of pulses of the

t.

ving objects

On the shaft.

he frequency of

condition the

Sawhneykararao houdhry

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ICS NOTES-43 UNIT-5


Vibration :

If the displacement - time variation is continuous with some degree of repetitive nature, then the

type of motion is called Vibration.

Measurement of motion and vibration parameters are very important

In predicting the fatigue failure of machine components or a structure. Measurement of vibration

parameters are also useful in reducing the structure vibration or noise level.

Motion Measuring Instruments:

1. Relative Motion Measurement Devices:

measure the motion of a body with respect to a fixed reference.

2.Absolute Motion Measurement Devices:

Measurement of motion of a bridge

e.g. Seismic instrument.

Vibrometers :

which gives an output usually voltage, that is proportional to either displacement or velocity.

Accelerometers:

Out put is a function of acceleration.

Simple Vibrometers

Vibrating Wedge:

Cantilever (or) Reed Vlbrometer

Simple Accelerometer:

Acceleration Level Indicator

1.Vibrating Wedge:

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ICS


Used to measure the vibr

When the member is vib

By observing the locatio

Amplitude of vibration

2.Cantilever (or) Reed Vi

NOTES-43 UNI

E

ation when amplitude of vibration is greater tha

ating, the wedge successively assumes two extr

of the point where the images overlap,

brometer :

-5

Page 92

0.8 mm.

eme positions

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ICS


It consists of a small cant

vibrating surface whose

In operation, the length o

until resonance occurs.

At resonance, the natural

body. ,

The natural frequency of

Acceleration Level Indi

Instrument actual

been reached or n

It consists of a pre loade

In operation, when the e

preload setting, electrical

some form of indicator.

NOTES-43 UNI

E

ilever beam mounted on the block which is pla

requency is to be measured.

f the beam is slowly adjusted by means of a scr

frequency of the beam is equal to the frequenc

the beam is given by

ator :

y indicates whether the predetermined level of

ot.

cantilever-spring and an electrical contact

fect of inertia forces acting on the spring and

contact will be broken, and this action may th

-5

Page 93

ed against the

w arrangement

of the vibrating

cceleration has

ass exceeds the

n be used to trip

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ICS


The Seismic Instrumen

It consists of a mass whi

damper to the housing fr

The base of the housing

characteristics are to stud

A displacement transduc

housing.

By proper selection of sp

used to measure either ac

NOTES-44 UNI

E

:

h is connected through the parallel arrangemen

me.

rame is attached to the vibrating body whose v

ied.

r is used to measure the relative motion betwe

ring-mass and damper combinations, the relativ

celeration or displacement of a vibrating body.

-5

Page 94

of spring &

bration

n mass and the

motion may be

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ICS


Strain Gauge Accelerometer:

It consists of a cantilever

One end of the cantilever

mass 'm'.

A viscous fluid is filled i

When the attachment is

mass occurs which produ

The strain of the cantile

The strain gauges mount

vibration /acceleration if

NOTES-44 UNI

E

, mass and strain gauges.

is fixed to the housing frame and other end is c

side the housing to provide damping.

onnected to the vibrating body, vibrational dis

ces strain in the cantilever beam.

er beam is proportional to the vibration/acceler

d on the beam indicates the strain which is the

it is calibrated prior to use.

-5

Page 95

onnected to the

lacement of

tion.

irect measure of

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ICS NOTES-44 UNIT-5

Page 96

R.Pola Rao,Asso.Prof,Dept of ME

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ICS


Variable Resistance Vi

Consists of a seismic ma

When the housing frame

attached to the mass mov

circuit.

The change in resistance

measurement.

Piezoelectric Accelerom

Piezoelectric accelerome

crystal along with electro

NOTES-45 UNI

E

ration Sensor

s, spring. damper and potentiometer

is connected to a vibrating body, seismic mass

es along with the body thereby changing the res

is calibrated in terms of vibration to facilitate th

eter:

er consists of a seismic mass, spring, damper a

de arrangement which are connected as shown.

-5

Page 97

nd the slider

istance of the

e direct

d a piezoelectric

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ICS NOTES-45 UNIT-5


when the housing frame is connected to the vibrating body, a force is exerted on the

piezoelectric crystal by the mass-spring arrangement.

Due to this force a voltage is generated which is the measure of vibration.

REFERENCE BOOKS:




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ICS


LVDT- Accelerometer:

The mass which is attach

An auxiliary power supp

During the vibration of

windings occurs which p

This output voltage of th

direct measurement.

Capacitance Vibration

NOTES-46 UNI

E

ed to the frame is made to act as the core of the

ly is given to the primary winding.

ody , relative displacement between the seismi

oduces voltage in the secondary coil.

LVDT is calibrated in terms of the vibration t

ensor

-5

Page 99

LVDT.

mass and the

facilitate the

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ICS NOTES-46 UNIT-5


When the housing frame is attached to the vibrating body relative movement between the

mass and the housing frame occurs.

This movement causes a change in distance between the capacitor plates, thereby

changing its capacitance.

The change in capacitance is the measure vibration.

REFERENCE BOOKS:




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ICS NOTES-47 UNIT-6


Strain gauge and different types of electrical resistance strain gauges:

Strain gauge is defined as a transducer used to measure strain and associated stress in

experimental stress analysis. When a metal wire (or conducting wire) is stretched or

compressed, its length and diameter changes due to which the resistance and also the

resistivity of the wire will change. This effect is known as piezo-resistive effect. All

the strain gauges operates on the principle of piezo-resistive effect. Hence these are

also referred as piezo-resistive gauges.

Classification

Electrical resistance strain gauges are mainly classified into two types. They are,

(i) Unbonded strain gauges

(ii) Bonded strain gauges.

Bonded strain gauges are further divided into,

(i) Foil type strain gauge

(ii) Wire type strain gauge

(iii) Semiconductor type strain gauge.

Different methods used for the measurement of strain:

If is defined as the relative change in dimensions i.e, change in length of a given line

segment (wire or conductor) to the actual length of that line segment. When a load is

applied to a simple bar whose length is L, dimensional change will occur in the length

of the bar. Therefore the strain applied to the bar is given by,

L

L ∆

=Strain

length

lengthin

Actual

Change=ε

The various methods available for the measurement of strain are,

1. Photo-elasticity method

2. Brittle lacquers

3. Strain gauges

(a) Non-electrical strain gauges

(i) Mechanical

(ii) Optical.

(iii) Photo-elastic.

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ICS NOTES-47 UNIT-6


(b) Electrical strain gauges

(i) Resistance gauges

1. Metallic

2. Semiconductor.

(ii) Capacitance gauges

(iii) Inductance gauges.

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Bonded strain gauge and Unbonded strain gauge.

(i) Bonded Strain Gauge

When the strain gauges is directly placed or bonded so on the surface of the

(component) which is subjected to stress or which under study, then this arrangement

is known as bonded wire strain gauge. A bonded wire strain gauge designed to

measure pressure or force is illustrated below.

Figure

In the above arrangement a fine resistance wire 0.25mm (in diameter) is bonded on

the surface of the device under observation. When a force or pressure is applied to the

device, the physical dimensions of it will change. Since the strain gauge element is

pasted on its surface, the dimensions of the strain gauge changes due to which the

resistance of the gauge changes. The measure of change in resistance will become the

measure of applied pressure or force.

This change in resistance of the gauge can be measured by connecting the gauge in

anyone of the four arms of balanced Wheatstone bridge as shown below.

Figure: Bonded Strain Gauge connected in Wheatstone Bridge

Specifications:

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1. Typical size - Typically 3mm x 3mm but some times bigger than 2.5mm x 12.5 mm

2. Resistance - 120 Ω to 1000 Ω

3. Maximum excitation voltage -5 V to 10 V,

4. Construction material - Nickel-Copper, Nickel-Chromium or Nickel-Ferrous alloys.

Advantages

1. Accuracy is high.

2. These can be available in different shapes.

3. High sensitivity and high stability.

4. Perfect bonding can be done.

5. Can measure high pressures.

Disadvantage

These are sensitive to change in temperature.

Applications

I. These can be used in the applications of stress analysis

These can be used along with different transducers for different measurement

application

(ii) Unbonded Strain Gauge

The strain gauges which are not directly bonded on the surface of the device which is

subjected to stress or which is under study are known as unbonded strain gauge

Figure: Unbonded Strian Gauge

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The unbonded strain gauge is placed on frames A and B with the help of insulated

pins. These two frames are movable with respect to each other and this arrangement

can be connected in one of the four arms of Wheatstone bridge. When the pressure

measured is applied, the frame, A moves with inspect to frame B. This causes change

in the length and cross section of the strain gauge which in turn oauses its resistance

to change. Due to this Change in resistance the bridge will be unbalanced and

Produces some output voltage which indicates the change in resistance, which in turn

gives the value of applied pressure.

Specifications

1. Typical size = 0.003 mm in dia. and 25 mm in length.

2. Resistance = 12 Ω, 350 Ω to 1000 Ω

3. Maximum excitation voltage = 5 V to 10 V

4. Construction material - Nichrome, Constantan, Nickel, Iso elastic

Advantages:

1. It has greater accuracy

2. This gauge can be used in the range of ± 0.15% strain

Disadvantages

1. It requires more space

Application:

1. It can be used in the measurement of pressure, acceleration and force.

2. It can be used in those systems where gauge can be placed at difference places

and requires measurement of pressure or stresses frequently or more number

of times

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(i) Foil Type Strain gauge

Metal Foil Strain gauges:

In this type of strain gauges a metal foil is used to sense the applied strain. The

materials used for its construction are nickel, nichrome, platinum, isoelastic

(nickel+chromium + molybdenum), constantan (nickel+copper). The gauge factor and

characteristics of foil strain, gauges are similar to the wire strain gauges.

Figure: Foil Type Strain Gauges

The metal foil gauges can be easily etched on a flexible insulating carrier film. In the

construction of etched foil strain gauge, first a layer of strain sensitive material is

bonded to a thin sheet of backelite or paper. The part of metal which is to be used as

wire element is covered with some masking material and then to this, unit an etching

solution is applied. Therefore, the unmasked part of the metal will be removed

thereby leaving the required grid structure. By this method of construction, the etched

foil strain gauges are made in thinner sizes.

Different forms of metal foil strain gauges are shown as follows,

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Figure: Different Forms of Metal Foil Strain Gauges

When a force or pressure is applied to the sensing element of metal foil strain gauge

the physical dimensions of it will change. Since, the strain gauge element is pasted on

its surface, the dimensions of the strain gauge changes due to which the resistance of

the gauge changes. The measure of change in resistance will become the measure of

applied pressure. or force (this change in resistance of the gauge can be measured by

connecting the gauge in anyone of the four arms of balanced Wheatstone bridge).

Semicoductor Strain Gauge

A typical semiconductor strain gauge is formed by the semiconductor technology i.e.,

the semiconducting wafers or filaments of length varying from 2 mm to 10 mm andthickness of 0.05 mm are bonded on suitable insulating substrates (for example

teflon). The gold leads are usually employed for making electrical contacts. The

electrodes are formed by vapour deposition. The assembly is placed in a protective

box as shown in the figure below.

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Figure: Semiconductor Strain Gauge

The strain sensitive elements used by the semiconductor strain gauge are the

semiconductor materials such as silicon and germanium.

When the strain is applied to the semiconductor element a large change in resistance

occur which can be measured with the help of a Wheatstone bridge. The strain can be

measured with high degree of accuracy due to relatively high change in resistance.

A temperature compensated semiconductor strain gauge can be used to measure small

strains of the order of 10-6

i.e., micro-strain.

This type of gauge will have a gauge factor of 130 ±10% for a semiconductor material

of dimension 1 x 0.5 x 0.005 inch having the resistance of 350 Ω.

Advantages of Semiconductor Strain Gauge

1. The gauge factor of semiconductor strain gauge is very high, about ± 130.

2. They are useful in measurement of very small strains of the order of 0.01

micro-strain due to their high gauge factor.

3. Semiconductor strain gauge exhibits very low hysteresis i.e., less than 0.05%.

4. The semiconductor strain gauge has much higher output, but it is as stable as a

metallic strain gauge.

5. It possess a high frequency response of 1012

Hz.

6. It has a large fatigue life i.e., 10 x 106operations can be performed.

7. They can be manufactured in very small sizes, their lengths ranging from 0.7

to 7.0 mm.

(iii)Wire Type Strain Gauges

These are available in bonded and unbonded type. In bounded type, the strain gauge is

directly pasted on the surface of the structure under test. To paste the strain gauge on

the structure, adhesives are used which are responsible for transmitting the strain from

the structure to the gauge wires.

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These gauges are basically fabricated in four varieties. Namely

1. Flat grid type.

2. Wrap around type.

3. Single wire gauge.

4. Woven type gauge.

1. Flat Grid Wire Gauge

In flat grid wire gauges the fine wire is arranged in the form of a grid (i.e, wound back

and forth) and then pasted on a backing material (ex: epoxy, paper etc) with the help

of adhesive as shown in figure (1). Transverse strain's cause changes in resistance at

the ends of each section where the wires are looped around. By maintaining the length

of loops at minimum or joining them with some material which is less sensitive to

strain compared to actual material used in the fabrication of gauge, the cross

sensitivity can be reduced.

In order to get maximum transfer of strain from structure under test to gauge, the grid

structure should be placed as cl9se, as possible to the structure under test. This also

helps to maintain the hysteresis and creep at minimum.

Figure: Flat Grid Wire gauge

2. Wrap Around Wire Gauge

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Figure: Wrap Around Wire Gauge

In wrap around wire gauges, the fine wire is wound on a thin strip or a flattened tube

of paper. There can be made smaller in length for same value of resistance compared

to flat grid type.

However in this type the wire grid is in two planes, the gauge has very high surface

thickness. Due to this creep and hysteresis increases.

3. Single wire gauges

This type of wire gauges are mainly designed to avoid cross-sensitivity factor. These

are formed when single wires are stretched across and laid (as illustrated in figure

(6)).

In order to avoid looping of the same material, thick copper wires are attached

(welded) at the ends. Due to this cross-sensitivity reduces to a great extent.

Figure: Single Wire Gauge

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4. Woven type gauges

This gauge is formed when a silk insulated Eureka wire is wound as the weft on a

textile or rayon wrap. These gauges can measure large strains and also used to

carryout tests on leather and fabrics.

Figure: Woven Type Gauge

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Principle of resistance strain gauge and equation for the gauge factor:

Strain gauge consists of a fine metallic wire of 0.025 mm diameter and in appropriate

length. The resistance of a single wire is given by.

A

L R

ρ = (1)

p = Resistivity of material of conductor

L=Length of wire

A =Cross-sectional area of wire.

When it is strained within the elastic limit, the resistance of the wire will change due

to,

1. Dimensional changes (L, A).

2. Change in value of resistivity, (This property is called piezo-resistive effect).

Taking logarithm on both sides of equation (1), we get,

log R = log ρ + log L - log A

Differentiating the above equation

A

dA

L

dLd

R

dR−+=

ρ

ρ (2)

4

2 D A

π = ( Is the area of cross section)

dD DdA .24

π =

4

.

2

2 D

dD D

A

dA

π

π =

D

dD

A

dA.2=

When a wire is subjected to longitudinal stress, its length increases (longitudinal

strain) and its diameter decrease (lateral strain).

Poisson's ratio is defined as the ratio of lateral strain to longitudinal strain.

Strain

Strain

al Longitudin

Lateralv

−=

LdL

DdDv

/

/ −=

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L

dLv

D

dD−= (4)

L

dLv

A

dA2−=

By substituting the above value in equation (2) we get,

L

dLv

L

dLd

R

dR2++=

ρ

ρ

)21( v L

dLd ++=

ρ

ρ

Dividing the above equation by L

dL

v LdL

d

LdL

RdR21

/

/

/

/ ++=

ρ ρ

The term LdL

RdR

/

/ is called Gauge factor (G) of the strain gauges.

++=∴

dL/L

/ d2v1G

ρ ρ

The term

LdL

d

/

/ ρ ρ represents pizo-resistance effect

For all the wires drawn from metals and metallic alloys Poisson's ratio is taken as 0.3,

piezo-resistive effect is almost zero for metals. .

The typical value of gauge factor is,

03.021 ++= xG

= 1.6

G = 1.6 for metals

Gauge factor is an index of sensitivity of the strain gauges. Higher the gauge factor

more is the output.

L

L R

R

G∆

∆

=

Strain.G R

R=

∆∴

R∆ *G

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A resistance strain gauge with a a gauge factor 2.04 is fastened to a beam which

is subjected to a strain of 1x10-6

. If the original resistance of the gauge is 120Ω

Calculate the change in resistance.

Given that,

Strain, ε = 1x 10-6

Gauge factor, Gf = 2.04

Original resistance of gauge, R =120Ω

Therefore change in resistance RG R f ε =∆

12010104.2 6 x x x R −=∆∴

= 0.2448 mΩ

Ω=∆−3

102448.0 x R

A single electrical resistance strain gauge of resistance 120Ω and having a gauge

factor of 2 is bonded to steel having an elastic limit stress of 400 MN/m2

and

modulus of elasticity 200 GN/m2. Calculate the change in resistance due to a

change of temperature of 20°C. Coefficient of linear expansion of steel is 12 x 10-6

/ 0C.

Given that,b = 0.02 m

d = 0.003m

E = 200GN/m2

X = 12.7 x 10-3

m

Movement of inertia

3

12

1 xbxd I =

3)003.0(02.0121 x x=

4121045 m x −

=

Deflection,

EI

Fl x

3

3

=

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3

3

l

EIxF =

3

3129

)25.0(

107.124510102003−−

=x x x x x

= 22 N

Bending moment

Nm xF M x 3.315.022 ===

Stress,

2

003.0

1045

3.3

2 12x

x

d

I

M S

−==

= 11o MN/m2

Strain,

9

6

10200

10110

x

x

E

S

L

L==

∆=ε

= 0.55 x 10-3

Gauge factor,

231055.0

120152.03==

∆

∆

=− x

x

L

L R

R

G f

Gf = 23

A strain gauge of nominal resistance 200 Ω is fixed on the flat surface of a short

column of 2 cm x 2 cm cross-sectional area. The column is subjected to an axial

force of 100 N. The strain gauge forms one arm of a bridge with other arms all

equal to 200Ω. Find open circuit voltage of the bridge excited by 10 V. Given

Young’s modulus of elasticity is 2.1 x 1011

N/m2.

Given,

Force, F = 100N

Cross-sectional area, A =2 x 2 x 10-4

m2

Normal resistance of gauge, Rg =200Ω

Young's modules, E = 2.1x1011

N/m2

Input Voltage, ei =10 V

Let, the Gauge factor, Gf =2

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Stress, S = F/A=24

4 / 1025

104

100m N x

x=

−

Strain, 6

11

4

1019.1101.2

1025 −=== x

x

x

E

S ε

Change in resistance,

g f g RG R ..ε =∆

= 2 x 1.19x10-6

x 200

Ω=∆−41076.4 x Rg

The output voltage, Oe∆ is given by,

102004

1076.4

.4

4

x x

x xe R

Re i

g

g

O

−

=∆

=∆

V xeO6

1095.5−

=∆ .

Two electrical strain gauges are bonded to a duralumin cantilever and connected

in a bridge as adjacent arms. Each gauge has a resistance of 100Ω and a gauge

factor of 2.1. The input voltage is 4V. The stress is 200 MN/m2. Find the current

through the detector if its resistance is 400Ω. Modulus of elasticity of duralumin

is 70 GN/m2.

FigureGiven that,

Each gauge has a resistance of, R =100Ω

The gauge factor, Gf =2.1

The input voltage, V = 4 V

The stress, ε =200 x 106

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The meter is connected at the output terminals of the bridge and has a resistance R m =

400Ω

The output voltage o

m

oxe

R

RV

+

=

1

1

Where, eo = open circuit voltage across the bridge for two active strain gauges

i

f

O e E

Ge .

2

.ε =

E = Modulus of elasticity

V x x x

x xeO 012.04

10702

102001.29

6

==

Output Voltage,

012.0

400

1001

1 xV O+

=

012.025.1

1 xV O =

012.08.0 x=

mV V O

6.9=

Thus, the meter current,

m

Om R

V I =

400

106.93−

=x

I m

63 102410024.0 −−== x x

A I m

µ 24=

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Calibration of resistive strain gauges:

The main objective of strain gauge calibration is to establish a relation between

measured strain and display system in which scale is divided into arbitrary units.

However, the process of strain gauge calibration requires some necessary adjustments

in order to get accurate scale reading.

This technique involves a known value of change to introduce in the resistance of

anyone arm of four arms bridge in order to simulate a certain value of strain. The

change made in the resistance to carry out this is calculated provided the values of

unstrained resistance of the strain gauge and its Gf (Gauge factor) are known.

Figure: Strain Gauge Calibration

In practical process of strain gauge calibration a known value of change is introduced

in the resistance of gauge by connecting a high value of resistor (R sh) in parallel to it

and measuring the resultant output due to this change. The RSh is used when the strain

gauge resistance is being Rg

When switch (S) is open the resistance of arm ‘a’ = Rg

When switch is closed the resistance of arm,

shg

shg

R R

R R

a +=

Therefore, the resulting variation in resistance of arm “a” due to shunt resistance (Rsh)

is given by,

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)(shg

shg

gg R R

R R R R

+−=∆

shg

g

g R R

R R

+=∆

2

(1)

And an equivalent strain is given by,

g

g

f

e R

R

G

∆=∈ .

1

)(

112

shg

g

g f R R

R x

R x

G +=

)( shg f

g

R RG

R

+= (2)

And shunt resistance,e f

e f g

shG

G R R

∈

∈−=

)1( (3)

In practical e f G ∈ is very small (nearly )1<<∈e f

G therefore it can be neglected

e f

g

shG

R R

∈≅∴ (4)

The value of e∈ can be calculated from equation (3) or (4). If the relation between

strain and readout is linear a single value of shunt resistance, (Rsh) is required for

calibration. Hence the R h value should be selected so that the indicator provides

maximum scale deflection or full scale deflection.

This procedure of calibration is also used if the bridge contains more than one active

gauge.

Let n =Number of active gauges in a bridge.

Then for a gives Rsh the represented strain is approximatelyn

1of the strain given in

equation (2)

Therefore the effective strain in this case is given by

)(shg f

g

e R RnG

R

+=∈

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Properties of materials used for strain gauges:

The materials used for strain gauges should contain below given properties in order to

obtain very high reproducibility, long, good, accuracy, good sensitivity etc.

• High gauge factor

• High specific resistance

• Constant strain sensitivity over several ranges of values of strain

• Low temperature coefficient of resistance

• High yield point

• Good corrosion resistance.

Some of the materials which are popularly used in the fabrication of strain gauges and

their properties are,

1. Nichrome

(i) It is composed with 80% of nickel and 20% of chromium.

(ii) Gauge factor: 2.5

(iii)Resistivity: 100 x 10-8

0Ωm

(iv) Resistance temperature coefficient : 0.1 x 10-3 /

0C

(v) Upper temperature value: 12000C

2. Isoelastic

(i) It is composed with, 36% of nickel, 8% of chromium, 0.5% of

molybdemum etc


(iii) Resistivity: 105 x 10-8Ωm

(iv) Resistance temperature coefficient: 0.175 x 10-3 /

0C

(v) Upper temperature value: 12000C.

3. Constantan

(i) It is composed with 45% of nickel and 55% of copper


(iii) Resistivity: 48 x 10-8

Ωm.

(iv) Resistance temperature coefficient:± 0.02 x10-3 /

0C

(v) Upper temperature value: 40000C

4. Nickel

(i) Gauge factor: - 12

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ICS NOTES-52 UNIT-6


(ii) Resistivity: 6.5 x 10-8Ωm

(iii) Resistance temperature coefficient: 6.8 x 10-3 /

0C.

5. Platinum

(i) Gauge factor: 4.8

(ii) Resistivity: 10x 10-8Ωm

(iii) Resistance temperature coefficient: 4.0 x 10-3 /

OC.

Desirable characteristics of bonding material:

The material is used to bond (paste) the strain gauge on to the surface of the device

which, is subjected to stress or which is under study is known as bonding material.

The bonding materials are also known as adhesives, cements. Thermoplastic cement,

thermosetting cement, special ceramic cement, silicon varnish, nitro cellulose are

some of the examples of bonding materials.

The bonding material transmits the strain from the device under test to the gauge

(sensing element). Therefore the selection of bonding material is very important.

The desirable characteristics of bonding material are,

(i) The bonding material must have high insulation resistance, high creep

resistance.

(ii) It should possess good shear strength to transmit the strain from the device

under test to gauge (sensing element).

(iii) It should have insulator properties with high elasticity.

(iv) It should be able to dry up in a small period of time.

(v) It should be insensitive to environmental conditions such as temperature,

humidity, moisture etc.

(vi) It should be able to provide linear change in resistance per unit strain.

(vii) It should be able to spread easily and provide good bonding adhesion.

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.

Single strain gauge element - active and inactive direction.

A single length of wire can be used as the sensing element in a strain gauge. However,

the circuits which are used for measuring the changes in resistance impose certain

restrictions on the minimum resistance that a strain gauge should possess. This value

depends on the gauge current and gauge length for given value of gauge factor, higher

resistance gauges offer high resistance changes and draw lower current and hence,

less heat dissipation problems. An element with grid structure is generally used for

increasing the resistance of the gauge.

Figure

Gauge sensitivity -strain gauge:

If a wire or conductor is stretched or compressed, the resistance of the conductor

changes because of dimensional changes of length and cross-sectional area. Therefore

the gauge sensitivity is described in terms of characteristic called the gauge factor

which is defined as the unit change in resistance per unit change in length.

Figure: Strain Gauge on a cantilever to measure Strain

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Two arm and four arm conditions used for strain measurement:

Measurement of Strain Using Two Strain Gauges

Consider two strain gauges placed on a cantilever beam (as shown in figure (1)) for

the measurement of strain.

Figure: Two Strain Gauges on a Cantilever to Measure StrainIn the above figure, when strain is applied the strain gauge mounted on the side i.e.,

Rga experiences positive strain or tension where as the strain gauge mounted on the

bottom side i.e., Rgc experiences negative strain or compression. When these two

active gauges are connected in two separate arms of the Wheatstone bridge the

resulting bridge is known as ‘half bridge’ and is shown below figure (2).

Figure: Half Bridge for Measurement of Strain

The effects of temperature can be eliminated by having Rb = Rd and connecting two

identical strain gauges

Let,

R R R R Rd gcbga−−−=

When strain is not applied the potential of both points

N and P are same i.e.,2

ie

. Therefore the output of the bridge will be zero. i.e., e0 = 0

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ICS NOTES-53 UNIT-6


When strain is applied to the cantilever the resistance of gauge placed on top of the

cantilever increase while that of the resistance of gauge placed on bottom side

decreases.

When strain is applied the resistance of Rga is given by,

∆+=

R

R R Rga

1

When strain is applied the resistance Rga is given by,

∆−=

R

R R R

gc 1

Since, Rb =Rd =R

The potential of point, P =2

ie

The potential of point,

∆−+

∆+

∆+

=

R

R R

R

R R

R

R R

e N i

11

1

2

1 R

R

ei

∆+

=

Due to the applied strain the resulting change in output is given by,

222

1 R

R

ee R

R

ee i

i

io

∆

=−

∆+

=∆

2

∈=∆∴

f

io

Gee

In this way, the output of half bridge is two times that of the output of quarter bridge.

Due to this the sensitivity is doubled and effects of temperature are also eliminated.

The gauge sensitivity of a half bridge will become,

f gg GkRS 2=

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Figure(1): Strain Gauges on a Cantilever

Four strain gauges are mounted on a cantilever beam (as shown above) to measure

strain. Assume that all the four strain gauges are of similar type. When strain is not

applied the resistance of all the gauges is same i.e,

Rga = Rgb =Rgc=Rgd =R

When these active gauges are connected in a Wheatstone bridge, each gauge in, each

arm then the resulting bridge is known as full bridge and is shown below.

Rga = Rgb =Rgc=Rgd =R

Figure(2): Full bridge for Measurement of Strain

When strain is not applied the potential of both points N and P are same i.e,2

ie

.

Therefore the output of the bridge will be zero i.e, e0 =0. When strain is applied the

resistance values of strain gauges will change. Thus,

The resistance of Rga and Rgd =

∆+

R

R R 1

The resistance of Rgb and Rgc =

∆−

R

R R 1

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When strain is applied the potential of point

∆−+

∆+

∆+

=

R

R R

R

R R

R

R R

e N i

11

1

2

1 R

R

ei

∆+

=

And

Potential of point, (when strained )

∆++

∆−

∆−

=

R

R R

R

R R

R

R R

ePi

11

1

2

1 R

R

ei

∆−=

=

Due to the applied strain the resulting change in output is given by

2

1

2

1 R

R

e R

R

ee iio

∆−

−

∆+

=∆

ei f o G

R

Re ε =

∆=∆

Four active arm bridges are required if the gauges are employed as secondary

transducers to provide highest sensitivity. Therefore the gauge sensitivity of a full

bridge will become,

f ggGkRS 4=

.

Types of strain gauge arrangement for measuring strain:

Strain gauge can be used in different possible arrangements for measuring strain.

They are,

(i) In this first arrangement a single strain gauge SG1 is fixed on the elastic member

(which is under test). Here die strain gauge measures the axial strain in the elastic

element. When the strain is applied e resistance of the gauge changes and it can be

measured with the help of Wheatstone bridge (i.e, by connecting the gauge in any one

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ICS NOTES-53 UNIT-6


of the four arms of the bridge). This arrangement is not compensated for changes in

temperature.

Figure(1): Measurement of Strain Using Single Strain gauge Arrangement

(ii) In the other method two active strain gauges SG1, SG2 are used to measure strain

as shown in figure (2). In this arrangement SG1 and SG2 are placed at right angles to

each other. This arrangement is also known as Poisson’s arrangement. When strain is

applied, the SG1 experiences axial tensile strain and SG2 experiences transverse

compressive strain. The compressive strait is v times the tensile strain. Since the two

gauges experiences strains of opposite nature the signal enhancement factor will be

(1 + v).

Where,

v is Poisson's ratio.

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ICS NOTES-53 UNIT-6


Figure (2): Strain Gauge on a Cantilever to Measure Strain

This arrangement is compensated for temperature. As the variations in temperature

identically gauges, the set output will be zero affects the two gauge, the set output will

be zero

(iii) In another arrangement, two strain gauges are placed such that both will

experience equal amount of axial tensile strains. To measure resistance changes in the

gauges due to axial tensile strains, the gauges are connected in any two opposite arms

of the bridge as illustrated below.

Figure (3): Arrangement of Two Strain Gauges in strain Measurement

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This arrangement requires two dummy strain gauges to compensate changes in

temperature.

(iv) Strain can also be measured by employing four active strain gauges. In this

arrangement gauges SG2 and SG4 are placed at right angles to gauges SG1, and SG3.

This arrangement compensates the changes in temperature and its signal

enchancement factor is 2( 1 + v)

Figure (4): Measurement of Strain using four Strains gauges

Measurement of torque with strain gauge:

Strain gauge torque transducers are frequently used for the measurement of torque.

The figure shows the strain gauge torsion meter. This arrangement uses four strain

gauges each mounted at 90° to each other. The most widely used strain gauges in this

technique is bonded-wire type. These strain gauges are diametrically opposite<to each

other. These strain gauges are arranged in a Wheatstone bridge circuit.

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ICS NOTES-53 UNIT-6


Figure

In this type of meter if characteristics of all the four strain gauges are matched then

the meter is insensitive to bending and pull effects. The Galvanometer is shown in thefigure is used to indicate the torsion deflection if any changes occur in the strain

gauge circuit. Strain gauges R1 and R4 are subjected to tensile stress and strain gauges

R2 and R3 are subjected to compressive stress, when the shaft is under torsion.

Therefore the torque of the rotating shaft is obtained with the help of compressive and

tensile stresses. The torque of the, strain gauge torque transducer is given by,

( )Nm

2

0 φ π

l

R RGT i

−=

Where,

T = Torque in Nm

G = Modulus of rigidity

Ro = Outer radius of the shaft

Ri = Inner radius of the shaft

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ICS NOTES-53 UNIT-6


φ = Angular deflection of the shaft

l = Length of the shaft

The angle made by the gauges with shaft is 450. Therefore the shaft given by,

( )44

0

45

i

o

R RG

TR

−

±=∈

π

Advantages:

1. This system is temperature compensated.

2. Automatic compensation is offered by this meter for bending and pull effects.

3. Maximum sensitivity is provided by this system for a particular torque.

Slip rings and speed sensors attached to the strain gauge torque transducers, are

known as commercial type, and are used for the measurement of torque in the range

of 0.6 to 104

mkgf.

A mild steel shaft is used to connect a motor drive to a constant load torque. A

foil strain gauge having, a resistance of 120 ΩΩΩΩ and a gauge factor 2 is mounted on

a shaft with its active axis at an angle of 45 degrees to the axis of the shaft. The

shear modulus of steel is 80 GN/m2. The shaft radius is 15 mm and the change in

strain gauge resistance due to the load is 0.24ΩΩΩΩ. Find the load torque.

Answer:

Given that,

∆R = 0.24Ω

R = 120 Ω

G = 80 x 109

N/m2

Gf = 2

Figure

Angular of shear,3

2

Gr

T

π θ =

An area of the shaft surface, originally square with the sides of unit length, is

deformed by strain to a parallelogram. The original length of diagonal is 2 .

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If the angle of shear, θ is small then the length of diagonal of parallelogram is longer

and the diagonal of the square.

Difference in length =2

θ

22

2 θ θ

ε ==∆

= L

L

θ θ

ε ===∆

22 xG

R

R f

R

R∆=θ

120

24.0=

rad102 3−= x

Torque

2

3θ π Gr T =

( )2

10210151080 339 −−

=x x x x xπ

= 848 NM

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ICS NOTES-54 UNIT-6


Measurement of torque with strain gauge:

Strain gauge torque transducers are frequently used for the measurement of torque.

The figure shows the strain gauge torsion meter. This arrangement uses four strain

gauges each mounted at 90° to each other. The most widely used strain gauges in this

technique is bonded-wire type. These strain gauges are diametrically opposite<to each

other. These strain gauges are arranged in a Wheatstone bridge circuit.

Figure

In this type of meter if characteristics of all the four strain gauges are matched then

the meter is insensitive to bending and pull effects. The Galvanometer is shown in the

figure is used to indicate the torsion deflection if any changes occur in the strain

gauge circuit. Strain gauges R1 and R4 are subjected to tensile stress and strain gauges

R2 and R3 are subjected to compressive stress, when the shaft is under torsion.

Therefore the torque of the rotating shaft is obtained with the help of compressive and

tensile stresses. The torque of the, strain gauge torque transducer is given by,

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ICS NOTES-54 UNIT-6


( )Nm

2

0 φ π

l

R RGT i

−=

Where,

T = Torque in Nm

G = Modulus of rigidityRo = Outer radius of the shaft

Ri = Inner radius of the shaft

φ = Angular deflection of the shaft

l = Length of the shaft

The angle made by the gauges with shaft is 450. Therefore the shaft given by,

( )44

0

45

i

o

R RG

TR

−±=∈π

Advantages:

1. This system is temperature compensated.

2. Automatic compensation is offered by this meter for bending and pull effects.

3. Maximum sensitivity is provided by this system for a particular torque.

Slip rings and speed sensors attached to the strain gauge torque transducers, are

known as commercial type, and are used for the measurement of torque in the range

of 0.6 to 104

mkgf.

A mild steel shaft is used to connect a motor drive to a constant load torque. A

foil strain gauge having, a resistance of 120 ΩΩΩΩ and a gauge factor 2 is mounted on

a shaft with its active axis at an angle of 45 degrees to the axis of the shaft. The

shear modulus of steel is 80 GN/m2. The shaft radius is 15 mm and the change in

strain gauge resistance due to the load is 0.24ΩΩΩΩ. Find the load torque.

Given that,

∆R = 0.24Ω

R = 120 Ω

G = 80 x 109

N/m2

Gf = 2

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Figure

Angular of shear,3

2

Gr

T

π θ =

An area of the shaft surface, originally square with the sides of unit length, is

deformed by strain to a parallelogram. The original length of diagonal is 2 .

If the angle of shear, θ is small then the length of diagonal of parallelogram is longer

and the diagonal of the square.

Difference in length =2

θ

22

2 θ

θ

ε ==∆

= L

L

θ θ

ε ===∆

22 xG

R

R f

R

R∆=θ

120

24.0=

rad1023−= x

Torque

2

3θ π Gr T =

( )2

10210151080 339 −−

=x x x x xπ

= 848 NM


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ICS NOTES-55 UNIT-6


Strain gauge rosette and different types:

Many transducer applications and stress analysis techniques use a combination of

strain gauge. This combination of two or more strain gauge elements is known as

rosette. With conventional strain gauges, it is not possible to indicate the direction of

the applied stress.

Therefore, it is required to develop strain gauge measurement system which can

determine strain and stress without knowing the direction of stress and strain. This

problem can be avoided by employing three strain gauges as a unit known as rosette.

The different forms of rosettes are illustrated below.

Figure (1): 3-element Rosette, 600 Planer (Foil Type)

Figure (2): 3-element Rosette, 450

Stacked (Wire Type)

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Stacked (Foil Type)


Planer (Foil Type)


Planer (Foil Type)

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Shear Planer Foil

If the measurement system contains 3-gauge rosette, then it is possible to calculate all

the required data, as this measurement is aim to give stress at a point, ideally the 3

gauges have to be superimposed on that point. This type of construction is known as

stacked rosette. This construction is feasible, but is places the top gauge at some

distance from the surface of the specimen. This increases its self heating, because it is

insulated from the underlying specimen. This underlying metal specimen serves like a

heat sink. If these limitations are greater, then planar rosettes are used.

(a) Rectangular Strain Gauge Rosette

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Figure (1): Three Element Rectangular Rosette

In three element rectangular rosettes the strain gauges are positioned at 00, 450 and

900

as illustrated in figure (1).

Let,

Strain measured by strain gauge 1 = ε S1

Strain measured by strain gauge 2 = ε S2

Strain measured by strain gauge 3= ε S3

The principle strains are,

( )Q

vvS S mS S m .

)1(2)1(2, 21

minmax+

∈±

−

∈+∈∈=

Where,

( ) ( )[ ] 2 / 12

32

2

21 S S S S Q ∈+∈+∈+∈=

Maximum shear stress is given by,

xQv E T m

)1(2max

+=

Orientation angle φ of principle stress is given by,

0

21

312 9002

2tan +<∈−∈

∈−∈−∈= φ φ

S S

S S S Where ( )3122

1S S S ∈+∈>∈

(b) Delta Strain Gauge Rosette

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Figure (2): Three Element Delter Rosette

In three element delta rosette the strain gauges are positioned as shown in figure (2)

Let the strains measured by strain gauge 1,2,3 are εS1, εS2 and εS3 respectively.

The principle strains are,

( ) ( ) ( ) ( )[ ][ ]2 / 12

13

2

32

2

21321minmax 2223

1, S S S S S S S S S ss ∈−∈+∈+∈+∈−∈±∈+∈+∈=∈∈

( )[ ] PS S S ss ±∈+∈+∈=∈∈ 321minmax3

1,

Where,

( ) ( ) ( )[ ] 2 / 12

13

2

33

2

21 222 S S S S S S E E E E E E P −+−+−=

The principal stresses are

++

−

++= xP

vv

E E E E S S S S S m

1

1

13. 321

minmax

Maximum shear stress is given by,

pv

T m .)1(3

max+

∈

Orientation angle φ of principal stress is given by,

( )[ ] 0

321

2 / 1

23 9002

3tan +<

∈−∈−∈

∈−∈= φ φ

S S S

S S when 23 S S >∈∈

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ICS NOTES-56 UNIT-7


Humidity: The quantity of water vapour contained in the atmosphere

Moisture : The quantity of water absorbed or adsorbed by a solid or Liquid.

Absolute humidity : The mass of water vapour present in a unit volume of gas or air.

Relative humidity: Ratio of amount of water vapour pressure actually present in a given volume

of gas to the amount of water vapour pressure required for saturation.

The relative humidity of saturated air is equal to 100%.

Dew point temperature: When the temperature of air is reduced by continuous Cooling at

constant pressure, the water vapour in the air starts condensing at a Particular temperature ,which

is referred as the Dew point temperature.

Dry bulb temperature: The sensing bulb of it is in direct contact with air and measures the

temperature which is known as dry bulb temperature.

Wet bulb temperature: The sensing bulb of it is covered with wet wick or moistened with pure

water and this covering is brought in contact with air. The temperature, measured by this

thermometer is known as wet bulb temperature.

Importance of Humidity Control:

Continuous monitoring and controlling of humidity is very essential in many process industries

such as paper industries, pharmaceutical industries, chemical industries, food industries, garment

industries, leather industries etc. This is because the presence or variation of humidity the effects

the behavior, properties and composition, quality of many substances.

To prevent the food products to become dry, spoilage of dried milk, eggs and for

successful storage of fruits, meat etc,.

To reduce the affects of surface leakage in electrical installations.

To maintain proper environmental conditions for human comforts.

For proper drying of wood, color printing. ,

Textile and paper industries require high humidity conditions. Any variations in humidity

may cause the nature, behaviour, characteristics of paper pulp and synthetic fibres to

change.

In pharmaceutical industries, the humidity should be carefully controlled to avoid

growing of any bacteria in the process of making pharmaceuticals

Classification of Humidity Measuring Instruments

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1.Sling Psychrometer

2.Arption hygrometers

(a) Mechanical humidity sensing absorption hygrometer.

(b) Electrical humidity sensing absorption hygrometer.

3.Dew point meter.

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Classification of Humidity Measuring Instruments

1.Sling Psychrometer

2.Arption hygrometers

(a) Mechanical humidity sensing absorption hygrometer.

(b) Electrical humidity sensing absorption hygrometer.

3.Dew point meter.

1.Sling Psychrometer :

It measures both dry and wet bulb temperatures.

These measured temperatures give the measure of humidity present in air .

This instrument uses two thermometers, one is dry bulb thermometer and

the other is wet bulb thermometer

The dry bulb thermometer is so called because the sensing bulb of it is in direct contact

with air and measures the temperature which is known as dry bulb temperature.

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The wet bulb thermometer is so called because the sensing bulb of it is covered with

woven cotton wick or moistened with pure water and this

covering is brought in contact with air.

The temperature, measured by this thermometer is known as wet bulb temperature.

These two thermometers are held in a frame which is covered by glass casing. and a

swivel handle is attached to this glass casing

Operation:

Arrangement is to be rotated at 5 m/s to 10 m/s in order to obtain necessary air motion.

When the psychrometer rotates, the thermometer whose sensing bulb is in direct contact

with air measures and indicates dry bulb temperature

When the air passes on the wick present on the bulb of the thermometer ,the moisturepresent in the wick starts evaporating and a cooling effect is produced at the bulb.

Wet bulb temperature is always less than dry bulb temperature.

The psychrometer frame–glass covering-thermometer should be rotated between the

specified period of time.

If it is rotated fro longer period of time, the wet wick will dry very fast, therefore wet

bulb temperature will not be at its minimum value.

If it is rotated for short period of time, proper wet bulb temperature cannot be measured.

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Absorption Hygromete

(a) Mechanical humidity

(b) Electrical humidity se

(a). Mechanical Humidi

Operating principle -inv

materials like wood, pa

moisture from atmospher

This variation in linearatmosphere

An animal hair is used as

The hair is separated fro

This hair arrangement is

other end is attached to a

The link carries a pointer

When the hygrometer is

surrounding air.

Due to this the length of

This increase or decrease

to the pointer.

NOTES-58 UNI

E

sensing absorption hygrometer.

nsing absorption hygrometer.

ty Sensing Absorption Hygrometer

olves the change of linear dimensions of s

per, human hair, animal membrane, etc., w

e.

dimensions is used to measure the humidit

humidity sensor.

one another and arranged parallely.

attached to an arm which is pivoted at one e

mechanical link.

which moves over a scale calibrated in terms o

placed in the atmosphere the hair absorbs the

hair increase or decreases (in a linear direction)

of hair arrangement is transmitted to the arm a

-7

Page 144

me hydroscopic

hen they absorb

y present in the

nd, where as the

humidity.

umidity from its

.

d link and hence

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ICS NOTES-58 UNIT-7


The pointer moves on the calibrated scale there by indicating the humidity content

present in the atmosphere.

If the hygrometer employs membrane as a humidity sensing element then it is known as

membrane hygrometer

(b). Electrical Humidity Sensing Absorption Hygrometer

Operating principle- variation of resistance with variation in humidity .

The two electrodes are coated with hygroscopic salt.

It is a lithium chloride conductor and acts as humidity sensing element.

The leads of two electrodes are connected in one of the four arms of a balanced

Wheat stone bridge circuit.

The two electrodes are placed in the atmosphere whose humidity is to be measured.

When the humidity of atmosphere changes, the lithium chloride absorbs or losses

moisture.

Therefore the resistance of the lithium chloride conductor changes.

When the humidity in the atmosphere increases, the resistance decreases and vice versa.

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ICS NOTES-58 UNIT-7


Due to this the balance condition of Wheatstone bridge will get disturbed and the bridge

produces some output voltage which gives the measure of relative humidity present in the

atmosphere.

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ICS NOTES-60 UNIT-7


Moisture Measurement - For Granular Materials :

To measure moisture present in granular material a cup shaped electrode arrangement is

used.

The granular material is poured into the cup to which electrode leads are connected.

A piston provided by spring arrangement is used to close the cup in order to maintain

maximum pressure in the material, and actual moisture is measured by weighting it after

it becomes completely dried.

For each material and its electrode arrangement, resistance – moisture content

characteristics are established provided the device is calibrated properly.

Elastic force meters

Elastic force meters are force measuring devices.

These can be used to measure both static force and dynamic force.

When the force to be measured is applied to these meters, the elastic sensing element

sense the applied force and produces displacement.

The measure of this displacement gives the amount of force applied at the input.

These elastic sensing elements can be available in the form of diaphragms, cylinders,

rings, strips etc.

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Load cell

NOTES-60 UNI

E

-7

Page 150

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Primary devices that enable measurements of both static and dynamic forces are known

as load cells.

Load cells use elastic member as primary devices and strain gauge as secondary devices

in the measurement of static and dynamic forces.

The load cell which uses a cantilever beam (elastic member) as primary transducer and

strain gauge as secondary transducer is shown in figure

The strain experienced by the strain gauges R2 and R4 is opposite in nature of strain that

is experienced by gauges Rl and R3 .

When force is applied to cantilever beam it will bend, with this the resistance of the

gauges will be changed.

Here electrical output will be obtained with the use of Wheatstone bridge.

AE

P =ε Strian

L

EAk , =Stiffness

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ICS NOTES-61 UNIT-7


Hydraulic load cell.

The principle of operation of hydraulic load cell -When a force is applied on a liquidmedium, the pressure of the liquid increases.

This increase in pressure is a measure of the applied force when calibrated.

It is also known as a hydraulic plunger.

Hydraulic oil is filled in a closed chamber whose height is 0.75 mm.

The force to be measured is applied on the diaphragm.

The applied force move the diaphragm downwards and thus closes the chamber from the

top.

The pressure of the liquid increases due to the applied force. This increase in pressure of

the liquid is measured by employing mechanical or electrical pressure gauge.

When full load is applied the maximum of 0.05 mm deflection occurs.

This type of load cell can measure force upto 500 tonnes.

Pneumatic Load Cell.

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ICS NOTES-61 UNIT-7


Principle - works on the balancing of force

i.e., the applied unknown downward force is balanced by upward force of air pressure. The

pressure at which the downward force is balanced by upward force indicates the amount of

applied force.

When the unknown force is applied to the top of the diaphragm, in diaphragm deflects

towards down.

Therefore the flapper moves downwards and closes the opening of nozzle.

Air is supplied through and air pressure regulator to the other side of the diaphragm.

Since the flapper shut off the nozzle opening, the back pressure increases in the system.

This increased back pressure also act on the diaphragm.

The air pressure value is regulated until the diaphragm comes back to its pre-loaded

position.

At this balanced stage of diaphragm, the pressure indicated by the pressure meter(pressure gauge) gives the amount of force applied.

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ICS NOTES-62 UNIT-7


Strain Gauge Torsion Meter

This arrangement uses four strain gauges each mounted at 900 to each other.

Strain gauges are diametrically opposite to each other and these strain gauge are

arranged in a Wheatstone bridge circuit.

Strain gauge R1 and R4 are subjected to tensile stress and strain gauge R2 and R3 are

subjected to compressive stress, when the shaft is under torsion.

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ICS NOTES-62 UNIT-7


Therefore, the torque of the rotating shaft is obtained with the help of compressive and

tensile stresses.

The angle made by the gauge with shaft is 45

0

, strain is given by.

Electrical Torsion Meter

Rotating shaft employs two slotted discs and two transducers which can be photoelectric

or magnetic transducers.

The two wheels are mounted on a shaft as shown.

When no torque is applied on the shaft the teeth of both wheels are correctly aligned

with each other.

Under this condition the voltage pulses induced in both the transducers are same i.e., thetime interval between the pulses is zero.

When the shaft is subjected to torque to the measured, the teeth of both wheels will not

align.

It causes voltage pulses to induce in the two transducers with a time difference.

Nml

R RGT

i 2

)( 0 φ π −

=

)( 44

0

045

i R RG

TR

−±=π

ε

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This time difference or time interval between the pulses is proportional to the torque

applied on the shaft.

Here the time interval is measured by connecting the output of transducer to as electronic

circuitry using leads.

Mechanical Torsion Meters

It contains a shaft mounted between two drums and two flanges.

One drum is provided with a torque calibrated scale and the other has a pointer.

A stroboscopic light source is used to note down the readings on the rotating shaft.

One of the two ends of the rotating shaft is mounted on the driving engine where as the

other end is attached to the driven load.

The angular displacement (angular twist) of the shaft over a fixed length is proportional

to the torque exerted on the shaft.

This angle of shift gives the amount of torque applied and is indicated by the movement

of the pointer on the calibrated scale.

Since the calibrate scale is marked on the rotating drum it is difficult to note down the

readings directly.

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ICS


To overcome this difficul

calibrated scale and the fl

is appeared.

The movement at which

is taken.

As the applied torque var

It can measure and indic

the shaft power.

Optical Torsion Meter

This optical system meas

The deflected light beam

applied torque.

In contains a shaft on wh

distance apart.

A tension strip is placed

Two mirrors are fixed o

reflected onto the torque

When the shaft is subject

NOTES-62 UNI

E

ty the flash light from the stroboscope is focuse

ashing frequency is varied and adjusted until a

the stationary image appears, the reading on th

ies, the angle of twist also varies.

ate the varying angle of twist and it can also be

ures the angular deflection of light beam.

is proportional to the angular twist of the shaft

ich two castings namely P and Q are mounted a

and attached between the two castings

the castings such that the light falling on the m

calibrated scale through an optical system.

ed to torque, a relative movement takes place b

-7

Page 158

d onto the

tationary image

calibrated scale

sed to indicate

and hence to the

a known

irrors is

tween P and Q.

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ICS NOTES-62 UNIT-7


Therefore the mirrors fitted on the castings will changes their position. Due to this,

angular deflection of light beam occurs.

The measure of the deflected light beam gives the angular twist of the shaft

Used to measure the torque continuously in Steam Turbines and I.C engines.

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ICS


Hydraulic Dynamomet

It is an absorption type o

This dynamometer uses

i.e., to dissipate mechani

Therefore, it is also kno

It contains ,

A rotating disk which is

semi elliptical groove wh

A stationary casing mou

system is fixed to it so as

Same as the rotating disk

The semi-elliptical groov

recesses of the casing.

As the driving shaft of th

chamber which causes v

causes the casing of the d

NOTES-63 UNI

E

r

dynamometer.

luid friction for their operation

al energy.

n as fluid friction dynamometer.

attached to the driving shaft of the test machine

ich allow water or steam to flow through them.

nted on antifriction bearings . It has a braking a

to make the casing rotate freely.

of the casing also contains semi-elliptical groo

es of the disk match with the corresponding se

prime mover rotates the liquid follows a helic

rtices and eddy currents to develop in the liqui

ynamometer to rotate in the direction of the sha

-7

Page 160

. The disk -has

m and a balance

es on it.

i-elliptical

l path in the

which in turn

ft.

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ICS NOTES-63 UNIT-7


The power absorption is maximum if the casing is full and is minimum if the amount of

liquid is minimum.

The total power absorption of this device changes approximately as

(i) Cube of rotational speed

(ii) Fifth power of rotating disk diameter.

The absorbing element contains a force sensing element such as load cell placed at the

end of the arm/whose radius is 'r'.

Torque T=F x r

Dynamometer:

A dynamometer is a brake but in addition it as a device to measure the frictional

resistance.

Knowing the frictional resistance, we may obtain the torque transmitted and hence

the power of the engine.

Types:

1. Absorption dynamometers:

Entire energy or power produced by the engine is absorbed by the friction resistances of

the brake and is transformed into heat.

a) Prony brake dynamometer, and

b) Rope brake dynamometer.

2. Transmission dynamometers:

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ICS NOTES-63 UNIT-7


The energy or power produced by the engine is transmitted through the dynamometer to

some other machines.

a) Epi cyclic-train dynamometer,

b) Belt transmission dynamometer, and

c) Torsion dynamometer.

Prony brake dynamometer:

It consists of two wooden blocks placed around a pulley fixed to the shaft of an engine

whose power is required to be measured.

The blocks are clamped by means of two bolts and nuts.

A helical spring is provided between the nut and the upper block to adjust the pressureon the pulley to control its speed.

Torque on the shaft T = W.L = F.R N-m

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ICS NOTES-63 UNIT-7


Rope brake dynamometer:

It is most commonly used for Measuring the brake power of the engine.

It consists of one, two or more ropes wound around the flywheel or rim of a

pulley fixed rigidly to the shaft of an engine.

The upper end of the rope is attached to a spring balance while the lower end of the rope

is kept in position by applying a dead weight .

Wooden blocks are placed at intervals around the circumference of the flywheel to

prevent the slipping of the rope over the flywheel.

Net load on the brake F = (W – S) N

Torque on the shaft T = F.R N-m

=(W-S).R

Brake Power = = =

Eddy Current Dynamometer:

60

)(2 RS W N −π

60

2 NT π

60

))(( d DS W N +−π

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ICS


Principle-Whenever a conductin

the current .

Current flows in a short c

The current flowing wit

the form of heat.

It has

(i)A toothed rotor made

tested or a test engine.

(ii)A cast iron stator that

Working-When the drivi

Due to this a constant ch

Therefore eddy currents

These induced currents o

The moment of resistanc

rotor) is determined by m

This moment of resistanc

NOTES-63 UNI

E

element moves through a magnetic flux an e.

ircular path inside the conductor.

in the conductor is known as eddy current and i

f steel mounted on the driving shaft of the pow

carries an exciting coil

ng shaft rotates which in turn rotates the rotor.

ange in flux density occurs at all points on the s

re induced in the stator

bstruct (resist) the rotation of the rotor.

e (i.e., the moment at which eddy currents resis

eans of braking arm and balance system.

e is used to measure the torque and the shaft po

-7

Page 164

.f is induced in

s dissipated in

r source to be

tator.

the rotation of

wer.

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ICS NOTES-63 UNIT-7


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ICS NOTES-64 UNIT-8


Control system system :

A system in which input has a command over the output or a system in which

input has a control over the output is called a control system.

Figure: A Control System

A water tap can be taken as simple and best example of a control system. Here the flow

of water (i.e., e output) is mechanically controlled by the movement valve (i.e., input).

Different examples of control system are,

A Switch

FigureOne among the examples of a control system is ach in which the input is

mechanically pressing or ap-ag force on button with the fingers and output is flow on-

flow of current.

A Driving System

Figure: Driving System of an Automobile

Controller SystemInput Input

LinkagesAnd

Carborator

EngineCalibration

InputOutput

Variable

speed

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Here the input is the Acceleration which is given by a human being to the vehicle which

controls or regulates the output i.e., the speed of the vehicle. The desired speed can be

obtained by controlling the Acceleration.

(iii) Biological Control System

In this system a person with his finger points towards a particular object and the

output is desired pointed direction. The control signal here is the position of the object.

The-diagrammatic representation is shown below.

Figure: Biological Control System

Examples of control system include temperature measurement~ of thermometer,

refrigerator, washing machine, electric frying pans, household devices with thermoset

like iron etc.

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Requirements of control system :

Basically there are three main requirements of a control system. They are,

(i) Accuracy

(ii) Stability

(iii) Speed of response.

Accuracy

Accuracy of a system is defined as the difference between the measured output

and the true input. A system with more accuracy is highly expensive.

It is given by formula,

Accuracy =Measured output - True input

In many systems accuracy is expressed as percentage,

inputTrue

inputTrue-outMeasure%Accuracy =

In day-to-day scenario's none of the systems are 100% accurate because there will

be a difference between the measured output and the true input.

Stability

A system is said to be stable if it produces bounded output for a bounded input

also the output also the output reaches to zero state in the absence of the input,

independent of initial conditions. In these type of systems the response is finite for a

given input.

Stability is categorized into two type (i). Absolutely stable (ii) Conditionally

stable. Absolutely stable system is one whose output is stable for all variations of its

parameters and conditionally stable system is one whose output is stable for limited

variations in its parameters.

Speed for Response

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The time taken by the system to sense the applied output and deliver us output is

known as the sped of response. For an ideal system the speed of response is infinity i.e.,

the system must response to the applied input instantaneously.

For an ideal system accuracy is 100%, it is perfectly stable and the speed for

response is infinity but such system never exists in real time scenarios.

meter which lets the rider to obtain the desired speed.


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Different types of control systems:

Control system is basically categorized into two types.

(i) Open loop system (system without feedback)

(ii) Closed loop system (system with feedback).

Let us discuss each of them detail.

(i) Open Loop System

A control system which does not automatical1y corrects error in the outputs of the system

of known as an open loop system. Error here inc1udes the environmental variations of

disturbances. The general block diagram of open loop system is,

Figure: Open loop Control system

An open loop system is also called a system with no feedback to the input. Since

without feedback the system is not aware of what errors in the output are occurring. Thus

the output changes due to disturbances is not followed by change in input to correct the

output.

Any changes in system are corrected manually,

Traffic Control System

Traffic control system is the best example for open loop control system. Here the

timers which are included in the signals does not depend upon the quantity of traffic

rather they display control signals depending upon the time allotted for each way.

(i) Closed Loop Systems

A system which automatically correct errors in the output is called a closed loop

system. In a closed loop system output effects the input so as to maintain the desired

output. The back diagram of closed loop system is shown figure

ControlSystemInput

Error

Output

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An open loop system can be modified to closed loop system by just adding a

feedback path.

When input is applied to a closed loop system a output is obtained this output is

sent as feedback to the input where the error detector checks for the errors. If error are

encountered then they are corrected and hence desired output is obtained.

Example

An Automobile Steering System

An automobile steering system is an example for closed loop system. Its block

diagram representation is shown in figure (3).

Figure (3): Automobile Steering System

The eyes will have the general view of movement of car. If any error is

encountered, it is corrected by the brain and hands. This result is feedback to the eyes.

obtain the desired speed.


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Distinguish between open loop and dosed loop systems:

Open Loop System

The features of this systems are,

1. The variations in the output effected by noise are not corrected automatically.

2. There is no comparison between obtained value and the desired value of the

variable.

3. Due to the disturbances the output may not be the desired one.

4. These systems are highly inaccurate and are unreliable.

5. Excluding above factors, these systems are easy of construct and are

economical.

Example:

Figure: Temperature Control System (open Loop)

The above block diagram represents a closed loop temperature control system.

Description

The relay circuit in the block diagram operates as a switch, which is automatically

controlled by a computer or a microprocessor. The time slot during wl1ich the relay

behaves as a short circuit (i.e., ON switch), in order to generate heat by coil is the critical

parameter for obtaining desired temperature. ,

The reference input (set point) is feeded in the controller with the help of a

keyboard or any other input device. The temperature in the electrical furnace is sensed by

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the sensor. The output of sensor is an analog signal and is converted to digital by means

of analog to digital converter and finally given to the computer. The computer compares

this signal with the reference input. If any difference occurs then the computer sends an

error signal. This error signal is converted into analog with the help of DAC (digital to

analog converter) and applied to relay circuit through 'amplifier, Depending on error

signal the relay circuit changes its state (switches ON or OFF). This process continues

until desired temperature is obtained. When temperature is at desired point no error signal

is generated by the controller.

REFERENCE BOOKS:

1.A course on mechanical measurements and instrumentation- A.K Sawhney2.Instrumentation and Control systems- S.Sudhakarreddy&P.Divakararao


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Working of one automatic control system used in practice:

Automata systems are the one controlled automatically (i.e., not manually). One among

the automatic control system is the feedback controlled thermal system is shown in the

figure below.

Description

The thermal system comprises of a steam control valve, an automatic controller, a

heating element and a thermometer.

Cold water is passed in the thermal system through the inlet shown in the figure.

Depending upon the temperature of water desired, the steam valve is opened and steam is

supplied into the tank. Due to this the temperature of water increases. The thermometer

employed is used to measure the temperature of hot water. This measured value is given

as feedback to the automatic controller (generally a regular). The controller compares the

measured temperature with the' desired temperature. If any difference is encountered then

an error signal is generated by the controller and is given to the control valve. Finally

depending upon the type of error signal the control valve performs the operation and

hence descried hotness of water is obtained.

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Controlling of water level in a boiler:

Automatic control systems are employed for controlling water level in a boiler. This is

shown diagrammatically as.

The boiler is provided with an inlet, specified for flow of water in the tank, A

pneumatic valve is employed at the inlet for adjusting the flow of water. This valve is

opened or closed so as to obtain the desired water level in the tank. Depending upon the

position of valve water gets accommodated in the tank. This obtained level of water in

the tank is measured, and applied to automatic controller. Here any increase and decrease

in the water level moves the ball up and down respectively. This up and down movement

of ball gives the status of liquid level to the controller. The controller compares the

obtained level with the desired level. If any difference occurs then an error signal is

generated By the controller and is given as a feedback to pneumatic valve. Depending

upon the error signal the operation is performed and desired water level in the boiler is

obtained.

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Servomechanism :

Servomechanism

A feedback control system in which the variable to be controlled (output) is either

mechanical position or velocity, acceleration i.e., time derivative of displacement is

known as servomechanism. It uses feedback mechanism to correct the performance of

the system. In this system the error-correcting signal&(feedback) help in controlling the

mechanical positions. The main purpose of servomechanism is to provide accurate

control of motion automatically. The response time is of order of milliseconds.

Applications of servomechanism include power assisted steering and control large cars,

ships etc.

Position Control System using Servomotor

Position control system with servomechanism is depicted below.

In the figure above the generator 'G' is used to power the servomotor. To the shaft

of servomotor a load is connected through gears wheels. Here we need to obtain the

desired position of the load. Electrical signals obtained are converted to mechanical

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motion by means of potentiometers. The input potentiometer is used to set desired load

position ' θd' and feedback Potentiometer is used for the actual, load" position ' θa'. The

difference between the two angular positions i.e., 'θd' 'θa' generates the error signal which

is amplified and fed to the generator. The generation circuit hence drives the servomotor.

The motor stops rotating if the error signal is zero i.e ., if the desired load position is

obtained.

REFERENCE BOOKS:

1.A course on mechanical measurements and instrumentation- A.K Sawhney2.Instrumentation and Control systems- S.Sudhakarreddy&P.Divakararao


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Working of a variable speed D.C. drive control system and its characteristics and

applications:

The block diagram representation of a variable speed D.C. drive control system is shown

below.

Potentiometer is used to vary the speed that is desired. The output of

potentiometer is given to the system. The motor included in this system is a D.C. shunt

motor, here the motors armature voltage is varied and field current is kept constant to

achieve required speed. This obtained speed is delivered to the load and is also measured

by the tachometer. The value measured by the tachometer is compared with the desired

speed and if difference is encountered, an error signal is generated; This error signal is

amplified is given to the shunt motor to obtain the desired speed.

Applications

1. Used in roll stabilization of ships.

2. Power steering apparatus of an automobile.

3. Variable speed control system of D.C. motor.

4. Machine tool position control.

5. For guided missiles, aircraft and manufacturing machinery.

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Block diagram of the arrangement and the uses of feedback in the application of:

(a) The Speed of Steam Engine

The speed of a steam engine is controlled by Pressure Controlling the steam

supply.

For controlling the steam supply centrifugal watt governor is used. Centrifugal

watt governor set-up consists of a string, spring and two flying balls. An actuator is also

attached to the governor, for controlling the opening of steam supply valve. The set-point

of the speed is also set and fed to the governor through a balancing rod.

The string of the govern pr is connected to the shaft of the steam engine.

Whenever the speed of the shaft exceeds the set-point the arrangement of the string andthe spring exeI1scentrifugal force on the flying balls which cause the balls to move

outwards. This outward movement of the balls applies power on the piston of the

hydraulic actuator which in turn decreases the opening of the valve of the steam supply.

Reduction in valve opening decreases the steam supply and thereby reduces the engine

speed and brings it to the set-point.

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Similarly, when there is a decrease in speed, the governor arrangement leads to

opening of the valve. This increases the steam supply and thus increases the speed.

When the output is not feedback to the input, automatic opening and closing of

valve will not take place and the steam supply is not controlled. Therefore the speed of

the steam engine is not controlled at desired value.

(b) Control of Pressure in a Furnace

The control system for controlling pressure, in a furnace consists of pressure

gauge, actuator and a damper mechanism.

The damper is placed inside the chimney in between regulates the flow of gases.

The damper mechanism regulates the flow of gases.

The pressure inside the furnace is measured through a pressure gauge and

compared to the set-point pressure. If there is a deviation from the set-point, the

corresponding correction signal (electrical signal) is applied to the actuator. The actuator

converts this electrical signal into a physical signal and applies it to the damper

mechanism. The displacement of the damper either increases or decreases the pressure

inside the furnace, according to the correction signal. Thus, in this way the pressure in a

furnace is controlled.

If the system does not contain feedback arrangement, the error signal' will not be

applied to the damper through the actuator. Therefore any changes in pressure inside the

furnace will not be controlled and desired pressure will not be achieved automatically.

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(a) The Temperature of Water Being Heated by Steam.

- For answer refer Unit-VIII, Q6.

(b) The Speed of an Automobile Vehicle

Figure above depicts the block diagram representation of a closed loop

automobile system. In our scenario we are controlling the speed of the automobile.

Description

Let us consider a rider riding a bike then the speed desired by the rider will be as

an input to the system. Depending upon the decision made by human brain the hands

will apply force on the acceleration. Receiving all these signals the vehicles engine will

perform the specified function.

The speed generated by the engine is the output. This speed is calculated with the

help of speedometer and is sighted towards human eye. If the actual speed or obtained

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speed is different from the desired speed then an error signal is made by the human eyes.

This error signal disturbs the human brain and hence on further acceleration or

retardation the desired speed is obtained.

The feedback in this case is the display scale of speedometer to the human eye. Itis a critical parameter which lets the rider to obtain the desired speed

REFERENCE BOOKS: 1.A course on mechanical measurements and instrumentation- A.K Sawhney

2.Instrumentation and Control systems- S.Sudhakarreddy&P.Divakararao 3 Instrumentation Measurement & Analysis B C Nakra and K K Choudhry

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ics notes by polarao sir

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