unit v - metrology and instrumentation notes

7/26/2019 Unit v - Metrology and Instrumentation Notes

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Unit V - Course Material SPRX1003 Metrology & Instrumentation

Department of Mechanical and Production Engineering Sathyabama University Page 1

UNIT V

MEASUREMENT OF POWER, FLOW AND TEMPERATUREForce.

The mechanical quantity which changes or tends to change the motion or shape of a body

to which it is applied is called force.

Load cells

Load cells are devices used for force measurement through indirect methods.

Principle of working of load cells.

Force applied to the elastic member of the cell results in a proportional displacement or strain is

sensed by calibrated mechanical or electromechanical means.

Principle of working of load cells.

Force applied to the elastic member of the cell results in a proportional displacement or strain issensed by calibrated mechanical or electromechanical means.

Devices used to measure force

1. Scale and balance

a. Equal arm balance

b. Unequal arm balance

c. Pendulum scale

2. Elastic force meter Proving ring

3. Load cell

a. Strain gauge load cell

b. Hydraulic load cell

c. Pneumatic load cell


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Basic principle of elastic force meter.

When a steel ring is subjected to a force across its diameter, it deflects. This deflection is

proportional to applied force when calibrated.

Basic principle of equal arm balance.

It works on the principle of moment comparison. The beam of the equal arm balance is in

equilibrium when clockwise rotating moment is equal to anticlockwise rotating moment.

Basic principle of hydraulic load cell.

When a force is applied on a liquid medium contained in a confined space, the pressure of

the liquid increases. The increase in pressure of liquid is proportional to the applied force.

Instruments used for the measurement of torque.

1.Optical torsion meter

2.Electrical torsion meter

3.Strain gauge torsion meter

4.Mechanical torsion meter

Basic principle of Mechanical torsion meter.

When a shaft is connected between a driving engine and driven load, a twist occurs on the

shaft between its ends. This angle of twist is measured and calibrated in terms of torque.

Types of strain gauges.

1.Unbonded strain gauge

2.Bonded strain gauge

3.Fine wire strain gauge4.Metal foil strain gauge

5.Piezo-resistive strain gauge

unbonded strain gauge.

These strain gauges are not directly bonded on to the surface of the structure under study.

Hence they are termed as unbonded strain gauges.


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bonded strain gauge.

These strain gauges are directly bonded on to the surface of the structure under study.Hence they are termed as unbonded strain gauges.

Gauge factor.

It is the ratio of change in resistance to the change in length.

Few materials used in binding of strain gauges.

1.Ceramic cement

2.Epoxy resin

3.Nitrocellulose.

Need for using strain gauge in wheatstone network circuits.

The need for the strain gauge in wheatstone network circuit is that the change in resistance

due to strain in the gauges can neither be measured or made to give an output which can easily

displayed or recorded.

Strain gauge rosettes

The arrangement of strain gauges in the shape of rose is referred to as a strain gauge rosette.

Purpose of temperature measurement

1. It is one of the most common and important measurements.

2. In process industries which involve chemical operations.

3. In studying the temperature of molten metal in foundries.

Instruments used to measure temperature.

1.

Bimettalic thermometers

2. Resistance thermometers

3.

Thermistors

4.

Thermocouples

5. Pyrometer


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Thermistor

It is a bulk semiconductor resistance temperature sensor.

Two distinct instruments commonly referred to as pyrometers.

1. Total radiation Pyrometers.

2. Optical pyrometers.

Applications of bimetallic thermometer.

1. Bimetallic thermometer is used in control devices.

2. Used for process applications such as refineries, oil burners, tyre vulcanizers, etc.

Principle of pressure thermometer

When liquids, gases or vapours are heated they expand and when they are cooled they

contract. This is the basic behind the construction of pressure thermometers.

Principle of bimetallic thermometer.

When a bimetallic helix fixed at one end free at the other end is subjected to temperature

changes, the free end of the bimetallic helix deflects proportional to change in temperature. This

deflection becomes a measure of change in temperature.

Advantages of bimetallic thermometer.

1. Their accuracy is between 2% to 5% of the scale.

2. Simple, robust, inexpensive.

Basic principle of resistance thermometers

When an electric conductor is subjected to temperature change the resistance of the

conductor changes. This change in resistance of the conductor becomes a measure of the change in

temperature when calibrated.

Advantages of thermistors

1. Fairly good operating range (100C to 300C).


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2. Have ability to withstand electrical and mechanical stresses.

Metal used for thermocouple wire.

1. Chromel - constantan

2. Iron constantan

3. Chromel Alumel

4. Copper constantan

5. Platinum Rhodium

Quantity meter and flow meter.

Quantity meter measures the rate of flow by measuring the total quantity of fluid over aperiod of time and dividing it by the time considered.

Flow meter measures the actual flow rate.

Advantages of venturimeter.

1. Low head loss about 10% of differential pressure head.

2. High co-efficient of discharge.

3. Capable of measuring high flow rates in pipes having very large diameter.

4. Characteristics are well established so they are extensively used in process and other industries.

Two types of hot wire anemometer.

1. Constant current type

2. Constant temperature type.

Pyrometer

Three definitions Any instrument used for measuring high temperatures by means of the radiation emitted

by a hot object

A thermometer designed to measure high temperatures

A device measuring the temperature of an object by means of the quantity and character

of the energy which it radiates

Types of pyrometers

There are two types of pyrometers


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(i)Optical Pyrometer

1892 introduced by Lechatelier, which it measured radiation from dull red to white hot Used for

measuring kiln and furnace temperature

Today an optical pyrometer is used in which the color of an electrically heated filament is matched

visually to that of the emitted radiation. Based on the principle of using the human eye to match

the brightness of the hot object to that calibrated inside the instrument

It is made from a small magnifying optical device. Filters that reduce wavelength to 0.65-0.66 and

other filters reduce intensity. These restrictions prevent the device from measuring object that are

glowing (700 C)

(ii)Radiation Pyrometer

Non-contact temperature sensors measure temperature from the amount of thermal

electromagnetic radiation received from a spot on the object of measureMeasures the rate energy

emission per area unit.

Absolute, gauge and differential pressures - zero reference

Although pressure is an absolute quantity, everyday pressure measurements, such as for tire

pressure, are usually made relative to ambient air pressure. In other cases measurements are made

relative to a vacuum or to some other ad hoc reference. When distinguishing between these zero

references, the following terms are used:

Absolute pressure is zero referenced against a perfect vacuum, so it is equal to gauge

pressure plus atmospheric pressure.

Gauge pressure is zero referenced against ambient air pressure, so it is equal to absolute

pressure minus atmospheric pressure. Negative signs are usually omitted.

Differential pressureis the difference in pressure between two points.

The zero reference in use is usually implied by context, and these words are only added whenclarification is needed.

Atmospheric pressure is typically about 100 kPa at sea level, but is variable with altitude and

weather. If the absolute pressure of a fluid stays constant, the gauge pressure of the same fluid

will vary as atmospheric pressure changes. For example, when a car drives up a mountain, the tire

pressure goes up. Somestandard values of atmospheric pressure such as 101.325 kPa or 100 kPa

have been d, and some instruments use one of these standard values as a constant zero reference

instead of the actual variable ambient air pressure. This impairs the accuracy of these instruments,

especially when used at high altitudes.
http://en.wikipedia.org/wiki/Pascal_(pressure)http://en.wikipedia.org/wiki/Standard_conditions_for_temperature_and_pressurehttp://en.wikipedia.org/wiki/Standard_conditions_for_temperature_and_pressurehttp://en.wikipedia.org/wiki/Pascal_(pressure)


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Use of the atmosphere as reference is usually signified by a (g) after the pressure unit e.g. 30 psi g,

which means that the pressure measured is the total pressure minus atmospheric pressure.There

are two types of gauge reference pressure: vented gauge (vg) and sealed gauge (sg).34. What are the different units of pressure

Pressure Units

pascal

(Pa)

bar

(bar)

technical

atmosphere

(at)

atmosphere

(atm)

torr

(Torr)

pound-force

per

square inch

(psi)

1 Pa 1N/m2 105 1.0197105 9.8692106 7.5006103 145.04106

1 bar 100,000 106dyn/cm2 1.0197 0.98692 750.06 14.5037744

1 at 98,066.5 0.980665 1kgf/cm2 0.96784 735.56 14.223

1 atm 101,325 1.01325 1.0332 1atm 760 14.696

1 torr 133.322 1.3332103 1.3595103 1.3158103 1 Torr;

1mmHg19.337103

1 psi 6.894103 68.948103 70.307103 68.046103 51.715 1lbf/in2

Static and dynamic pressures

Static pressure is uniform in all directions, so pressure measurements are independent of direction

in an immovable (static) fluid. Flow, however, applies additional pressure on surfaces

perpendicular to the flow direction, while having little impact on surfaces parallel to the flow

direction. This directional component of pressure in a moving (dynamic) fluid is calleddynamic

pressure. An instrument facing the flow direction measures the sum of the static and dynamic

pressures; this measurement is called the total pressure or stagnation pressure. Since dynamic

pressure is referenced to static pressure, it is neither gauge nor absolute; it is a differential

pressure.

While static gauge pressure is of primary importance to determining net loads on pipe walls,

dynamic pressure is used to measure flow rates and airspeed. Dynamic pressure can be measured

by taking the differential pressure between instruments parallel and perpendicular to the flow.

Need of calibrating a pressure gauge

Pressure gauges are either direct- or indirect-reading. Hydrostatic and elastic gauges measure

pressure are directly influenced by force exerted on the surface by incident particle flux, and arecalled direct reading gauges. Thermal and ionization gauges read pressure indirectly by
http://en.wikipedia.org/w/index.php?title=Pressure_unit&action=edit&redlink=1http://en.wikipedia.org/wiki/Atmospheric_pressurehttp://en.wikipedia.org/wiki/Pascal_(unit)http://en.wikipedia.org/wiki/Bar_(unit)http://en.wikipedia.org/wiki/Technical_atmospherehttp://en.wikipedia.org/wiki/Technical_atmospherehttp://en.wikipedia.org/wiki/Atmosphere_(unit)http://en.wikipedia.org/wiki/Torrhttp://en.wikipedia.org/wiki/Pound-force_per_square_inchhttp://en.wikipedia.org/wiki/Pound-force_per_square_inchhttp://en.wikipedia.org/wiki/Pound-force_per_square_inchhttp://en.wikipedia.org/wiki/Newton_(unit)http://en.wikipedia.org/wiki/Newton_(unit)http://en.wikipedia.org/wiki/Dynehttp://en.wikipedia.org/wiki/Kilogram-forcehttp://en.wikipedia.org/wiki/Kilogram-forcehttp://en.wikipedia.org/wiki/Atmosphere_(unit)http://en.wikipedia.org/wiki/Atmosphere_(unit)http://en.wikipedia.org/wiki/Torr#Manometric_units_of_pressurehttp://en.wikipedia.org/wiki/Pound-forcehttp://en.wikipedia.org/wiki/Pound-forcehttp://en.wikipedia.org/wiki/Static_pressurehttp://en.wikipedia.org/wiki/Dynamic_pressurehttp://en.wikipedia.org/wiki/Dynamic_pressurehttp://en.wikipedia.org/wiki/Total_pressurehttp://en.wikipedia.org/wiki/Stagnation_pressurehttp://en.wikipedia.org/wiki/Stagnation_pressurehttp://en.wikipedia.org/wiki/Total_pressurehttp://en.wikipedia.org/wiki/Dynamic_pressurehttp://en.wikipedia.org/wiki/Dynamic_pressurehttp://en.wikipedia.org/wiki/Static_pressurehttp://en.wikipedia.org/wiki/Pound-forcehttp://en.wikipedia.org/wiki/Torr#Manometric_units_of_pressurehttp://en.wikipedia.org/wiki/Atmosphere_(unit)http://en.wikipedia.org/wiki/Kilogram-forcehttp://en.wikipedia.org/wiki/Dynehttp://en.wikipedia.org/wiki/Newton_(unit)http://en.wikipedia.org/wiki/Pound-force_per_square_inchhttp://en.wikipedia.org/wiki/Pound-force_per_square_inchhttp://en.wikipedia.org/wiki/Pound-force_per_square_inchhttp://en.wikipedia.org/wiki/Torrhttp://en.wikipedia.org/wiki/Atmosphere_(unit)http://en.wikipedia.org/wiki/Technical_atmospherehttp://en.wikipedia.org/wiki/Technical_atmospherehttp://en.wikipedia.org/wiki/Bar_(unit)http://en.wikipedia.org/wiki/Pascal_(unit)http://en.wikipedia.org/wiki/Atmospheric_pressurehttp://en.wikipedia.org/w/index.php?title=Pressure_unit&action=edit&redlink=1


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measuring a gas property that changes in a predictable manner with gas density. Indirect

measurements are susceptible to more errors than direct measurements.

Various direct methods of measurement of force.

Force is very basic engineering parameter the measurement of which can be done in many

ways as follows:

(i) Direct Methods: Involves a direct comparison with a known gravitational force on a

standard mass, say by a balance.

(ii) Indirect Methods: Involves the measurement of effect of force on a body, such as

acceleration of a body of known ma subjected to force.

(i) Direct Methods

(a) Use of Analytical Balance

Analytical balance consists of an arm that rotates about a pivot. Two forces W1 W2 (or)

weights are added at the two ends as shown in figure.

Let W1be the know force and W2be the unknown. Let G be the gravity center of the arm

and WGbe its weight. When W1= W2, the arm is unbalanced. This unbalance is indicated by angle

the pointer making with the vertical.

For equilibrium, the requirement is

WG.XG= W1W1W2W2


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(b) Use of Pendulum Scale

This uses the Principle of multiple leverage. The input, a direct force or a force Proportionalto weight is transmitted from a suitable agency and applied to the lord rod. As the load is applied,

the sectors rotate about A (Figure) moving the counter weights outward. This movement increases

the counterweight effective moment until the load and balance moments are equalized. Motion of

the equalizer bar is converted to indicator movement by a rack and pinion.

Indirect methods of measurement of force (i) Acceleration method (ii) Using elastic loadedmembers (iii) Using cantilever elastic member.

(a) Use of Acceleration

A force will make a body accelerate. By measuring the acceleration, the force may be

determined, from the equation F=ma, when m mass of the body used. To measure acceleration,

accelerometers are used.


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(b) Use of Elastic Loaded Members

This uses the principle of finding strain produced in a body to measure the force applied.

For measuring displacement, strain gauges are mounted as shown in figure. The body is subjected

to a force and the gauges measure the strain so produced.

From basic mechanics of materials, force F produces a displacementFl

AE

Where

l Length of the specimen

A Cross-sectional area

E Youngs modulus

1 2

2 4

FAnd strain ,

AE

F ,

AE


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being poisons ratio. If the output of the circuit is e, it is given by

F1 2 3 4

F

V.Ge = ( )

4V.G Fe = (l )

2 AE

(c) Use of Cantilever Elastic member

In a cantilever beam, if the point of application of load is known, the bending moment

caused by it can be interpreted as force applied.

It is established that due to force, F, deflection of a cantilever at a length l from th e point of

application of force, is given as3

W I

3 EI

where E Youngs modulus of beam material,

I Moment of inertia of beam section =

3bd

12 From bending equation,

Moment at section xx xM x z (z-section modulus)

2

x x

bdM x

6


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xx x

1x 2

Strain is given byE

6.Fli.e.,

E.bd

Gauges R1, R3 measure tensile strain and

R2, R4 measure compressive strain.

Indirect methods of measurement of force (i) Using proving Rings (ii) Using load cells.

(i) Use of proving Rings

Proving rings are steel rings used for calibration of material testing machines in situations

where, due to their bulkness, dead weight standards cannot be used.

P ring is a circular ring of rectangular section and may support tensile or comprehensive force

across its diameter.

the change in radius in the direction of force, is given by

3K 4 F.d

16 2 EI

where d is the outer diameter of the ring and

K is stiffness.

Deflection of the ring is measured using a precision micrometer. To get precise measurements,

one edge of the micrometer is mounted on a vibrating reed which is plucked to obtain a vibratory

motion. The micrometer contact is then moved forward until a noticeable damping of the

vibration is observed.


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Maximum deflection is typically of the order of 1% of the outside diameter of the ring. Proving

rings are normally used for force measurement within the range of 2 kN to 2 mN.

(ii) Use of Load Cell

Force transducers intended for weighing purposes are called load cells. Instead of using total

deflection as a measure of load, strain gauge load cells measure load in terms of unit strains. A

load cell utilizes an elastic member as the primary transducer

and strain gauges as secondary transducer. Figure

shows one such load cell arrangement.


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Working of a DC Dynamometer for the measurement of torque.

Mechanical Dynamometer:

These come under the absorption type. An example for this kind is prony brake.

In Prony brake, mechanical energy is converted into heat through dry friction between the

wooden brake blocks and the flywheel (pulley) of the machine. One block carries a lever arm. An

arrangement is provided to tighten the rope which is connected to the arm. Rope is tightened so as

to increase ht frictional resistance between the blocks and the pulley.

If F Load applied and

Power dissipated2 NT 2 NFr

P60 60

r - Lever arm

N Speed of flywheel (rpm)

Torque T = F.r

The capacity of Prony brake is limited because:

1. Due to wear of wooden blocks, friction coefficient varies. So, unsuitable for large powers

when used for long periods.

2.To limit temperature rise, cooling is to be ensured.

D.C. Dynamometer

D.C. dynamometer is usable as an absorption as well as transmission dynamometer. So, it

finds its use in I.C. Engines, steam turbines and pumps. A d.c. dynamometer is basically a d.c.

motor with a provision to run it as a d.c. generator where the input mechanical energy, after

conversion to electrical energy, can either be dissipated through a resistance grid or recovered foruse. When used as an absorption dynamometer it acts as d.c. generator. (figure) Cradling in

trunnion bearings permits the determination of reaction torque.


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The torque is measured by measuring a balancing force (by means of a load cell, for

example) at a fixed known torque arm. When used as a transmission dynamometer it performs as

a d.c. motor. It then measures the torque and power input to the machine, for example, a pump

that absorbs power.

Its good performance at low speeds and ease of control makes it an efficient means of torque

measurement.

Working of a eddy current or inductor dynamometer for the measurement of torque.

Eddy Current or Inductor Dynamometers:

This is an example for absorption type dynamometers.

Principle: When a conducting material moves through a magnetic flux field, voltage is generated,

which causes current to flow. If the conductor is a wire forming a part of a complete circuit will be

caused to flow through that circuit, and with some form of commutating device a form of a.c. or

d.c. generator may result.


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An eddy current dynamometer is shown in figure. It consists of a metal disc or wheel

which is rotated in the flux of a magnetic field. The field if produced by field elements or coils

excited by an external source and attached to the dynamometer housing which is mounted in

trunnion bearings. As the disc turns, eddy currents are generated. Its reaction with the magnetic

field tends to rotate the complete housing in the trunnion bearings. Water cooling is employed.

Measuring instruments used for temperature measurement and the working of bimetallic

thermometers.

Temperature measuring instruments may be classified on the basis of:

1. Nature of change produced in the temperature sensing elements.

2. Electrical and non-electrical operation principle.3. Temperature range of the instrument.

Classification based on the Nature of Change Produced.

1. Glass thermometers

2. Pressure gauge thermometers

3. Differential expansion thermometers

4. Electrical resistance thermometers

5. Thermo couples

6. Optical pyrometers


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7. Radiation pyrometers

8. Fusion pyrometers

9. Calorimetric pyrometers

Based on Electrical and non-electrical Principles

1. Primarily electrical or electronic in nature

2. Not primarily electrical or electronic in nature.

Bimetallic Thermometers:

Principle Involved : These use the principles of metallic expansion when temperature changes.

A bimetallic strip is shown in figure which is straight initially. When temperature changes,

its shape also changes into an arc.

Fig. Deformation of bimetallic Strip

The displacement of the free end can be converted into an electric signal through use of

secondary transducers like variable resistance, inductance and capacitance transducers. Figure

shows a strip of bimetal in the form of a spiral. The curvature of the strip varies with temperature.

This causes the pointer to deflect. A scale is provided which has been calibrated to show thetemperature directly.

This kind of spiral is mostly used in devices measuring ambient temperature and air-

conditioning thermostats.

Advantages of Bimetallic Thermometers

1. Simple

2. Inexpensive


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3. Accuracy of 0.5% to 2%

Limitations

1. Not usable above 400C because of possibility of warping

Application Areas of Bimetal Thermometers

1. Refineries

2. Vulcanizers

3. Oil burners, etc.

Working of thermocouples and thermistors

i) Thermocouples

Principles Involved : When heat is applied to the junction of two dissimilar metals, an e.m.f. is

generated. (Figure)


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The e.m.f. produced E can be written as,

E = k.

Where - Difference in temperature of two junctions

This means that the e.m.f. produced is directly proportional to the temperature difference.

So, if the conjunction is maintained at constant temperature the thermocouple reading will be a

direct measure of temperature. (figure)

ii) Thermistors:

Thermistor is a temperature sensitive variable resistor made of a ceramic like

semiconducting material. They are made of metal oxides and their mixtures like oxides of cobalt,

copper, nickel, etc. Unlike metals, thermistors respond negatively to temperature. They behave as

resistors with a high negative temperature coefficient of resistance. Typically, for each 1 C rise intemperature, the resistance of a thermistor decreases by about 5%. This high sensitivity to

temperature changes makes the thermistor useful in precision temperature measurements. The

resistance of thermistors vary from 0.5 to 0.75M . Variation of resistivity with temperature is

shown in figure.


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The temperature vs resistance relation is given by

0

0

l 1R R e

T T

Where R Resistance at temperature TK

R0 - resistance at temperature T0K

B Constant (3400 K to 4600 K)

Thermistors come in different configurations some of which are shown in figure.

Application Area of Thermistor

1. Measurement of thermal conductivity

2. Measurement of gas composition

3. Measurement of flow and pressure of liquids.


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Flow is measured using Orifice meter and Venturi meter.

i) Orifice Meter:

Let a1Area at section I-I

a0Area of orifice

CdDischarge coefficient

Then, Flow rate d 1 02 2

1 o

C a aQ

A a

ii) Venturimeter:

This is just like an orifice meter. It has three distinct parts, namely convergent cone, throat

and divergent cone. A manometer measures the pressure difference between two sections as

shown in figure.

Let a1 - Area at the inlet (1-1)

A2 - Area at the section (2-2)

x - Pressure head difference

Cd - Discharge coefficient

Then, Q = d 1 22 2

1 2

C a a 2 g x

a a


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Flow is measured using Rotameter and Pitot tube.

i) Rotameter:

A rotameter is a variable area type flow meter. It consists of a vertical tapered tube with a

float which is free to move within the tube. The fluid goes from the bottom to the top. When no

fluid flows, the float rests at the bottom of the tube. The float is made of such a diameter that it

completely blocks the inlet. When flow starts in the pipeline and fluid reaches the float, the

buoyant effect of fluid makes the float lighter. The float passage remains closed until the pressure

of the flowing material plus the buoyance effect exceeds the downward pressure due to the float

weight. Thus, depending on flow, the float assumes a position. Thus the float gives the reading of

flow rate.

ii) Pitot Tube:

Principle: Transformation of kinetic energy of a liquid into potential energy in the form of a static

head.

Figure shows a pitot tube installed in a pipeline where it acts like a probe. The tube consists of two

concentric tubes, the inner tube with its open ends faces the liquid.


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The outer tube has a closed end and has four to eight holes in its wall. The pressure in the outer

tube is the static pressure in the line. Total pressure is sum of static pressure and the pressure due

to the impact of fluid.

If P - Pressure at inlet (Stagnation pressure)

Ps - Static pressure

- Density, then

Velocity v =0

2 / (P P ), from which flow rate is determined.

Hydraulic and Pneumatic systems for the measurement of force.

Hydraulic and Pneumatic Systems:

If a force is applied to one side of a piston or diaphragm, and a pressure, either hydraulic orpneumatic, is applied to the other side, some particular value of pressure will be necessary to

exactly balance the force. Hydraulic and pneumatic load cells are based on this principle.

For hydraulic systems, conventional piston and cylinder arrangements may be used.

However, the friction between piston and cylinder wall and required pickings and seals is

unpredictable, and thus good accuracy is difficult to stain. Use of the floating piston with a

diaphragm-type seal practically dominates this variable.


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Figure shows a hydraulic cell in section. This cell is similar to the type used in some materials-

testing machines. The piston does not actually contact a cylinder wall in the normal sense, but a

thin elastic diaphragm, or bride ring, of steel is used as the positive seal, which allows small piston

movement. Mechanical stops prevent the seal from being overstrained.

When force acts on the piston, the resulting oil pressure is transmitted to some form of

pressure sensing system such as the simple Bourdon gage. If the system is completely filled with

fluid, very small transfer or flow will be required. Piston movement may be less than 0.002 in at

full capacity. In this respect, at least, the system will have good dynamic response; however,

overall response will be determined very largely by the response of the pressure sensing element.

Very high capacities and accuracies are possible with cells of the type. Capacities to

5,000,000 Ibf (22.2MN) and accuracies of the order of % of reading or 1/10% of capacity.

Whichever is greater, have been attained. Since hydraulic cells are somewhat sensitive to

temperature change, provision should be made for adjusting the zero setting. Temperature

changes during the measuring process cause errors of about % per 10F change.

Pneumatic load cells

Pneumatic load cells are quite similar to hydraulic cells in that the applied load is balanced

by a pressure acting over a resisting area, with the pressure becoming a measure of the applied

load. However, in addition to using air rather than liquid as the pressurized medium, these cells

differ from the hydraulic ones in several other important respects.

Pneumatic load cells commonly use diaphragms of a flexible maternal rather than pistons

and they are designed to regulate the balancing pressure automatically. A typical arrangement is

shown in figure.


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Air pressure is supplied to one side of the diaphragm and allowed to escape through a

position controlling bleed valve. The pressure under the diaphragm, therefore, is controlled both

by source pressure and bleed valve position. The diaphragm seeks the position that will result in

just the proper air pressure to support the load, assuming that the supply pressure is great enough

so that its value multiplied by the effective area will at least support the load.

We see that as the load changes magnitude, the measuring diaphragm must change its

position slightly. Unless care is used in the design, a nonlinearity may results, the cause of which

may be made clear by referring to figure.


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As the diaphragm moves, the portion between the load plate and the fixed housing will alter

position as shown. If it is assumed that the diaphragm is of a perfectly flexible material, incapableof transmitting any but tensile forces, then the division of vertical load components transferred to

housing and load plate will occur at points A or A, depending on diaphragm position. We see

then tat the effective area will change, depending on the geometry of this portion of the

diaphragm. If a complete semicircular roll is provided, as shown in figure (b) this effect will be

minimized.

Since simple pneumatic cells may tend to be dynamically unstable, most commercial types

provide some form of viscous damper to minimize this tendency. Also additional chambers and

diaphragms may be added to provide for tare adjustment. Single-unit capacities to 80,000 Ibf (356

kN) may be obtained, and by use of paralleled units practically any total load or force may be

measured. Errors as small as 0.1% of full scale may be expected.

Working of pressure thermometers with a neat sketch.

Pressure Thermometers:

Figure shows the essentials of the practical pressure thermometer. The necessary parts are

bulb A, tube B, pressure sensing gage C, and some sort of filling medium. Pressure

thermometers are called liquid-filled, gas filled, or vapor filled, depending on whether the filling

medium is completely liquid, completely gaseous, or a combination of a liquid and its vapour. A

primary advantage of these thermometers is that they can provide sufficient force output to

permit the direct of recording and controlling devices. The pressure-type temperature sensing

system is usually less costly than other systems. Tubes as log as 200ft may be used successfully.


27/34



Expansion (or contraction) of bulb A and the contained fluid or gas, caused by temperature

change, alters the volume and pressure in the system. In the case of the liquid-filled system, the

sensing device C acts primarily as a differential volume indicatory, with the volume incrementserving as an analog of temperature. For the gas-or vapour-filled systems, the sensing device

serves primarily as a pressure indicator, with the pressure providing the measure of temperature.

In both cases, of course, both pressure and volume change.

Ideally the tube or capillary should serve simply as a connecting link between the bulb and

the indicator. When liquid or gas-filled systems are used, the tube and its filling are also

temperature sensitive, and any difference from calibration conditions along the tube introduces

output error. This error is reduced by increasing the ratio of bulb volume to tube volume.

Unfortunately, increasing bulb size reduces the time response of a system, which may introduce

problems of another nature. On the other hand, reducing tube size, within reason, does not

degrade response particularly because, in any case, flow rate is negligible. Another source of errortht should not be overlooked is any pressure gradient resulting from difference in elevation of

bulb and indicator not accounted for by calibration.

Temperature along the tube is not a factor for vapour-pressure systems, however, so long

as a free liquid surface exists in the bulb. In this case, Daltons law for vapou rs applies, which

states that if both phases (liquid and vapour) are present, only one pressure is possible for a given

temperature. This is an important advantage of the vapour-pressure system. In many cases,

though, the tube in this type of system will be filled with liquid, and hence the system is

susceptible to error caused by elevation difference.


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Principle and working of thermistors.

Resistance elements sensitive to temperature are made of metals generally considered to begood conductors of electricity. Examples are nickel, copper, platinum and silver. A temperature

measuring device using an element of this type is commonly referred to as a resistance

thermometer, or a resistance temperature detector, abbreviated RTD. Of more recent origin are

elements made from semiconducting materials having large and usually negative resistance

coefficients. Such materials are usually some combination of metallic oxides of cobalt, manganese,

and nickel. These devices are called thermistors.

One important difference between these two kinds of material is that, whereas the

resistance change in the RTD is small and positive (increasing temperature causes increased

resistance), that of the thermistor is relatively large and usually negative. In addition, the RTD

type provides nearly a linear temperature resistance relation, whereas that of the thermistor is

nonlinear. Still another important difference lies in the temperature ranges over which each may

be used. The practical operating range for the thermistor lies between approximately - 100C to

275C (-150F to 500F). The range for the resistance thermometer is much greater, being from

about - 260C to 1000C (-435F to 1800F). Finally, the metal resistance elements are more time

stable than the semiconductor oxides; hence they provide better reproducibility with lower

hysteresis.

Resistance Thermometers (RTDs)

Evidence of the importance and reliability of the resistance thermometer may be had by

recalling that the International Temperature Scale of 1990 specifies a platinum resistance

thermometer as the interpolation standard over the range from -259.35C to 961.78C (-484.52F to

1763.20F).

Certain properties are desirable in material used for resistance thermometer elements. The

material should have a resistivity permitting fabrication in convenient sizes without excessive

bulk, which would degrade time response. In addition, its thermal coefficient of resistivity shouldbe high and as constant as possible, thereby providing an approximately linear output of

reasonable magnitude.

The material should be corrosion resistant and should not undergo phase changes in the

temperature range of corrosion resistant and should not undergo phase changes in the

temperature range of interest. Finally, it should be available in a condition providing reproducible

and consistent results. In regard to this last requirement, it has been found that to produce

precision resistance thermometers, great care must be exercised in minimizing residual strains,

requiring careful heat treatment subsequent to forming.


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As is generally the case in such matters, no materials is universally acceptable for resistance-

thermometer elements. Undoubtedly, platinum, nickel, and copper are the materials most

commonly used, although others such as tungsten, silver and iron have also been employed. Thespecific choice normally depends upon which compromises may be accepted. The temperature

resistance relation of an RTD must be determined experimentally. For most metals, the result can

be accurately represented as

0 o 0R(T) R 1 A T T B T T 2 where

R(T) = the resistance at temperature T,

R0 = the resistance at a reference temperature T0

A and B = temperature coefficients of resistance depending on material.

Over a limited temperature interval (perhaps 50C for platinum) a linear approximation to

the resistance variation may be quite acceptable.

R(T) = R0(1+ A(T T0))

But for the highest accuracy, a high order polynomial fit is required.

The resistance element is most often a metal wire wrapped around an electrically insulatingsupport of glass, ceramic or mica. The latter may have a variety of configurations, ranging from a

simple flat strip, as shown in figure to intricate bird-cage arrangement (3). The mounted element

is then provided with a protective enclosure. When permanent installations are made and when

additional protection from corrosion or mechanical abuse is required, a well or socket may be

used, such as shown in figure.


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More recently, thin films of metal-glass slurry have been used as resistance elements. These films

are deposited onto a ceramic substrate and laser trimmed. Film RTDs are less expensive than the

wire RTDs and have a larger resistance for a given size; however, they are also somewhat lessstable (4). Resistance elements similar in construction to foil strain gages are available as well. The

resistance grid is deposited onto a supporting film, such as Kapton, which may then be cemented

to a surface. These sensors are generally designed to have low strain sensitivity and high

temperature sensitivity. Table describes characteristic of several typical commercially available

resistance thermometers.

The use of a pyrometer, its working principle and Applications

A pyrometer is a non-contacting device that intercepts and measuresthermal radiation,a process

known aspyrometry.This device can be used to determine thetemperatureof an object's surface.The word pyrometer comes from the Greek word for fire, "" (pyro), and meter, meaning to

measure. Pyrometer was originally coined to denote a device capable of measuring temperatures

of objects aboveincandescence(i.e. objects bright to the human eye).

Principle of operation

A pyrometer has an optical system and detector. The optical system focuses thethermal radiation

onto the detector. The output signal of the detector (Temperature T) is related to the thermal

radiationor irradiancej*of the target object through the StefanBoltzmann law, the constant of

proportionality, called theStefan-Boltzmann constantand theemissivity of the object.This output is used to infer the object's temperature. Thus, there is no need for direct contact

between the pyrometer and the object, as there is withthermocoupleandResistance temperature

detector(RTDs).

Applications

Pyrometer are suited especially to the measurement of moving objects or any surfaces that can not

be reached or can not be touched.

In Industry: Temperature is a fundamental parameter in metallurgical furnace operations.

Reliable and continuous measurement of the melt temperature is essential for effective control ofthe operation. Smelting rates can be maximized, slag can be produced at the optimum

temperature, fuel consumption is minimized and refractory life may also be lengthened.

Thermocouples were the traditional devices used for this purpose, but they are unsuitable for

continuous measurement because they rapidly dissolve.

Over-the-bath Pyrometer: Continuous pyrometric measurement from above the bath surface is

still employed, but is known to give poor results because of emissivity variations, interference by

gases and particulate matter in the intervening atmosphere, and dust accumulation on the optics.
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Tuyre Pyrometer:The Tuyre Pyrometer is an optical instrument for temperature measurement

through the tuyeres which are normally used for feeding air or reactants into the bath of the

furnace.

Different types of pressure measuring instruments

Many instruments have been invented to measure pressure, with different advantages and

disadvantages. Pressure range, sensitivity, dynamic response and cost all vary by several orders of

magnitude from one instrument design to the next. The oldest type is the liquid column (a vertical

tube filled with mercury) manometer invented byEvangelista Torricelli in 1643. The U-Tube was

invented byChristian Huygens in 1661.

Hydrostatic Gauges

Hydrostaticgauges (such as the mercury column manometer) compare pressure to the hydrostatic

force per unit area at the base of a column of fluid. Hydrostatic gauge measurements are

independent of the type of gas being measured, and can be designed to have a very linear

calibration. They have poor dynamic response.

Piston Gauges

Piston-type gauges counterbalance the pressure of a fluid with a solid weight or a spring. Another

name for piston gauge isdeadweight tester.For example, dead-weight testers used for calibration

ortire-pressure gauges.

Liquid column

The difference in fluid height in a liquid column manometer is proportional to the pressure

difference.

Liquid column gauges consist of a vertical column of liquid in a tube whose ends are exposed todifferent pressures. The column will rise or fall until its weight is in equilibrium with the pressure

differential between the two ends of the tube. A very simple version is a U-shaped tube half-full of

liquid, one side of which is connected to the region of interest while thereference pressure (which

might be theatmospheric pressure or a vacuum) is applied to the other. The difference in liquid

level represents the applied pressure. The pressure exerted by a column of fluid of height hand

density is given by the hydrostatic pressure equation, P= hg. Therefore the pressure difference

between the applied pressure Pa and the reference pressure P0 in a U-tube manometer can be

found by solving Pa P0 = hg. If the fluid being measured is significantly dense, hydrostatic

corrections may have to be made for the height between the moving surface of the manometerworking fluid and the location where the pressure measurement is desired.
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Based on the use and structure following type of manometers are used

1. Simple Manometer

2.

Micromanometer3.

Differential manometer

4.

Inverted differential manometer

McLeod gauge

A McLeod gauge isolates a sample of gas and compresses it in a modified mercury manometer

until the pressure is a fewmmHg.The gas must be well-behaved during its compression (it must

not condense, for example). The technique is slow and unsuited to continual monitoring, but is

capable of good accuracy.

Useful range: above 10-4torr (roughly 10-2Pa) as high as 106Torr (0.1 mPa),

0.1 mPa is the lowest direct measurement of pressure that is possible with current technology.

Other vacuum gauges can measure lower pressures, but only indirectly by measurement of other

pressure-controlled properties. These indirect measurements must be calibrated to SI units via a

direct measurement, most commonly a McLeod gauge.

Aneroid Gauges

Aneroidgauges are based on a metallic pressure sensing element which flexes elastically under

the effect of a pressure difference across the element. "Aneroid" means "without fluid," and the

term originally distinguished these gauges from the hydrostatic gauges described above.

However, aneroid gauges can be used to measure the pressure of a liquid as well as a gas, and

they are not the only type of gauge that can operate without fluid. For this reason, they are often

called mechanicalgauges in modern language. Aneroid gauges are not dependent on the type of

gas being measured, unlike thermal and ionization gauges, and are less likely to contaminate the

system than hydrostatic gauges. The pressure sensing element may be a Bourdon tube, a

diaphragm, a capsule, or a set of bellows, which will change shape in response to the pressure of

the region in question. The deflection of the pressure sensing element may be read by a linkageconnected to a needle, or it may be read by a secondary transducer. The most common secondary

transducers in modern vacuum gauges measure a change in capacitance due to the mechanical

deflection. Gauges that rely on a change in capacitances are often referred to as Baratron gauges.

Bourdon Gauges

A Bourdon gauge uses a coiled tube, which, as it expands due to pressure increase causes a

rotation of an arm connected to the tube. In 1849 the Bourdon tube pressure gauge was patented

inFrancebyEugene Bourdon.
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The pressure sensing element is a closed coiled tube connected to the chamber or pipe in which

pressure is to be sensed. As the gauge pressure increases the tube will tend to uncoil, while a

reduced gauge pressure will cause the tube to coil more tightly. This motion is transferred throughalinkage to agear train connected to an indicating needle. The needle is presented in front of a

card face inscribed with the pressure indications associated with particular needle deflections. In a

barometer, the Bourdon tube is sealed at both ends and the absolute pressure of the ambient

atmosphere is sensed. Differential Bourdon gauges use two Bourdon tubes and a mechanical

linkage that compares the readings.

In the following illustrations the transparent cover face of the pictured combination pressure and

vacuum gauge has been removed and the mechanism removed from the case. This particular

gauge is a combination vacuum and pressure gauge used for automotive diagnosis:

the left side of the face, used for measuringmanifold vacuum,is calibrated incentimetres of

mercury on its inner scale andinches of mercury on its outer scale.

the right portion of the face is used to measure fuel pump pressure and is calibrated in

fractions of 1 kgf/cm on its inner scale and pounds per square inch on its outer scale.

Diaphragm Gauges

A pile of pressure capsules with corrugated diaphragms in an aneroidbarograph.

A second type of aneroid gauge uses thedeflection of a flexiblemembrane that separates regions

of different pressure. The amount of deflection is repeatable for known pressures so the pressure

can be determined by using calibration. The deformation of a thin diaphragm is dependent on the

difference in pressure between its two faces. The reference face can be open to atmosphere to

measure gauge pressure, open to a second port to measure differential pressure, or can be sealed

against a vacuum or other fixed reference pressure to measure absolute pressure. The deformation

can be measured using mechanical, optical or capacitive techniques. Ceramic and metallic

diaphragms are used.

Useful range: above 10-2Torr (roughly 1Pa)

For absolute measurements, welded pressure capsules with diaphragms on either side are often

used.

Shape:

Flat

corrugated

flattened tube

capsule
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Bellows Gauges

In gauges intended to sense small pressures or pressure differences, or require that an absolutepressure be measured, the gear train and needle may be driven by an enclosed and sealed bellows

chamber, called an aneroid, which means "without liquid". (Early barometers used a column of

liquid such as water or the liquid metal mercury suspended by a vacuum.) This bellows

configuration is used in aneroid barometers (barometers with an indicating needle and dial card),

altimeters,altitude recordingbarographs,and the altitude telemetry instruments used in weather

balloonradiosondes.These devices use the sealed chamber as a reference pressure and are driven

by the external pressure. Other sensitive aircraft instruments such asair speed indicators and rate

of climb indicators (variometers)have connections both to the internal part of the aneroid chamber

and to an external enclosing chamber.
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unit v - metrology and instrumentation notes

Documents