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QUESTIONS AND ANSWERS
1. STATIC AND DYNAMIC CHARACTERSTICS OF MEASUREMENT SYSTEM?WHEN IT IS USED MEASURE RAPIDLY VARYING QUANTITES WITH
REFERNCE TO :
(i) ACCURACY
(ii) PRECISION
(iii) RESOLUTION
(iv) THRESHOLD
(v) SENSITIVITY
A)Static Characteristics
Static characteristics refer to the characteristics of the system when the
input is either held constant or varying very slowly. The static
characteristics of measuring instruments describe the performance of
the instruments related to the steady-state input/output variables only.
The various static characteristics are destined for quantitative
description of the instruments perfections and they are well presented in
the manufacturer's manuals and data sheets.
Dynamic Characteristics
Dynamic characteristics is the relation between system input and output
when the measured quantity is varying rapidly. It is necessary to find
dynamic response characteristics input varies from instant to instant, so
dynamic I/p. Dynamic input may be Periodic, Transient, random.
The static and dynamic characterstics with reference to
accuracy, precision, resolution, threshold and sensitivity are:(i) ACCURACY
Accuracy of a measuring system is defined as the closeness of the
instrument output to the true value of the measured quantity. It is also
specified as the percentage deviation or inaccuracy of the measurement
from the true value. For example, if a chemical balance reads 1 g with an
error of 10 -2 g, the accuracy of the measurement would be specified as
1%.
The accuracy of the instruments can be specified in either of the
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following forms:
1. Percentage of true value =(measured value - true value /true
value)*100
2. Percentage of full - scale deflection =( measured value - true
value/maximum scale value)*100
(ii) PRECISION
Precision is defined as the ability of the instrument to reproduce acertain set of readings within a given accuracy. For example, if aparticular transducer is subjected to an accurately known input and ifthe repeated read outs of the instrument lie within say 1 %, then theprecision or alternatively the precision error of the instrument would bestated as 1%. Thus, a highly precise instrument is one that gives thesame output information, for a give input information when the readingis repeated a large number of times.
(iii) RESOLUTIONIt is defined as the smallest increment in the measured value that can bedetected with certainty by the instrument. In other words, it is the degreeof fineness with which a measurement can be made. The least count ofany instrument is taken as the resolution of the instrument. For example,a ruler with a least count of 1 mm may be used to measure to the nearest
0.5 mm by interpolation. Therefore, its resolution is considered as 0.5mm. A high resolution instrument is one that can detect smallestpossible variation in the input.
(iv) THRESHOLDIt is a particular case of resolution. It is defined as the minimum value ofinput below which no output can be detected. It is instructive to notethat resolution refers to the smallest measurable input above the zero
value. Both threshold and resolution can either be specified as absolute
quantities in terms of input units or as percentage of full scale deflection.Both threshold and resolution are not zero because of various factors likefriction between moving parts, play or looseness in joints (more correctlytermed as backlash), inertia of the moving parts, length of the scale,spacing of graduations, size of the pointer, parallax effect, etc.
(v) SENSITIVITYSensitivity (also termed as scale factor or gain) of the instrument isdetermined from the results of static calibration. This staticcharacteristic is defined as the ratio of the magnitude of response(output signal) to the magnitude of the quantity being measured (input
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signal). Sensitivity is represented by the slope of the input-output curveif the ordinates are represented in actual units.
2. PRESSURE MEASURING DEVICES USE ELASTIC MEMBERS FOR SENSING
PRESSURE AT THE PRIMARY STAGE? DESCRIBE DIFFERENT TYPES OF
ELASTIC MEMBERS WITH REFRENCE T0
(i) BOURDON TUBES
(ii) DIAPHRAGMS
(iii) BELLOWS
A) Most pressure measuring devices use elastic members for sensingpressure at the primary stage. These elastic members are of many typesand convert the pressure into mechanical displacement which islater converted into an electrical form using a secondary transducer.
(i) Bourdon TubesThese are designed in various forms like:
(i) C type(ii) Spiral
(iii) twisted tube(iv) helical
The Bourdon tubes are made out of an elliptically sectioned flattenedtube bent in such a way as to produce the above mentioned shapes.One end of the tube is sealed or closed and physically held. The otherend is open for the fluid to enter. When the fluid whose pressure is to bemeasured enters the tube, the tube tends to straighten out on account ofthe pressure. This causes the movement of the free end and thedisplacement of this end is amplified through mechanical linkages. The
amplified displacement of the free end is used to move a pointer over ascale calibrated in units of pressure. Bourdon tubes normally measure
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gauge pressure. The materials used for Bourdon tubes are brass,phosphor bronze, beryllium copper, and steel.
(ii) DIAPHRAGMSThe movement of a diaphragm is a convenient way of sensing lowPressures. A diaphragm is a circular disc of thin, springy metal firmlyfixed at its rim. The unknown pressure is applied to one side of thediaphragm and since the rim of the diaphragm is rigidly fixed there is adeflection of the diaphragm. The displacement of the centre of thediaphragm is directly proportional to the pressure and therefore can beused as a measure of pressure.The diaphragms are of two types:
(i) Flat(ii) Corrugated
(iii) BELLOWSThe bellows element consists of a cylindrical metal box with corrugated
walls of thin springy material like brass, phosphor bronze, or stainlesssteel. The thickness of walls is typically 0.1 mm. Bellows are used inapplications where the pressures involved are low. The pressure insidethe bellows tends to extend its length. This tendency is opposed by thespringiness of the metals, which tends to restore the bellows to its
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original size. Pressure on the outside of the bellows tends to reduce itslength and this tendency also, is opposed by the springiness of metal.
3. IN THE MEASUREMENTS SYSTEMS DESCRIBE VARIOUS TYPES OF
ERRORS IN PERFORMANCE PARAMETER DEPENDING ON THE TYPE OF
INSTRUMENTS AND NATURE OF APPLICATION OF INSTRUMENTS?
A) ERRORS IN PERFORMANCE PARAMETERS
The various static performance parameters of the instruments are
obtained by performing certain specified tests depending on the
type of instrument, the nature of the application, etc. Some salient static
performance parameters are periodically checked by means of a staticcalibration. This is accomplished by imposing constant values of 'known'
inputs and observing the resulting outputs.
RANDOM ERRORS SYSTEMATIC ERRORS
Types of Errors
Error is defined as the difference between the measured and the true
value (as per standard). The different types of errors can be broadly
classified as follows.
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Systematic or Cumulative Errors
Such errors are those that tend to have the same magnitude and sign for
a given set of conditions. Because the algebraic sign is the same, they
tend to accumulate and hence are known as cumulative errors. Since
such errors alter the instrument reading by a fixed magnitude and withsame sign from one reading to another, therefore, the error is also
commonly termed as instrument bias. These types of errors are caused
due to the following:
Instrument errors:
Certain errors are inherent in the instrument systems. These may be
caused due to poor design / construction of the instrument. Errors
in the divisions of graduated scales, inequality of the balance arms,
irregular springs tension, etc., cause such errors. Instrument errorscan be avoided by (i) selecting a suitable instrument for a given
application, (ii) applying suitable correction after determining the
amount of instrument error, and (iii) calibrating the instrument against
a suitable standard.
Environmental errors:
These types of errors are caused due to variation of conditions external
to the measuring device, including the conditions in the area
surrounding the instrument. Commonly occurring changes in
environmental conditions that may affect the instrument characteristics
are the effects of changes in temperature, barometric pressure, humidity,
wind forces, magnetic or electrostatic fields, etc.
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Loading errors:
Such errors are caused by the act of measurement on the physical system
being tested. Common examples of this type are: (i) introduction of
additional resistance in the circuit by the measuring millimetre whichmay alter the circuit current by significant amounted.
Accidental or Random Errors:
These errors are caused due to random variations in the parameter or
the system of measurement. Such errors vary in magnitude and may be
either positive or negative on the basis of chance alone. Since these
errors are in either direction, they tend to compensate one another.
Therefore, these errors are also called chance or compensating type of
errors. The following are some of the main contributing factors to
random error.
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4) EXAMPLES OF INSTRUMENTS TO ENNUMERATE THE DEVELOPMENT OF
SUCH DEVICES IN THE MEASUREMENTS OF VARIABLE AND QUANTIES?
A)The history of development of instruments encompasses three phases of
instruments, viz.
(i) mechanical instruments
These instruments are very reliable for static and stable conditions.
Major disadvantage is unable to respond rapidly to measurements of
dynamic and transient conditions. This is due to the fact that these
instruments have moving parts that are rigid, heavy and bulky and
consequently have a large mass. Another disadvantage of mechanical
instruments is that most of them are a potential source of noise and
cause noise pollution.
Ruler and scales
Callipers
Venire calliper
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Micrometre
Feeler gauge
(ii) Electrical instruments
Electrical methods of indicating the output of detectors are more rapid
than mechanical methods. Electrical system normally depends upon a
mechanical meter movement as indicating device. This mechanical
movement has some inertia and therefore these instruments have a
limited time (and hence, frequency) response.
Voltmeter
Ammeter
Ohm meter
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Wattmeter
Power-factor meter
(iii) electronic instrumentsSince in electronic devices, the only movement involved is that of
electrons, the response time is extremely small on account of very smallinertia of electrons. This is particularly important in the area of Bio-instrumentation since Bio-electric potentials are very weak i.e., lowerthan 1 mV.
Digital Millimetres
Capacitors
Function Generators
Transistors
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Diodes
5) ANSWER THE FOLLOWING QUESTIONS:
(i) GIVE THE CLASSIFICATION OF MEASUREMENTS AND EXPLAIN
EACH CLASS WITH EXAMPLE (DIRECT OR INDIRECT
MEASUREMENT)?
A) Measurement
Measurements provide us with a means of describing variousphenomena in quantitative terms. It has been quoted "whatever exists,exists in some amount". The determination of the amount ismeasurement.
The methods of measurement may be broadly classified into twoCategories:Direct MethodsThese methods, the unknown quantity (also called the measurand) isdirectly compared against a standard. The result is expressed as anumerical number and a unit. Direct methods are quite common for themeasurement of physical quantities like length, mass and time.Examples are:
(i) Dip Stick(ii) Resistance tapes(iii) Sight glass(iv) Floats(v) Ultrsonic
In-Direct MethodsIn engineering applications Measurement Systems are used. Thesemeasurement systems use indirect methods for measurement purposes.
A measurement system consists of a transducing element which convertsthe quantity to be measured into an analogous signal. The analogous
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signal is then processed by some intermediate means and is then fed tothe end devices which present the results of the measurement.Examples are:
(i) Hydrostatic head methods
(ii) Load cells(iii) Capacitance(iv) Conductivity
(ii) WHAT DO YOU MEAN BY IPTS (INTERNATIONAL PRACTICALTEMPERATURE SCALE)?
A) A temperature scale adopted by international agreement in 1968, and
revised in 1990, based on thermodynamic temperature and using
experimental values to define 16 fixed points. The lowest is the triplepoint of an equilibrium mixture of orthohydrogen and parahydrogen (-
259.34C) and the highest the freezing point of copper (1084.62C).
(iii) DIFFERENT TYPES OF TEMPERATURE SCALES?
A) Fahrenheit, Celsius, Kelvin and Rankin are the four most commonly
used temperature scales. The scales use degrees with ratios defined by
the boiling and freezing points of water and a value called absolute zero.
Fahrenheit ScaleThe Fahrenheit scale, named after physicist Daniel Gabriel Fahrenheit,
was used in most English-speaking countries until the 1970s, when mostof those countries switched to the Celsius scale. When writing atemperature on the Fahrenheit scale, the number value is generallyfollowed by a degree sign and the letter "F."This scale features a water
boiling point of 212 F and a water freezing point of 32 F. Absolute zerohas a value of minus 459.67 F. The only point on the Fahrenheit andCelsius temperature scales at which Fahrenheit and Celsius equal each
other is at minus 40 F and, therefore, minus 40 Converting atemperature from Fahrenheit to Celsius requires subtracting 32 from theFahrenheit degree number; then that number needs to be divided by 9/5or 1.8.
Celsius ScaleThe Celsius, or Centigrade, scale received its name from astronomer
Andrew Celsius. This scale was the standard in science even before itspost-1970s prominence. It is based on a water freezing point of 0 C and a
water boiling point of 100 C. The 100-degree difference between those
values explains the alternate name of Centigrade. The Celsius value forabsolute zero is minus 273.15. To convert from Celsius to Fahrenheit
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requires multiplying the Celsius degree value by 9/5 or 1.8 and thenadding 32.
Kelvin Scale
The Kelvin scale was named for the physicist William Thomson, BaronKelvin. The scale has degrees equivalent in size to the Celsius scale, butthe Kelvin scale has an absolute zero of 0 compared to Celsius' minus273.15. The standard degree unit of thermodynamic temperature, Kelvintemperatures generally are written without a degree symbol between thenumbers and the Water boils at 373.15 K and freezes at 273.15.Conversion from Celsius to Kelvin requires adding 273.15 to the Celsiusreading. To convert from Kelvin to Celsius merely requires subtracting273.15 from the Kelvin reading.
(iv) HOW TO MEASURE TEMPERATURE (NON ELECTRICAL, ELECTRICAL,
RADIATION)?
A)Temperature is measured by observing the effect that temperaturevariation causes on the measuring device. Temperature measurementmethods can be broadly classified as follows:
(i) NON-ELECTRICAL METHODSThe non-electrical methods of temperature measurement can be basedon anyone of the following principles:1. change in the physical state,2. change in the chemical properties, and3. change in the physical propertiesExample are:
BIMETALLIC THERMOMETER
LIQUID-IN-GLASS THERMOMETER
PRESSURE THERMOMETERS
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MERCURY-IN-GLASS THERMOMETER
(ii) ELECTRICAL METHODS
Electrical methods are in general preferred for the measurement oftemperature as they furnish a signal which can be easily detected,amplified or used for control purposes. There are two main electricalmethods used for measuring temperature. They are:1. Thermo-resistive type i.e., variable resistance transducers and2. Thermo-electric type i.e., emf generating transducersExamples are:
ELECTRICAL RESISTANCE THERMOMETERS
METALLIC RESISTANCE THERMOMETERS
THERMO-ELECTRIC SENSORS
(iii) RADIATION METHODSRadiation Thermometers (Pyrometers, if you will) are non-contacttemperature sensors that measure temperature from the amount ofthermal electromagnetic radiation received from a spot on the object ofmeasurement.
Examples are:
INFRARED THERMOMETERS
PYROMETERS