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Nov 22, 2015
CONCEPTS AND ISSUES IN MEASUREMENT & INSTRUMENTATION
GENERALIZED CONFIGURATION AND FUNCTIONAL DESCRIPTIONS OF MEASUREMENT SYSTEMSLecture 4Instructor : Dr Alivelu M ParimiOUTLINEFunctional elements of an instrumentCLASSIFICATION OF INSTRUMENTS Active and passive transducersAnalog and digital modes of operationNull and deflection methodsInput output configuration of measurement systemMethods of correction of modifying and interfering inputs2 CLASSIFICATION OF INSTRUMENTS Active and passive transducers3Passive transducer A component whose output energy is supplied entirely or almost entirely by its input signal is commonly called a passive transducer.
Active transducerAn active transducer requires an auxiliary source of power which supplies a major part of the output power while the input signal supplies only an insignificant portion. Normally, the output magnitude of the active transducer is higher than that of the passive type.CLASSIFICATION OF INSTRUMENTS Analog and digital modes of operationIn analog measuring instruments, any change of the input results in the corresponding change of the reading on scale.
Analog devices process signals, that can assume any value across a continuous range and produce results that are also in continuous form.
Analog instruments generally cost less and are easy to maintain and repair.
4Analog and digital modes of operationDigital describes any system in which changes are in discrete steps. Digital meters give value of physical variable in numbers.
A well-known example of analog vs. digital is that of clocks: analog being the type with pointers that slowly rotate around a circular scale, and digital being the type with decimal number displays.5
Analog and digital modes of operation
6Analog and digital modes of operationAdvantages of Digital instrumentsReading directly in decimal numbersHuman errors like parallax & approximations in reading, especially in the cramped portion are eliminatedLesser power requirementsTolerant of spurious noiseCan be interfaced easily with digital devices for interfacing, recording for further analysis No moving parts, so wear and tear is eliminatedPortable, small in size, low costHigher input impedance so loading effect is minimized
7CLASSIFICATION OF INSTRUMENTS Null and deflection methodsIn a deflection type device the measured quantity produces some physical effect that engenders a similar but opposing effect in some part of the instrument. The opposing effect is closely related to some variable (usually a mechanical displacement or deflection) that can be directly observed by human senses. The opposing effect increases until a balance is achieved, at which point deflection is measured and the value of the measured quantity is inferred from this.
8Null and deflection methodsIn opposite to deflection type of instruments, the null or zero type electrical measuring instruments tend to maintain the position of pointer stationary. They maintain the position of the pointer stationary by producing opposing effect. Thus for the operation of null type instruments following steps are required:Value of opposite effect should be known in order to calculate the value of unknown quantity.Detector shows accurately the balance and the unbalance condition accurately.The detector should also have the means for restoring force.
Advantages and disadvantages of deflection and nulltype of measuring instruments:Deflection type of instruments is less accurate than the null type of instruments. It is because, in the null deflecting instruments the opposing effect is calibrated with the high degree of accuracy while the calibration of the deflection type instruments depends on the value of instrument constant hence usually not having high degree of accuracy.
Null point type instruments are more sensitive than the Deflection type instruments.
Deflection type instruments are more suitable under dynamic conditions than null type of instruments as the intrinsic responses of the null type instruments are slower than deflection type instruments.1000(b)1 mm 1 mmReference99 mm 10-500(c)0 mm 1 mm100 mm 10-5 Inaccuracy:Example A: (a) deflection, (b) difference, and (c) null measurements(a)Null method: linearity is not important;sensitivity and zero drift are important.0 1 mm100 mm 0.1 mm100 mm Inaccuracy: 0.1 mm Uncertainty:1 1 mmReferenceReferenceNull and deflection methodsNull and deflection methods12
Figure depicts the application of strain gage to measure the strain caused by load/pressure/displacement.
The gage subjected to load is the active gage and the identical gage not subjected to load is dummy gage. One arm of the Wheatstone bridge is fixed and other is variable.
When there is no load, the bridge is designed to be in balance condition, meaning the voltage across the central terminal is zero. When the gauge experiences a strain due to applied load/pressure, there is a change in the resistance, making the bridge unbalanced and causing change in detector voltage or current, whose measurement is related to the active gage resistance.
This deflection could be related to the load which caused change in resistance; hence it is Deflection mode of operation.
If upon change in resistance of active gage, variable resistance is adjusted so as to make the bridge balanced, then this becomes Null mode of operationStrain Gauge Bridge Circuit CLASSIFICATION OF INSTRUMENTS Contacting v/s Non-contactingA contacting type of instrument is one that is kept in the measuring medium itself. clinical thermometer, tachometer, thermocouple, mouse
Non-contacting or proximity type instruments are those that measure the desired input (measurand) without coming in close contact with the measuring medium. An optical pyrometer monitors the temperature of a blast furnace from a distanceUltrasonic/microwave based instruments for level measurementIn comparison to the contacting type instruments, non-contacting type instruments are more accurate and efficient. For the contacting type instruments, maintenance costs are high as they wear out due to friction.
CLASSIFICATION OF INSTRUMENTSManually Operated v/s Automatic
Any instrument which requires the services of human operators is a manual type of instrument. The instrument becomes automatic if the manual operation is replaced by an auxiliary device incorporated in the instrument.Intelligent v/s Dumb
In Intelligent instruments, processing of measured data to refine the data for presentation/storage, self correction, and further connection to web is carried by the instrument itself, while in Dumb instruments measurement is only made of variable and the observer must process the data.