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### Classification of Measuring Instruments

 Basic classification of measuring instruments: 1.     Mechanical instruments:- They are very reliable for static and stable conditions. The disadvantage is they are unable to respond rapidly to measurement of dynamic and transient conditions. 2.    Electrical instruments:- Electrical methods of indicating the output of detectors are more rapid than mechanical methods. The electrical system normally depends upon a mechanical meter movement as indicating device. 3.    Electronic instruments:- These instruments have very fast response. For example a cathode ray oscilloscope (CRO) is capable to follow dynamic and transient changes of the order of few nano seconds (10-9 sec). Other classification of instruments:-1.  Absolute instruments or Primary Instruments:- These instruments gives the magnitude of quantity under measurement in terms of physical constants of the instrument e.g. Tangent Galvanometer. These instruments do not require comparison with any other standard instrument• These instruments give the value of the electrical quantity in terms of absolute quantities (or some constants) of the instruments and their deflections.• In this type of instruments no calibration or comparison with other instruments is necessary.• They are generally not used in laboratories and are seldom used in practice by electricians and engineers. They are mostly used as means of standard measurements and are maintained lay national laboratories and similar institutions.• SSome of the examples of absolute instruments are:* Tangent galvanometer* Raleigh current balance* Absolute electrometer 2.  Secondary instruments:-These instruments are so constructed that the quantity being measured can only be determined by the output indicated by the instrument. These instruments are calibrated by comparison with an absolute instrument or another secondary instrument, which has already been calibrated against an absolute instrument. Working with absolute instruments for routine work is time consuming since every time a measurement is made, it takes a lot of time to compute the magnitude of quantity under measurement. Therefore secondary instruments are most commonly used. • They are direct reading instruments. The quantity to be measured by these instruments can be determined from the deflection of the instruments.• They are often calibrated by comparing them with either some absolute instruments or with those which have already been calibrated.• The deflections obtained with secondary instruments will be meaningless untill it is not calibrated.• These instruments are used in general for all laboratory purposes.• Some of the very widely used secondary instruments are: ammeters, voltmeter, wattmeter, energy meter (watt-hour meter), ampere-hour meters etc.Classification of Secondary Instruments:(a) Classification based on the various effects of electric current (or voltage) upon which their operation depend. They are:• Magnetic effect: Used in ammeters, voltmeters, watt-meters, integrating meters etc.• Heating effect: Used in ammeters and voltmeters.• Chemical effect: Used in dc ampere hour meters.• Electrostatic effect: Used in voltmeters.• Electromagnetic induction effect: Used in ac ammeters, voltmeters, watt meters and integrating meters.         Generally the magnetic effect and the electromagnetic induction effect are utilized for the construction of the commercial instruments. Some of the instruments are also named based on the above effect such as electrostatic voltmeter, induction instruments, etc.(b) Classification based on the Nature of their Operations We have the following instruments.• Indicating instruments: Indicating instruments indicate, generally the quantity to be measured by means of a pointer which moves on a scale. Examples are ammeter, voltmeter, wattmeter etc.• Recording instruments: These instruments record continuously the variation of any electrical quantity with respect to time. In principle, these are indicating instruments but so arranged that a permanent continuous record of the indication is made on a chart or dial. The recording is generally made by a pen on a graph paper which is rotated on a dice or drum at a uniform speed. The amount of the quantity at any time (instant) may be read from the traced chart. Any variation in the quantity with time is recorded by these instruments. Any electrical quantity like current, voltage, power etc., (which may be measured lay the indicating instruments) may be arranged to be recorded by a suitable recording mechanism.• Integrating instruments: These instruments record the consumption of the total quantity of electricity, energy etc., during a particular period of time. That is, these instruments totalize events over a specified period of time. No indication of the rate or variation or the amount at a particular instant are available from them. Some widely used integrating instruments are: Ampere-hour meter: kilowatthour (kWh) meter, kilovolt-ampere-hour(kVARh) meter.(c) Classification based on the Kind of Current that can be Measurand.                Under this heading, we have:• Direct current (dc) instruments• Alternating current (ac) instruments• Both direct current and alternating current instruments (dc/ac instruments).(d) Classification based on the method used.              Under this category, we have:• Direct measuring instruments: These instruments converts the energy of the measured quantity directly into energy that actuates the instrument and the value of the unknown quantity is measured or displayed or recorded directly. These instruments are most widely used in engineering practice because they are simple and inexpensive. Also, time involved in the measurement is shortest. Examples are Ammeter, Voltmeter, Watt meter etc.• Comparison instruments: These instruments measure the unknown quantity by comparison with a standard. Examples are dc and ac bridges and potentiometers. They are used when a higher accuracy of measurements is desired.