Electrical Measurements [PDF]

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Electrical Measurements 



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Classification of Measuring Instruments



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Essentials of Measuring Instruments



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Classification of Measuring Instrum Basic classification of measuring instruments:



1. Mechanical instruments:- They are static and stable conditions. The disadva unable to respond rapidly to measuremen transient conditions.



2. Electrical instruments:- Electrical indicating the output of detectors are mechanical methods. The electrical depends upon a mechanical meter movem device.



3. Electronic instruments:- These instr fast response. For example a cathode (CRO) is capable to follow dynamic and t of the order of few nano seconds (10-9 se



Other classification of instruments:1. Absolute



instruments or Primary Inst instruments gives the magnitude of measurement in terms of physical constants o e.g. Tangent Galvanometer. These instrumen



comparison with any other standard instrume



• These instruments give the value of the elec terms of absolute quantities (or some co instruments and their • In this type of instruments no calibration or other instruments is • They are generally not used in laboratorie used in practice by electricians and engi mostly used as means of standard measur maintained lay national laboratories and simila



• SSome of the examples of absolute instrum * Tangent galvanometer * Raleigh current balance * Absolute electrometer



2. Secondary instruments:-These instru constructed that the quantity being measu determined by the output indicated by the in instruments are calibrated by comparison instrument or another secondary instrum already been calibrated against an absolute in



Working with absolute instruments for rout consuming since every time a measurement is lot of time to compute the magnitude of measurement. Therefore secondary instru commonly used.



• They are direct reading instruments. T



measured by these instruments can be the deflection of the • They are often calibrated by comp either some absolute instruments or w have already been calibrated.



• The deflections obtained with secon will be meaningless untill it is • These instruments are used in general purposes. • Some of the very widely used secon are: ammeters, voltmeter, wattmeter (watt-hour meter), ampere-hour meters Classification of Secondary Instruments:



(a) Classification based on the various ef current (or voltage) upon which their o They are:



• Magnetic effect: Used in amme watt-meters, integrating • Heating effect: Used in ammeter • Chemical effect: Used in dc amp • Electrostatic effect: Used • Electromagnetic induction effec ammeters, voltmeters, watt meter meters. Generally the magnetic electromagnetic induction effect ar construction of the commercial in of the instruments are also name above effect such as electros



induction instruments, etc. (b) Classification Operations



based



on



the



N



We have the following • Indicating instruments: Indicat indicate, generally the quantity to means of a pointer which moves on are ammeter, voltmeter, wattmeter



• Recording instruments: These ins continuously the variation of any e with respect to time. In princ indicating instruments but so a permanent continuous record of made on a chart or dial. The recor made by a pen on a graph paper whic dice or drum at a uniform speed. T quantity at any time (instant) may b traced chart. Any variation in the q is recorded by these instruments quantity like current, voltage, power be measured lay the indicating inst arranged to be recorded by a su mechanism.



• Integrating instruments: These ins the consumption of the total quanti energy etc., during a particular per is, these instruments totalize specified period of time. No indicat variation or the amount at a parti



available from them. Some widely instruments are: Ampere-hour me (kWh) meter, kilo (kVARh) meter. (c) Classification based on the Kind of be Measurand. Under this heading, we have: • Direct current (dc) instruments



• Alternating current (ac) instrumen



• Both direct current and alte instruments (dc/ac instruments).



(d) Classification based on the method us Under this category, we have:



• Direct measuring instruments: Th converts the energy of the me directly into energy that actuates and the value of the unknown quanti displayed or recorded directly. Th are most widely used in engineering they are simple and inexpensive. A in the measurement is shortest Ammeter, Voltmeter, Watt meter e



• Comparison instruments: These measure the unknown quantity by c standard. Examples are dc and potentiometers. They are used



accuracy of measurements is desire



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