Classification of Electrical Instruments
Primary standards
Primary standards instruments give the magnitude of the 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. The primary standards which represent the elementary units are independently calibrated by absolute measurements at each of the national laboratories. One of the primary functions of the primary standards is the verifications and calibration of secondary standards.
Secondary standards
The secondary standards are the basic reference standards used in industrial measurement laboratories. They are sent periodically to the national standards laboratories for calibration and comparison against primary standards. The secondary standards are sent back to the industry by the National Laboratories with a certification regarding their measured values in terms of primary standards.
Classification of Instruments
Absolute instruments
Absolute instruments are those which give the value of the quantity to be measured, in terms of the constants of the instrument and their deflection only. No previous calibration or comparison is mandatory in their case. The example of such an instrument is tangent galvanometer, which gives the value of current, in terms of the tangent of deflection produced by the current, the radius and number of turns of wire used and the level component of earth’s field.
Secondary instruments
Secondary instruments are those, in which the value of electrical quantity to be measured can be determined from the diversion of the instruments, only when they have been pre-calibrated by comparison with an absolute instrument. Without calibration, the deflection of such instruments is meaningless. It is the secondary instruments, which are most generally used in everyday work; the use of the absolute instruments being simply confined within laboratories, as standardizing instruments.
Electrical measuring instruments may be classified according to their functions as;
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.
Integrating instruments:- These instruments information 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 of a particular expression is available from them.
Recording instruments:- These instruments record constant 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 may be read from the traced chart. Any variation in the quantity of time is recorded by these instruments.
Mechanical instruments:- They are very dependable for static and strong conditions. The disadvantage is they are unable to respond rapidly to the measurement of dynamic and transient conditions.
Electronic instruments:- These instruments have the very quick response. For example, a cathode ray oscilloscope (CRO) is capable to follow dynamic and transient changes of the order of few nanoseconds (10-9 sec).
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