Classification of Measuring Instruments.
Electrical measuring instruments may be classified as direct measur ing instruments (such as ammeters, voltmeters, watt meters) and comparison instruments.
Direct measuring instruments convert the energy of the unknown quantity directly into the energy that deflects the moving elements of the instrument, the value of unknown quantity being measured by reading the resulting deflection.
Direct measuring instruments are most widely used in engineering practice since they are the most simple and index pensive ones and enable the measurements to be made in the shortest possible time. Comparison instruments are used in cases when a higher accuracy of measurement is needed.
Electrical measuring instruments may also be classified according to the kind of the quantity being measured (such as ammeters, voltmeters, watt meters, energy meters, ampere-hour meters, frequency meters, ohm meters, phase meters, or power factor meters), kind of current for which they are designed (such as de or ac), the principle of operation of the moving system such as magnetic, thermal, chemical, electrostatic and electro magnetic induction instruments), their accuracy class, protection against the influence of external fields, service conditions, ability against mechanical effects, fields of application, method of installation and mounting or shape and size of the instrument ases and the degree of enclosure they provide.
According of BIS 1248 (1968), accuracy classes are 0.05, 1, 0.2, 0.5, 1, 1.5, 2.5 and 5. These figures are indication of erce tage error-instrument of accuracy class 0.2 means that the stringent can have an error up to +0.2% of full-scale deflection.
The electrical measuring instruments may, in very broad Ense, be divided into two classes namely absolute and second y instruments. Absolute instruments give the quantity under easureme to in terms of instrument constant and its deflection d do not need any comparison with any other standard instrument.
Secondary instrument gives deflection in proportion to the magnitude of electrical quantity under measurement di- recruitment. Absolute instruments are mostly used in standard labora tories and similar institutions as standardized instruments and secondary instruments are widely used in practice.S
Secondary instruments are further classified into three groups namely indicating instruments (such as ordinary ammeters, voltmeters, watt meters, frequency meters, power factor meters etc.), recording instruments (such as graphic recorders, galvanometer recorders, null balance recorders, etc.) and integrating instruments (such as ampere-hour meters and energy meters).
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