Classifications of Measuring Instruments

CLASSIFICATIONS OF MEASURING INSTRUMENTS

Measuring Instruments are classified into two broad categories:

1. Primary Instruments (or) Absolute Instruments

2. Secondary instruments

1. Primary Instruments

Primary Instrument (or) Absolute Instruments gives the value of electrical quantity to be measured, in terms of physical constants of the instruments and of its deflection. No necessity for comparing with another instrument. Examples are,

1.Tangent Galvanometer.

2. Rayleigh current balance meter.

2. Secondary Instruments

Secondary Instruments are those instruments which give the measurement of the different quantities of electricity by the deflection of the instrument, the deflection of which is calibrated either with an absolute instrument or with an instrument already calibrated.

The secondary instruments can be classified according to

1. The various effects of electric current (or) voltage upon which their operation depend.

2. The nature of reading given by the instrument

The classification of instruments according to the various effects utilised by the instrument are as follows:

(a) Magnetic effect (for ammeters, voltmeters).

(b) Electro‒dynamic effect (for ammeters, voltmeters but particularly for wattmeters).

(c) Electro‒magnetic effect (for ammeters, voltmeters, wattmeters and watt‒hour meters).

(d) Thermal effect (for ammeters and voltmeters).

(e) Chemical effect (for DC ampere ‒ hour. meters).

(f) Electrostatic effect (for voltmeters only).

According to the nature of reading given by the instruments, are classified as,

(i) Indicating Instruments: These instruments which measure values of the quantities of electricity by the deflection of the needle at the instant the readings are observed. The ammeters, voltmeters, wattmeters and power factor meters are indicating instruments in common use.

(ii) Integrating Instruments: Measure the total amount either the quantity of electricity or of electrical energy supplied to a circuit over a period of time. Energy meter is the example for Integrating Instruments.

(iii) Recording Instruments: While the above two types of instruments, cannot have any permanent reading on the dial or scale because of their nature; This recording instruments make a permanent record of electrical readings (Quantities) on a graph paper fitted within the instrument and latter moves in accordance with time. Thus at the end of a certain interval of time say a week, a month, quarter or an year, the paper may be removed from the instrument and the variations of quantity occurred at different instances during a certain interval of time can be studied. Examples: Recording pyrometers, Recording MD meters, Recording kVA meters.