Electrical Measuring Instruments - An Introduction

What are the Electrical Measuring Instruments?

The instruments which are used to measure electrical quantities are known as electrical measuring instruments. Measurement is a process of comparison between a standard and unknown and knowing the result of the unknown in terms of standard. The measuring quantity can be voltage, current, power and energy etc.

Classification of Measuring Instruments

They are divided into two parts

• Absolute instruments

• Secondary instruments

Absolute Instruments 

These instruments give the quantity to be measured in terms of instrument constant and its deflection. They do not require any comparison with any other standard instruments. Due to a smaller number of moving mechanical parts, these instruments are highly accurate. These instruments are rarely used because the measuring quantity varies with time.
Example- Tangent galvanometers, Rayleigh current balance absolute electrometers etc.

Secondary Instruments 

The deflection of these instruments gives the values of an electrical quantity to be measured directly. These instruments are required to be calibrated by comparison with either absolute instruments or secondary instruments. Which has already been calibrated already. Due to large numbers of moving mechanical parts, these instruments consume higher power in operation and are less accurate in the result.
Example - Ammeter, Voltmeter, Wattmeter, Energy meter.

Types of Secondary Instruments 

Figure 1- Classification of electrical  measuring instruments

Analog Instruments - Those instruments whose output varies continuously w.r.t. time, all the while maintaining a constant relationship with input.

Secondary Instruments - Those instruments whose output varies discretely w.r.t. time while maintaining a constant relationship with input.

Analog instruments are further classified on the basis at the end of a measurement.

Deflecting Instruments - Deflecting instruments are those which indicate the end of measurement with deflection of pointer way from zero position. A finite amount of energy is consumed to produce this deflection, these instruments are relatively less accurate.

Null Deflection Instruments are those which indicates their end of measurement with Zero or null deflection.  As no energy or power is consumed for zero position, they are highly accurate.

• Indicating Instruments

• Recording Instruments

• Integrating Instrument

Deflecting instruments are further classified on the basis of type output.

Indicating Instruments

These are the instruments which indicate the magnitude of an electrical quantity at the time when it is being measured. The indication is given by the pointer moving over a dial. 
Example- ordinary ammeter, voltmeter, wattmeter, frequency meter etc.

Recording Instruments

These are the instruments which record the variations in the electrical quantity over a definite period of time with the help of a pen and ink system.

Integrating Instrument

These instruments give the electrical output multiplied by time. Integrating instruments gives the sum of total electrical parameters consumed by a circuit over a period of time. 
Example - Ampere-hour meters and energy meters.

Author 

Rajeev Kumar is dedicated to teaching and research in the field of electrical and electronics engineering. He is always sharing valuable content of the engineering field on this website.

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