What are the Measuring Instruments? And Classification of Measuring Instruments

What are the Measuring Instruments? And Classification of Measuring Instruments

Definition

The instruments used for all electrical measurements are called measuring instruments. 

What are the Measuring Instruments? And Classification of Measuring Instruments

They include ammeters, voltmeters, wattmeters, energy meters etc.

The various electrical instruments may
mainly classified into two groups

1. Absolute instruments

2. Secondary instruments

1.Absolute instruments

Those are the instruments which indicate the quantity to be measured in terms of the constants of the instrument and in order to find out the quantity in the practical units it is necessary to multiply such deflections with an instrument constant.

No previous calibration or comparison is necessary in this case.

The most common absolute instrument is tangent galvanometer which gives the measured current in terms of tangent of the
deflected angle, the radius and the number of turns of the galvanometer.

Such instruments are rarely used such as in  laboratories as a standardizing instruments.

2.Secondary instruments

Those are instruments in which the value of electrical quantity to be measured can be determined from the deflection of instrument only when they have been precalibrated by comparison with an absolute instrument.

The deflection of the instrument gives directly
the quantity to be measured.

These instruments are most generally used in everyday work.

Secondary instruments can be further classified as follows:

1) Indicating instruments

2) Recording instruments

3) Integrating instruments

1)Indicating instruments

Those are the instruments which indicate the instantaneous value of the electrical quantity being measured at the time at which it is being measured.

Their indications are given by pointers moving over calibrated scales.

Examples: Ammeters, voltmeters and wattmeters are the indicating type measuring instruments.

2)Recording instruments

those are the instruments which give a continuous record of the variations of an electrical quantity over a selected period of time.

The pointer in these types of instruments is an
infed pen which leaves a trace on a paper put over a moving drum.

3)Integrating instruments

those are the instruments which measure the total quantity of electricity delivered in a particular time.

Examples: Ampere-hour and watt-hour meters are the examples of this type of measuring instruments.

Electrical measuring instruments can also be classified as follows:

According to the quantity being measured

1.Ammeter

Ammeter is use for measuring the magnitude of current.

2.Voltmeters

Voltmeter is use for measuring voltages.

3.Ohmmeters and resistance bridges

Ohmmeters and resistance bridges are use for measuring resistance

4.Wattmeters

Wattmeters are use for power measurements.

5.Watt-hour meters

Watt-hour meters are use for energy measurements.

6.Frequency meters

Frequency meters are use for frequency measurements.

7.Power factor meters

Power factor meters are use for power-factor measurements.

According to the kind of current.

Instruments are classified into as follows according to the kind of current

✓ D.C. METERS

✓ A.C. METERS

✓ A.C. / D.C. METERS

According to the principle of operation
Instruments are grouped into as shown in below

✓ Moving coil

✓ Moving iron

✓ Electrodynamic

✓ Induction

✓ Hot-wire

✓ Thermo-electric

✓ Rectifier types

According to the type of indication

Instruments can be classified as follow

✓ Indicating type

✓ Recording type

According to application

✓ Switch board

✓ Portable.

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