CLASSIFICATION OF MEASURING INSTRUMENTS

Electrical measuring instruments may be classified into two groups:
(a) Absolute (or primary) instruments.
(b) Secondary instruments.

Absolute Instruments

• These instruments give the value of the electrical quantity in terms of absolute quantities (or
some constants) of the instruments and their deflections.
• In this type of instruments no calibration or comparison with other instruments is necessary.
• They are generally not used in laboratories and are seldom used in practice by electricians and
engineers. They are mostly used as means of standard measurements and are maintained lay
national laboratories and similar institutions.
• Some of the examples of absolute instruments are:
* Tangent galvanometer
* Raleigh current balance
* Absolute electrometer.


 Secondary Instruments

• They are direct reading instruments. The quantity to be measured by these instruments can be
determined from the deflection of the instruments.
• They are often calibrated by comparing them with either some absolute instruments or with
those which have already been calibrated.
• The deflections obtained with secondary instruments will be meaningless untill it is not calibrated.
• These instruments are used in general for all laboratory purposes.
• Some of the very widely used secondary instruments are: ammeters, voltmeter, wattmeter,
energy meter (watt-hour meter), ampere-hour meters etc.
Classification of Secondary Instruments

(a) Classification based on the various effects of electric current (or voltage) upon which their
operation depend. They are:

• Magnetic effect: Used in ammeters, voltmeters, watt-meters, integrating meters etc.
• Heating effect: Used in ammeters and voltmeters.
• Chemical effect: Used in dc ampere hour meters.
• Electrostatic effect: Used in voltmeters.
• Electromagnetic induction effect: Used in ac ammeters, voltmeters, watt meters and
integrating meters.
Generally the magnetic effect and the electromagnetic induction effect are utilized for the construction
of the commercial instruments. Some of the instruments are also named based on the above effect such
as electrostatic voltmeter, induction instruments, etc.

(b) Classification based on the Nature of their Operations

We have the following instruments.
• 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.
• Recording instruments: These instruments record continuously 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 (instant) may be read
from the traced chart. Any variation in the quantity with time is recorded by these instruments.
Any electrical quantity like current, voltage, power etc., (which may be measured lay the
indicating instruments) may be arranged to be recorded by a suitable recording mechanism.
• Integrating instruments: These instruments record 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 at a particular instant are available from them. Some widely used integrating
instruments are: Ampere-hour meter: kilowatthour (kWh) meter, kilovolt-ampere-hour
(kVARh) meter.

(c) Classification based on the Kind of Current that can be Measurand.

Under this heading, we have:
• Direct current (dc) instruments
• Alternating current (ac) instruments
• Both direct current and alternating current instruments (dc/ac instruments).

(d) Classification based on the method used.

Under this category, we have:
• Direct measuring instruments: These instruments converts the energy of the measured
quantity directly into energy that actuates the instrument and the value of the unknown
quantity is measured or displayed or recorded directly. These instruments are most widely
used in engineering practice because they are simple and inexpensive. Also, time involved in
the measurement is shortest. Examples are Ammeter, Voltmeter, Watt meter etc.
• Comparison instruments: These instruments measure the unknown quantity by comparison
with a standard. Examples are dc and ac bridges and potentiometers. They are used when
a higher accuracy of measurements is desired.

(e) Classification based on the Accuracy Class of Instruments.

Groups of error in the measured quantity for instruments of various class of accuracy are
listed below:

Class of accuracy   0.2    0.5     1.0      1.5      2.5     5

Limit of error %   ± 0.2  ± 0.5  ± 1.0  ± 1.5   ± 2.5  ± 5