Page 806 - Basic Electrical Engineering

be calibrated as proportional to the level of liquid in the tank since the pointer
will move on the resistance due to rise of liquid level.

11.4 STATIC CHARACTERISTICS OF INSTRUMENTS

Static characteristics of instruments are defined in terms of accuracy,
precision, sensitivity, resolution, etc. Normally, these values do not change

with time once the instrument is manufactured. The quality of measurement
depends on these characteristics. The cost of an instrument will increase

when we want to achieve higher values of these characteristics. The terms
used to express the static characteristics of a measuring instrument are
explained below.

11.4.1 Accuracy

You must be acquinted with deflecting-type instruments like ammeters and

voltmeters used in laboratories. Let us consider a voltmeter which can read a
maximum voltage of 100 V. Its accuracy is mentioned in terms of its full-
scale deflection. As for example, accuracy of ±1% will mean that for a

reading of 100 V, the actual value of voltage could be 100, ±1% of 100 i.e.,
either 101 V or 99 V.

Accuracy, therefore, tells us about the nearness of the measured value
(indicated value of the instrument) to the actual or true value of the quantity
being measured.

Now, suppose an instrument whose measuring range is 0–100 V with an
accuracy of say, 1 per cent has been selected to measure a low value of

voltage, say 10 V.
Since the error of the instrument is expressed in terms of its full-scale

deflection, the maximum error could be 1 V. For a 10 V measurement using
the same voltmeter, could give rise to a possible error of 1 V which is 10 per

cent of 10 V.
The accuracy of measurement is drastically reduced from 1 per cent to 10
per cent. It is therefore advisable not to use instruments of higher range to

measure low values. In your laboratory while doing experiments you must