Page 807 - Basic Electrical Engineering

select instruments such that their range is appropriate to the range of values
being measured. That is to say, to measure 5 V you should not use a
voltmeter of range 0–100 V. Instead you should use a voltmeter of lower

range, say 0–10 V.

11.4.2 Precision

Precision is often confused with accuracy. Suppose with a voltmeter of 1 per
cent accuracy we take a number of readings of a particular voltage. If the
instrument is a high-precision one, the recorded values will not differ much.

Thus, for a high-precision instrument the spread of the number of readings
taken at a point of time while measuring a particular value will be very

narrow. Precision, therefore, means the degree of agreement of several
readings taken for the same value. High precision does not guarantee

anything about the accuracy of measurement. Although several readings
taken by a precision instrument may be very close to each other, like 1.1110,

1.1108, 1.1109, 1.1111, etc., the readings may have low accuracy, i.e., they
may vary significantly from the true value which could be, say 1.2. Precision
and accuracy of instruments are dependent on many design factors like

calibration and graduations of scale, the design of sharp-edged pointers, zero
adjustment, reduction of error due to parallax, etc. Precision is a measure of

reproducibility and repeatability. Reproducibility indicates how close the
output readings are for the same input where there are some changes in the
method of measurement and the person making the measurement also

changes. Repeatability indicates closeness of readings for the same input with
no changed conditions of measurement.

The degree of repeatability or reproducibility in measurement is also a way
of expressing the precision of an instrument. To further illustrate the

difference between accuracy and precision, let us assume that true value of
voltage to be measured is 100 V. The voltmeter readings are taken five times.

If the readings are, say 98.01 V, 98.05 V, 98.03 V, 98.01 V, 98.02 V, we can
say that the instrument is a precision instrument but not an accurate one as
there is considerable error in measurement, which is nearly2 per cent.