Accuracy And Precision


Accuracy of the instrument may be defined as its ability to respond to a true value of a measured variable under reference conditions- In other words, it can  also be explained as (he closeness with which an instrument reading approaches the true value of the quantity being measured. Moreover, the accuracy of measurement means conformity to the truth. The accuracy of an instrument may be expressed in different ways, viz., in terms of the measured variable itself,  span of the instrument, upper-range value, per cent of scale length of actual output reading.


Oversll Accurscy For the instruments composed of separate physical units like primary, secondary, manipulation, etc, overall accuracy is expressed by  combining individual accuracies of different elements.

For pressure spring thermometer having accuracy of bulb-capillary system as ± 0 . 5% and accuracy of Bourdon pressure gauge as ±1 %, the overall accuracy can be expressed as

a. least accuracy is within ± (0.5 I ) , i.e, within ± ! . 5

b. root square accuracy is within



It is a measure of re prod liability o f the measurements, given a fixed value o f a quantity or the degree of exactness for which an instrument is designed or  intended to perform. In other words, precision is a measure of the degree of agreement within the group of measurements. It is expressed in terms of conformity of the instrument, which is nothing but maximum deviation of an instrument's actual calibration curve as compared to its specified characteristic  curve. In general, the distinction between the words "accuracy* and 'precision* is usually very vague. But as far