Accuracy and Precision of Measuring Instruments

About the accuracy and precision of measuring instruments

Accuracy

Accuracy is the extent to which a reading might be wrong, and is often quoted as a percentage of the full-scale reading of an instrument.


If, for example, a pressure gauge of range 0-10 bar has a quoted inaccuracy of 1.0% f.s. (1% of full-scale reading), then the maximum error to be expected in any reading is 0.1 bar. This means that when the instrument is reading 1.0 bar, the possible error is 10% of this value. For this reason, it is an important system design rule that instruments are chosen such that their range is appropriate to the spread of values being measured, in order that the best possible accuracy is maintained in instrument readings. Thus, if we were measuring pressures with expected values between 0 and 1 bar, we would not use an instrument with a range of 0-10 bar.

 

Precision

Precision is a term used to describes an instrument's degree of freedom from random errors. If a large number of readings are taken of the same quantity by a high-precision instrument, then the spread of readings will be very small.

 

Precision is often, though incorrectly, confused with accuracy. High precision does not imply anything about measurement accuracy. A high-precision instrument may have a low accuracy. Low-accuracy measurements from a high-precision instrument are normally caused by a bias in the measurements, which is removable by recalibration.


The terms repeatability and reproducibility mean about the same but are applied in different contexts. Repeatability describes the closeness of output readings when the same input is applied repetitively over a short period of time, with the same measurement conditions, same instrument and observer, same location and same conditions of use maintained throughout. Reproducibility describes the closeness of output readings for the same input when there are changes in the method of measurement, observer, measuring instrument, location, conditions of use and time of measurement. Both terms thus describe the spread of output readings for the same input. This spread is referred to as repeatability if the measurement conditions are constant and as reproducibility if the measurement conditions vary.


The degree of repeatability or reproducibility in measurements from an instrument is an alternative way of expressing its precision.


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