Accuracy and Precision of Measuring Instruments
About the accuracy and precision of
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 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
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.
More Instrument Classification