value would be equal to the true value. The accuracy of the system can be defined
as the closeness of the measured value to the true value. A perfectly accurate system
is a theoretical ideal and the accuracy of a real system is quantified using measurement
system error E, where
E = measured value − true value
E = system output − system input
Thus if the measured value of the flow rate of gas in a pipe is 11.0 m3/h and the
true value is 11.2 m3/h, then the error E = −0.2 m3/h. If the measured value of the
rotational speed of an engine is 3140 rpm and the true value is 3133 rpm, then
E = +7 rpm. Error is the main performance indicator for a measurement system. The
procedures and equipment used to establish the true value of the measured variable
will be explained in Chapter 2.