Deviation
The set-point is compared to the measured value to give file deviation or error signal. The term absolute deviation is used when the deviation is just quoted as the difference between the measured value and the set value, e.g. a temperature control system might operate between 0~ and 30~ with an absolute deviation of 3~ This deviation is often quoted as a fractional or percentage deviation, this being the absolute deviation as a fraction or percentage of the span. Thus, a temperature control system operating between 0~ and 30~ with an error of 3~ has a percentage deviation of (3/30) x 100 - 10%. When there is no deviation then the percentage deviation is 0% and when the deviation is the maximum permitted by the span it is 100%.