CHAPTER 3 - MEASUREMENT ACCURACY

3. Capability Terms

The term linearity usually means the maximum deviation in tracking a linearly varying quantity, such as measuring head, and is generally expressed as percent of full scale. Discrimination is the number of decimals to which the measuring system can be read. Repeatability is the ability to reproduce the same reading for the same quantities. Sensitivity is the ratio of the change of measuring head to the corresponding change of discharge. Range is fully defined by the lowest and highest value that the device can measure without damage and comply with a specified accuracy. The upper and lower range bounds may be the result of mechanical limitations, such as friction at the lower end of the range and possible overdriving damage at the higher end of the range. Range can be designated in other ways: (1) as a simple difference between maximum discharge (Qmax) and minimum discharge (Qmin), (2) as the ratio (Qmax/Qmin), called rangeability, and (3) as a ratio expressed as 1:(Qmin/Qmax). Neither the difference nor the ratios fully define range without knowledge of either the minimum or maximum discharge.

Additional terms are related more to dynamic variability and might be important when continuous records are needed or if the measurements are being sensed for automatic control of canals and irrigation. Hysteresis is the maximum difference between measurement readings of a quantity established by the same mechanical set point when set from a value above and reset from a value below. Hysteresis can continually get worse as wear of parts increases friction or as linkage freedom increases. Response has several definitions in the instrumentation and measurement fields. For water measurement, one definition for response is the smallest change that can be sensed and displayed as a significant measurement. Lag is the time difference of an output reading when tracking a continuously changing quantity. Rise time is often expressed in the form of the time constant, defined as the time for an output of the secondary element to achieve 63 percent of a step change of the input quantity of the primary element.