When specifying a flowmeter, it is important to know the accuracy. The accuracy of a flow meter can be stated one of two ways; as a percentage of full scale (FS) or as a percentage of reading (RD, also referred to as percentage of rate). It is important to understand the meaning of both so that your flow meter performs as expected and with the desired accuracy.
Percentage of Full Scale (FS) Accuracy:
A flow meter that has an accuracy expressed in FS has a fixed error band across the flow range of the meter. For example, a flow meter with a 1% FS accuracy that has a range of 10GPM-100GPM has an error band +/- 1% of 100GPM, which is equal to +/- 1 GPM. Therefore, since the accuracy of the flow meter is +/- 1 GPM throughout the flow range, this type of meter is most accurate at full scale, 100GPM, and becomes less accurate as the flow decreases. When the meter is measuring flow at 50GPM or 50% FS, the actual flow will be within +/- 1 GPM, or anywhere from 49GPM51GPM. When you do the math, this works out to be +/- 2% accuracy at 50GPM, 1/50 = 2%. At the bottom end of the flow range, or 10GPM, the accuracy will be +/- 10%, 1/10=10%.