Peeling the Onion of Meter Accuracy Two Steps to Evaluating Flow Meter Data

It is a natural desire and has been past practice to evaluate a meter's accuracy by reviewing its output – a measured rate of flow (Q). Most users test a meter's Q reading merely by comparing it to the Q from other meters or primary devices, much the same way drivers compare speedometers by driving next to each other and comparing outputs. The readings in such comparisons will never agree exactly, just as two watches will not. Experience shows that a traditional Q-to-Q comparison seldom results in a clear understanding of source of any meter inaccuracy.The technique presented in this paper offers an easy process for evaluation of meter accuracy without a second flow measurement device. The first step in evaluating accuracy of any instrument is to evaluate its measurable components, not its output. For a speedometer, measurable components are distance and time; for an open channel flow meter, they are depth and velocity. A determination of precision and bias is then made for each measurable component. This concept of evaluating a meter's precision and bias has been incorporated into the EPA Environmental Technology Verification (ETV) program in the flow meter verification protocol.