Instrumentation and Accuracy
It is tempting to ask the question, "How accurate is this flowmeter?" The proper question is, "How accurate do you want it to be?" Sonic Nozzles, Venturis, Flow Nozzles, and Orifice Plates are known as Primary Flow Elements. A Primary Flow Element or Primary Flowmeter is any device which produces a signal that has a defined relationship to the fluid flow in accordance with known physical laws. It does not involve any type of translation or conversion to generate an observable signal. The differential pressure generated by a Venturi Flowmeter has an exact relationship with the flow. The trick is determining this relationship. This relationship can be estimated theoretically with mathematical equations or measured physically with calibration. The problem is not the accuracy of the differential pressure generated by a Venturi Flowmeter, but the accuracy in measuring this value and the accuracy of the calibration data. Any measurement of a physical value, pressure for example, involves accuracy (uncertainty). The measurement of the differential pressure is only as accurate as the transducer or pressure gauge used to measure it. So, if you want a very accurate flowmeter, use very accurate transducers, have the flowmeter physically calibrated, use proper installation configurations, and correct data reduction routines. The calibration report generated by physical calibration is used as a correction factor when calculating the flow using the measured values of pressure and temperature.