Accurate flow measurements are needed for test fluids, temperatures, pressures, flows, and installation conditions that differ from those used during NIST's calibrations; therefore, we strive to predict the performance of flow meters under a wide range of conditions. To do so, we study the operating principles of a wide range of flow meters. We concentrate on those meters that we use as 1) working standards (meters calibrated by NIST and used by manufacturers or secondary laboratories as reference flow meters), and 2) transfer standards (meters for inter-laboratory comparisons and proficiency tests).
Critical flow venturis (CFVs) are widely used for gas flows of 1 L/min or more, if a significant pressure drop is acceptable.
Laminar flow meters are often used for flows < 1 L/min.
Turbine flow meters are widely used for liquid flow comparisons.
Ultrasonic flow meters are used in liquids, natural gas, and smoke stacks.
Pitot tubes are used as air speed sensors and to conduct velocity profiles in liquid flows and smoke stacks.