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Understanding Flow Meters


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.

  • Theoretical discharge coefficient (Cd): A calculated value of the discharge coefficient allows 0.3% uncertainty from a CFV from a low uncertainty throat diameter measurement. In Johnson and Wright, 2008, the Fluid Metrology Group used low uncertainty dimensional metrology to show that theoretical Cds agree with experimental Cds within 0.09% for CFVs with throat diameters > 5 mm.
  • Species effects: In What is the “Best” Transfer Standard for Gas Flow?, Wright plots the differences in a CFV discharge coefficient for different gases. The largest differences (2%) are due to vibrational relaxation in CO2 as explained in Johnson et al., 2006 and Johnson et al., 2000. Some remaining species effects have been improved by better calculations of the real gas critical flow function (see Wright and Johnson, 2009), and that property is now available in NIST’s Standard Reference Database 23 - REFPROP.
  • Transient flows: Rapidly changing and pulsatile flows can introduce significant flow errors in gaseous re-fueling and reciprocating compressor applications. The fluid Metrology Group has used CFVs with fast instrumentation (see Wright, 2010 Performance of critical flow venturis under transient conditions) to test coriolis meter response to transient flows.
  • Thermal effects: CFVs smaller than 10 mm exhibit sensitivity to the environmental temperature because of the difficulty in measuring the correct temperature of a gas flow and thermal boundary layer effects. The Fluid Metrology Group developed a theoretical explanation for the thermal effects and presented experimental results in Thermal Effects on Critical Flow Venturis, Wright et al. 2015.

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.


  • When anemometers are calibrated in NIST’s large, low-turbulence wind tunnel and then used in smaller wind tunnels, their responses to blockage and wall effects may lead to errors. This is a subject of ongoing research.

End Date:


Lead Organizational Unit:


John Wright
301-975-5937 Telephone