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Properties for Accurate Gas Flow Measurements



John D. Wright


Accurate gas properties are needed to take full advantage of the low uncertainties provided by NIST’s Gas Flow Calibration Services. If a flowmeter user and NIST use different values for these properties (molecular mass, compressibility, density, viscosity, and critical flow factor), the user’s flow measurements will have errors. Since January 2009, calibrations conducted by NIST’s Fluid Metrology Group use the NIST-supported database REFPROP version 8.1 to reduce calibration data. Prior to 2009, NIST’s calibration data were reduced using REFPROP 7.0. Flowmeter users who recalibrate their meters at NIST and ignore the January 2009 change of database may erroneously conclude that either their meters or NIST’s standards are not stable. The newer database, REFPROP v. 8.1, calculates properties for mixtures that include up to 2 % water vapor. By measuring the dew point temperature of air used during our calibrations and by incorporating the REFPROP v. 8.1 dynamic link library (REFPROP.dll) into our Labview and Excel data acquisition and reduction programs, NIST reduced the uncertainty of air flow calibrations from 0.05 % to 0.025 %. NIST now reduces calibration data for critical flow venturis using the “real” critical flow factor generated by REFPROP v. 8.1 instead of approximate “ideal” critical flow factors. This change improved the mutual consistency of critical flow venturi calibrations performed in N2 and air from > 0.05 % to
Conference Dates
October 13-15, 2010
Conference Location
Conference Title
15th Flow Measurement Conference (FLOMEKO)


density, gas flow, properties, viscosity


Wright, J. (2010), Properties for Accurate Gas Flow Measurements, 15th Flow Measurement Conference (FLOMEKO), Taipei, -1, [online], (Accessed May 22, 2024)


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Created October 15, 2010, Updated February 19, 2017