Accurate gas properties are needed to take full advantage of the low uncertainties provided by NISTs 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 users flow measurements will have errors. Since January 2009, calibrations conducted by NISTs Fluid Metrology Group use the NIST-supported database REFPROP version 8.1 to reduce calibration data. Prior to 2009, NISTs 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 NISTs 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 < 0.012 %. In anticipation of future calibrations at higher pressures, we tabulate estimates of property uncertainties for pressures up to 70 MPa for the gases listed above as well as H2 and CH4 by comparing REFPROP to primary data sources..
Conference Dates: October 13-15, 2010
Conference Location: Taipei, -1
Conference Title: 15th Flow Measurement Conference (FLOMEKO)
Pub Type: Conferences
density, gas flow, properties, viscosity