The Metering Research Facility (MRF) was commissioned in 1995/1996 at Southwest Research Institute for research on, and calibration of natural gas flow meters. A key commissioning activity was the calibration of critical flow Venturi (sonic) nozzles by a gravimetric proving process flowing nitrogen or natural gas at different pressures. This paper concerns the calibration of the four sonic nozzles installed in the MRF Low Pressure Loop (LPL). Recently, a new project prompted a review of the relations used to calculate sonic nozzle discharge coefficient in the LPL data acquisition computer code. New calibrations of the LPL sonic nozzles were performed flowing natural gas over a lower range of pressure than used in the original commissioning tests. The combination of new and old gravimetric calibration data are shown to agree well with correlations published by Arnberg and Ishibashi (2001) and by Ishibashi and Takamoto (2001) for laminar, transitional and turbulent boundary layer flow in critical flow Venturi nozzles.

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