A study of kinematic viscosity approach with air as a gas medium for turbine flowmeter calibration

Calibrating a gas flowmeter requires a suitable working medium that represent its use in the real operating condition. However, factors like installation conditions, costs, and safety considerations may necessitate to utilize a different medium for calibration. The accuracy of measuring fluid flow by a flowmeter is greatly influenced by the kinematic viscosity property of the gas medium. In this particular study, a turbine flowmeter for compressed natural gas (CNG) application was calibrated using air as a substitute with an approach that simulated the kinematic viscosity property of CNG. Here, the pressure and temperature of the air were calculated to achieve the same kinematic viscosity value as CNG during the calibration process, which was accomplished by adjusting the setting of a regulating instrument in the calibration installation. Furthermore, argon (Ar) was also used for comparison with air, with a modification in working pressure to attain similar kinematic viscosity properties for both gases. The calculation results showed that the working pressure setting for the air used was 4.5 bars, while for argon was 3.38 bars to achieve similar kinematic viscosity. By utilizing air instead of CNG, a minimum and maximum volume flowrate difference of −0.0242 m3/h and 0.0016 m3/h, respectively, was observed, with an absolute average of 0.0086 m3/h or 0.8 %. This study reveals the effectiveness of the kinematic viscosity approach in calibrating a turbine flowmeter under working pressure of up to 4.5 barg. This process results in uncertainty measurement of under 1 % when calibrating gas flowmeters, using air as the medium.