Humidity effect on the calibration of discharge coefficients of critical flow Venturi nozzles in a pVTt facility

• The discharge coefficient measured by pVTt with humid air could reduce 1.17%.
• Vapor condensation and adsorption on the surface added humidity effects.
• An empirical model was developed to quantify the humidity effect.

The available research on the humidity effect when calibrating the discharge coefficients of critical flow Venturi nozzles has mostly treated the humid air as a mixture of vapor and dry air. In this paper, atmospheric air with normal humidity was used to calibrate the discharge coefficients of critical flow Venturi nozzles in a pVTt facility. The calibration results were found to depend on the initial pressure in the collection tank, which reduced the discharge coefficient by 1.17%. The results suggest that vapor condensation in the diffuser section of the CFVN and the inventory volume or collection tank and adsorption of water molecules on the surface of the collection tank are additional humidity effects in a pVTt facility. An empirical model was developed for the humidity effect with the critical pressure for each facility determined from the experimental results. With the humidity correction, the NIM and PTB results compare very well.