The Humidity Effect On The Discharge Coefficient Of Sonic Nozzles

The sonic nozzles are widely used in China as master meter due to its high-precision and low-cost characteristics, furthermore it is used to disseminate of the value of quantity for other types of gas flow meter. As the important parameter of the sonic nozzle, the discharge coefficient is determined by ideal and real mass flow going through the sonic nozzle. Although the ideal and real mass flow both influenced by humidity, the research of the humidity effect on the discharge coefficient of sonic nozzles had been focused on the ideal mass flow. Several humidity correction methods had been presented in this field, however, there are significant differences among methods.Considering the importance of sonic nozzles in the field of gas flow measurement, a suitable humidity correction method is required in order to eliminate the systematic error. In this paper, we improved the humidity control and measuring system based on the pVTt facility. For the experimental sonic nozzles, experiments were carried out in different humidity level. Finally, the discharge coefficient was corrected by four correction methods which were used. The results were achieved under working the humidity condition within (38-93) %, which showed that the most discrepancy is only 0.11% among four correction methods. Despite the apparent difference, four correction methods had excellent consistency with the ISO empirical formula.Based on the experimental results, a detailed analysis and comparison. was carried out among four correction methods, form the principle, calculation process, correction factor, influence factors, age of experimental parameters respectively, ISO method was superior to the other three methods.