Construction of gas flow standard facilities by means of positive pressure sonic nozzle

The sonic nozzle airflow standard facilities are applied widely throughout the world and can be divided into two categories, negative pressure and positive pressure methods. The latter is better than the former at calibrating flowmeters and analyzing the impact of pressure and density on performance of flowmeters. This paper discusses several key issues about establishing standard facilities by the positive pressure method. In order to ensure the stability of stagnation pressure and flow rate, it is vital important to choose rationally the parameters of stable pressure system which is jointly controlled by the air compressors, gasholders and self-operated pressure regulating valves. On the basis of the analysis of the heat transfer process, the water vapor in the compressed air is most likely to be saturated and cause dew to condense out of the compressor air. For this reason, the measures of filtration and drying should be included in the standard facilities. Critical flow function which changes with temperature and pressure must be calculated from ISO equation, instead of using a constant.