Step-down procedure of sonic nozzle calibration at low Reynolds numbers

The purpose of this investigation is to develop flow measuring technology for miniscule flow-rates, in order to calibrate sonic nozzles at lower pressures than the existing pressure ranges, without resorting to an expensive primary standard system. Once these sonic nozzles are calibrated successfully, calibration services for flow meters in industry for lower pressure ranges than the atmospheric pressure will be readily available. For this experiment, eight sonic nozzles (throat diameter of 0.2 mm) were manufactured according to the specifications of ISO 9300. Then four nozzles out of these were selected. The criterion for such a selection was the smallest standard deviation among the others, when the (Cd⋅A) ratios for various combinations of two nozzles were calculated. Regarding the experiment: First, all the four nozzles were calibrated for the pressure range 200-600 kPa by means of the primary standard system in KRISS (Korea Research Institute of Standards and Science). Then the step-down procedure was taken in order to reduce the stagnation pressure down to 3 kPa, so that the discharge coefficients for the pressure could be obtained in the range 3-200 kPa. Through this study, it was proven that the step-down procedure could be repeated as many times as desired with only small additional uncertainty.