The aim of this investigation is to develop the minute flow measuring technology to calibrate sonic nozzles at the low flow rate and low pressure without the expensive primary standard system. With this calibrated sonic nozzles, calibration service of flow meters to industry will be available. In this experiment, eight sonic nozzles with the throat diameter of 0.2 mm were manufactured according to the specifications of ISO 9300, and then four out of eight nozzles, which have most close values of the rate of discharge coefficients for a pair of nozzles, were selected. At first, all the four nozzles for the stagnation pressure range of 200～600 kPa were calibrated in the standard system. Then, for the rest of the whole experiments, the step-down procedure was adopted to reduce the stagnation pressure range of the nozzle, 3～600 kPa. It was proven that the step-down procedure can be done over and over with small additional uncertainty.
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