The choking pressure ratio (CPR) for a Critical Flow Venturi (CFV) is defined as the ratio of the maximum permissible exit pressure to the inlet pressure that can exist across the venturi and still maintain sonic velocity at the throat. Experimental data is presented for a series of small critical flow venturis with throat diameters from 0.79 mm (0.031") to 3.18 mm (0.125") and with various diffuser geometries. Test data was obtained over an inlet pressure range from sub-atmospheric 41 kPa (6.0 psia) to 965 kPa (140 psia). American National Standard ASME/ANSI MFC-7M-1987 entitled "Measurement of Gas Flow by Means of Critical Flow Venturi Nozzles" gives the design parameters for CFV's, and predicts the choking pressure ratio for throat Reynolds numbers exceeding 200,000. For lower Reynolds numbers, the data presented in this paper will indicate where more stringent design parameters must be followed to insure accurate measurement.
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