Performance test codes require primary mass-flow accuracies that in many applications require laboratory quality calibration of differential pressure meters. It is also true that many performance tests are conducted at Reynolds numbers and flows well above the laboratories' capacities, and sound extrapolation methods had to be developed. Statistical curve fits and regression analyses by themselves, absent fluid-dynamic foundations, are not valid procedures for extrapolation. The ASME PTC 19.5-2004 discharge coefficient equations reproduced in this paper for nozzles, orifices, and venturis are suitable for use whenever calibration data are to be applied in a flow measurement and/or extrapolated to higher Reynolds numbers as necessary. The equations may also be used for uncalibrated differential pressure meters by using nominal values. It is necessary to note that the metering runs must be manufactured with dimensions, tolerances, smoothness, etc., and installed in strict accordance with ASME PTC 19.5 for these equations to be valid. Note that for compressible flow, the value of the expansion factor term in the PTC 19.5 equation must be the one corresponding to the published PTC 19.5 equation.

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