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Title: Uncertainty In Theoretical Mass Flowrate Of Pure Gases Through Critical Flow Nozzles
Author: D. G. Stewart, J. T. R. Watson, A.M. Vaidya
Source: 1999 International Symposium on Fluid Flow Measurement
Year Published: 1999
Abstract: The critical flow nozzle is a very accurate and reliable flowmeter and as such is frequently used in laboratories to calibrate other flowmeters. The critical flow nozzle must initially be calibrated itself, usually against a gravimetric facility or some other primary standard, in order to obtain the coefficient of discharge. The uncertainty in Cd is also analysed. However, this uncertainty analysis does not take into account fully the thermodynamic uncertainty in the theoretical mass flowrate, most likely because it is not known. This paper rectifies this situation by developing a method to estimate the thermodynamic uncertainty in the theoretical mass flowrate of a gas through a critical flow nozzle. The method estimates this uncertainty by considering the uncertainties in density, speed of sound, enthalpy and entropy from the equation of state used to calculate the theoretical mass flowrate. Over the range 250-400 K and up to 15MPa the uncertainty in the theoretical mass flowrate for nitrogen, methane, argon, and carbon dioxide lies in the range 0.06/0 to 0.4/0.

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