Calibration of laminar flow meters for process gases

Abstract

We calibrated three models of commercially-manufactured, laminar flow meters (LFMs) at four pressures (100, 200, 300, and 400 kPa) with five gases (N2, Ar, He, CO2, and SF6) over a 10:1 flow range using NIST’s primary flow standards as references. We combined three items: (1) the calibration data acquired with N2, (2) gas-property data from NIST’s database REFPROP 9.0, and (3) a physical model for each LFM that accounts for the effects of viscosity, entrance and exit effects, gas expansion, gas non-ideality, and slip. This combination predicted the calibrations for the flow of Ar, He, CO2, and SF6 with a maximum error of 0.8% for Reynolds numbers Re<500. Under these conditions, the present LFM model allows prediction of calibration results for other gases with approximately three times more accuracy than conventional approaches that plot the flow coefficient as a function of the viscosity coefficient or Re. We represented the calibration data for SF6 in the range 500 <Re<2000 by adding an empirical quadratic function to the model for one of the LFMs.