Accurate Measurements of Process Gas Flow with Laminar Flow Meters
Thiago Cobu, Robert F. Berg, John D. Wright, Michael R. Moldover
We calibrated three models of commercially-manufactured, laminar flow meters (LFMs) with nitrogen at four pressures (100 kPa, 200 kPa, 300 kPa, and 400 kPa) over a 10:1 flow range using NISTs primary flow standards and a physical model. Without additional calibration, each LFM was used to measure the flow of three process gases (Ar, He, and CO2) over the same pressure and flow ranges with a maximum error of only 0.5 %. The calibration and flow measurements used the gas-property data from NISTs database REFPROP 8.0 and a physical model for each meter that accounts for the viscous pressure drop, compressibility and non-ideal gas behavior, slip flow effects, kinetic energy effects, gas expansion effects, and thermal effects. The three LFMs have design differences that illustrate the dependence of the model corrections on LFM construction and the gas used. The LFM model improves results for the conditions tested by a factor of 4 over the flow coefficient and viscosity coefficient plots commonly used to present LFM calibration data.