Quartz capillary flow meter for gases

Abstract

This article describes an accurate meter for gas flow rates below 1000 μmol/s. (1 μmol/s≅1.3 cm3/min at 0 °C and 1 atm). Two gauges measure the input and output pressures as the gas flows through a flow element consisting of either a single quartz capillary or an array of parallel quartz capillaries. A hydrodynamic model determines the molar flow rate from the two pressures, the gas temperature, and the gas properties. Measurements with nitrogen demonstrated that three flow elements could span the flow range from 0.1 to 1000 μmol/s with a standard uncertainty of less than 0.03%. Additional measurements with helium, argon, propane, and sulfur hexafluoride demonstrated that, by using the capillary radius determined for nitrogen, the model could predict the flow rates of gases with widely varying properties.