Capillary flow meter for calibrating spinning rotor gauges

Abstract

This article describes a capillary flow meter whose maximum flow rate of 0.2μmol∕s (∼0.2cm3∕min at ambient conditions) covers the range that is useful for calibrating spinning rotor gauges. Knowing the input pressure, output pressure, and temperature of the capillary yields the gas flow rate with a relative standard uncertainty as small as 0.04%. The flow meter, which requires no moving parts aside from valves, comprises a ballast tank, a coil of quartz capillary with an inner diameter of 0.1mm, and a commercial instrumentation package. Measurements near 0.1μmol∕s showed agreement with an independent primary flow meter to within 0.2%, comparable to the combined relative standard uncertainty of 0.11%. Additional measurements showed that operating the capillary flow meter with an exit pressure less than 1kPa allowed the flow to stabilize within minutes. However, the small exit pressure caused an unexpected enhancement of the slip correction in the hydrodynamic model of the capillary.