INRIM primary standard for microgas-flow measurements with reference to atmospheric pressure

Primary standard flowmeters are developed for the calibration of leak devices used in many applications in which is necessary to detect and quantify gas leakage from a material, a component or a system. At INRIM, a primary standard was designed and realized for the measurement of molar gas flow from 4 1010 mol/s to 2 107 mol/s with reference to atmospheric pressure. It is based on the constant-pressure–variable-volume method and is able to work with any tracer gas. The paper describes the primary standard, the mathematical model adopted for the estimate of the measurand and the uncertainty evaluation. The recent developments of the system, performed to improve the accuracy of the standard, are presented. The new layout of the flowmeter has allowed a study of the temperature effects on the gas flow measurements in the range between 15 C and 30 C.