Development and characterization of a volume flow measurement system for low-pressure gases

This paper presents the design and validation of a volume flow measurement system developed to measure volume flowrate of gas at pressures below 1 kPa. The system had to be developed because a suitable commercial transducer was not available for the unusual working environment, where outgassing and sublimation phenomena become relevant for many materials and the rarefied atmosphere changes dramatically its heat and mass exchange parameters. The development of a low-pressure compatible measurement system comes from the need of measuring the flow rate of a rotary vane pump working under an environmental absolute pressure in the range between 600 and 1000 Pa, mostly composed by CO2 i.e. the average condition of the Martian atmosphere at the surface. The pump will be a key component of the MicroMED particle analyzer, an instrument of the ExoMars 2020 ESA-Roscosmos mission payload. The measurement system is based on a control volume, made by a stainless-steel bellow, which has been designed accounting for the environmental requirements and the expected flow rate. The measurement system has been calibrated at ambient condition and tests in low-pressure have been performed to validate the measurement system and to derive the flow rate characteristics of a pump mock-up at different environmental pressures.