Real-Time Gas-Compensated Thermal Flow Sensor

In this paper, a thermal flow sensor is presented that is compensated real-time for changes in gas composition. The sensor comprises a thermal flow meter and a velocity-independent gas properties meter. The maximum error of the gas properties meter due to velocity fluctuations is less than 1 % which enables the measurement of thermal conductivity and volumetric heat capacity under dynamic flow conditions. These two parameters are obtained using a single sensing wire with two different measurement techniques, DC excitation and the $\mathbf{3}\omega$ method. Various gases, including pure gases and binary gas mixtures (CO2, Ar, air, He, Ar-air, and He-air), are applied, and the flow rate is corrected for each gas simultaneously using the measured physical parameters. The obtained maximum error between the measured and applied flow rate is 8.8% for CO2 and less than 5 % for the other gases.