Thermal Polymer Membrane Sensor to Measure Simultaneously Flow Rate and Gas Concentration of Binary Gas Mixtures

Micro machined thermal sensors have many advantages, e.g. long-term stability or low power consumption. In this work, a thermal sensor is presented that is capable to measure the gas concentrations of a binary gas mixture under flow conditions and its flow rate simultaneously. The sensor design corresponds to the calorimetric layout. The multi parameter monitoring is achieved by the use of two time independent excitation modes – constant power and constant temperature. A flow independent, but gas sensitive region is obtained in power mode, due to the developed sensor design. In this flow range, the thermal conductivity is measured by analyzing the temperature of the downstream element. Gas concentration of the binary gas mixture is derived from the theoretical relationship to thermal conductivity. Afterwards, the flow rate is detected by evaluating the temperature difference between the down- and upstream temperature sensors in temperature mode. This measurement technique can be applied in anaerobic distinctions to analyze biogas, which consists mainly of methane and carbon dioxide. This application has been proven under laboratory conditions, where a flowing mixture of methane and carbon dioxide has been analyzed in terms of flow rate and gas concentration of the binary gas mixture.