A microbridge heater for low power gas sensing based on the 3-Omega technique

The 3-Omega measurement technique was applied to a microbridge heater for the purpose of low power gas sensing. The sensor performance was evaluated for mixtures of CO2, Ar, He and CH4 in N2 in an isothermal chamber where concentrations were precisely controlled by mass flow controllers. A custom 3-Omega conditioning circuit controls the AC heating current and detection of the 3-Omega voltage signal. The amplitude and phase lag, and the in-phase and out-of-phase components of the 3-Omega signal are presented for each case and are related directly to gas concentrations. Advantageously, the phase lag can determine the gas concentration without calibration of an individual gas sensor. Using this technique and our current microbridge thermal conductivity detector (TCD), it is possible to resolve gas concentrations down to 0.07%. The frequency behavior of the 3-Omega signal as it pertains to the thermal environment and sensor sensitivity is discussed. The 3-Omega technique combined with the microbridge is capable of operating with power consumptions that are <10% of the power consumed by conventional Wheatstone bridge measurements at room temperature.