Metal-coated glass microfiber for concentration detection in gas mixtures using the 3-Omega excitation method

• 3-Omega technique using metal-coated fibers can determine concentration of gas mixtures.
• Gas concentrations can be determined without individual calibration of the sensors.
• The sensitivity was shown to be as low as 10 ppm with a power input of ∼20 mW.
• Offers better compromise between sensitivity and power input than existing sensors.

The 3-Omega measurement technique was applied to a fabricated metal-coated glass fiber 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 mass flow controllers precisely controlled concentrations. The metal-coated fiber was fabricated by depositing a thin layer of gold (∼150 nm) onto a glass fiber, using a custom designed deposition lathe installed in a standard sputtering system. 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 voltage signal are presented for different gas mixtures and are related directly to their concentrations. Using this gas sensing technique, we have demonstrated the uncertainty in concentration (i.e., sensitivity) to be better than 50 ppm, and as low at 10 ppm for some gases. The dependence of the different 3-Omega signals on the thermophysical properties of the system is briefly described. The low power, high sensitivity nature of the sensor is also demonstrated as the metal-coated fiber sensor consumes ∼10% of the power consumed by conventional thermal conductivity detectors (TCDs), and unlike those, it operates at near room temperatures.