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

Abstract

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 CO 2 , Ar, He and CH 4 in N 2 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 inphase 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.

A Robust Miniaturized Gas Sensor for H2 and CO2 Detection Based on the 3ω Method

“…In the case of CO 2 , the studies of Cai et al [24], and Kliche et al [10], which are both based on time-domain readout circuits, achieve a resolution limit of 94 ppm, and 2000 ppm, respectively. The work of Kommandur et al [32] is also based on the 3ω-method and obtains a resolution limit of 4970 ppm, which is comparable to the resolution limit of our study. Thus, this technique is not yet suitable for applications, where it is necessary to resolve lower CO 2 concentrations, e.g., for air-quality monitoring [54].…”

“…It appears that particularly dynamic or pulsed methods seem to be advantageous for the detection of CO 2 . This observation is based on the fact that Cai et al [24] and Kliche et al [10] were able to resolve very low concentrations using dynamic methods, whereas the resolution limit for 3ω-based studies [32,33] was significantly higher. In the case of H 2 , a (quasi)-static operation, such as low-frequency 3ω-method or steady-state operation, might be better.…”

“…A sinusoidal current is applied to the sensor element and the thermal conductivity of the surrounding gas mixture can be derived from the amplitude of the resulting temperature oscillations. In the context of a gas sensor, this principle, known as the 3ω-method, is still subject of ongoing research with just a limited number of publications so far [30][31][32][33][34]; especially for the detection of hydrogen, there are no scientific studies yet.…”

“…The 3ω-method has the advantage that it does not depend on the thermal characteristics of the sensor and can be operated at lower temperatures [32]. In combination with a miniaturization of the sensor which has a positive effect on the power consumption of the system this technique shows a good sensitivity and resolution limit [33].…”

Frequency-domain hot-wire sensor and 3D model for thermal conductivity measurements of reactive and corrosive materials at high temperatures

“…Such wirelike 3ω heater geometries have been used to study the wire itself, 44 gases and liquids, [45][46][47][48][49][50][51][52] as well as hybrid geometries such as metal-film coated glass wires. 53,54 Early 3ω hot-wire measurements, however, have been considered unreliable by transient hot-wire standards, 55 but this largely stems from over-simplified heat-transfer analysis for wire geometries in early frequency-domain hot-wire experiments, which did not take into account radial and axial heat conduction or the thermal mass of the heating wire.…”