Measurement of helium concentration in air by using a microheater on a polyimide substrate

Accurate gas concentration measurement is critical in achieving carbon neutrality and ensuring the safe utilization of hydrogen as a clean energy source. This study evaluates a 3-omega sensor incorporating a microheater deposited on a polyimide substrate for quantifying hydrogen concentration in air. Because hydrogen’s thermal conductivity is seven times that of air, the hydrogen concentration in an air–hydrogen mixture can be inferred by measuring the mixture’s effective thermal conductivity using the 3-omega method. Helium, whose thermal conductivity is approximately 83% of hydrogen, was used as a surrogate gas. The thermal conductivity of the gas mixture was determined by fitting the temperature oscillation amplitude obtained from the experiments to theoretical model. A comprehensive error analysis was performed to quantify the measurement uncertainty. The measurement uncertainty of helium measurements was extrapolated to the scenario in which hydrogen is mixed with air, enabling estimation of the expected measurement uncertainty as a function of hydrogen concentration. Results showed that the averaged measurement uncertainty for helium and hydrogen concentrations were 9.7% and 8.3%, respectively. These findings suggest design optimizations, such as reducing the substrate thickness or implementing a suspended microheater structure, as potential strategies to enhance measurement sensitivity for high thermal conductivity gas, such as hydrogen, in air.