Wideband microwave multiport-based system for low gas concentration sensing and its application for acetone detection

Abstract

A microwave system dedicated to low gas concentrations sensing at room temperature is proposed. An optimized transmission line type sensor coated with a thin film, that changes electrical properties under target gas exposure is developed. The sensor’s response, namely its reflection coefficient is measured using a novel wideband multiport reflectometer. The instrument exhibits an advantageous measurement uncertainty, which can be adjusted to precisely measure small reflection coefficient changes over a wide bandwidth. Broadband data is used to extract the system’s response to gas defined as the reflection coefficient’s phase slope over frequency. The developed sensor covered with acetone-sensitive copper-oxide film together with a reflectometer operating over 1.5 GHz – 4.5 GHz band were incorporated in a gas measurement system to test its response to low acetone concentrations in the range of 0.5 ppm – 5 ppm. Due to high sensor’s sensitivity, optimum measurement conditions provided by the dedicated reflectometer and wideband analysis of the collected data, a robust and low-noise signal proportional to acetone concentration is achieved. The tested concentrations result in measured system’s response in the range of 0.02°/GHz – 0.29°/GHz @ 20 °C and 50 % of relative humidity derived with uncertainty of ±0.022°/GHz being 3 times lower than for a commercial instrument. Additionally, humidity dependence, stability, repeatability, reproducibility, and selectivity are investigated.