PSE-Coated Interdigital Resonator for Selective Detection of Ammonia Gas Sensor

This paper presents, for the first time, a microwave gas sensor utilizing Poly (styrene-co-maleic acid) partial isobutyl/methyl mixed ester (PSE), which is a sensing material, as a coating on an interdigital resonator. The proposed sensor is based on a near-field transmission-line technique to detect the changing of the dielectric properties of the sensing material. The sensor was designed based on a DiClad 880 substrate with an interdigital resonator, operating at 4 GHz, on the middle of an ungrounded coplanar waveguide transmission line structure with rejection characteristic by coating the PSE solution as a sensed-gas material. The measured transmission coefficient,
$S_{\mathrm {21}}$
, was changed depending on the dielectric properties of the PSE change due to various concentrations of gas targeted in a range of 50 to 1,000 ppm (50 ppm, 200 ppm, 500 ppm, 1,000 ppm) under a static system in a closed chamber at room temperature. The extracted results of differential transmission coefficient change,
$\Delta S_{\mathrm {21}}$
, from four samples of ammonia concentration can be separated impressively. To confirm the selectivity performance of ammonia gas, three interfering gases, i.e., acetone, ethanol and methanol, were investigated. The measured results show that the proposed sensor can discriminate ammonia gas from three interfering gases over the entire concentration range in only three minutes. Moreover, it has many advantages, such as low cost, real-time monitoring and room temperature operating. The PSE-coated sensor described in this paper is suitable for a high-performance selectivity in ammonia gas sensing applications both in industrial processes and in agricultural products.