Influence of Water on the Response of a Microwave Sensor for Ammonia Detection at Room Temperature

This article proposes a dual approach to characterize the influence of water on the response of a microwave gas sensor designed to detect ammonia at room temperature. First, experiments carried out at constant low relative humidity (RH) (0%, 15%, and 25%) demonstrate the sensor’s ability to respond qualitatively and quantitatively to different ammonia injections despite the presence of water. Interpretations of these experiments demonstrate the additivity of the microwave response, which is due on the one hand to the target (punctual variations in dielectric properties) and on the other to the water (latent variation, baseline). The second approach consists of carrying out experiments at high and variable RH (above 60%). Tests carried out under such conditions demonstrate that the microwave response is totally correlated and synchronized with hygrometric variations. Variations due to ammonia injections (a few ppm) are made invisible by variations in water concentration (several thousand ppm). Finally, it has been demonstrated that the initial resonance frequency of the sensor (after conditioning of the device but before target injection) is directly related to the water concentration present, and that variations in this initial resonance frequency are linear with RH.