A Thin Film Porous Alumina-Based Cross-Capacitive Humidity Sensor

The capacitive humidity sensor is normally fabricated by an interdigitated electrode (IDE) or parallel plate structure. In this paper, the principle of the cross-capacitance proposed by Thompson-Lampard theorem has been utilized for the first time to fabricate a precise and accurate relative humidity (% RH) sensor. A thin film of porous alumina (gamma-Al2O3) was deposited on the inner wall of the quartz tube for adsorbing humidity. The four symmetrical silver electrodes of the cross-capacitor are formed around the quartz tube by the screen printing technique. The sensor has been tested for different levels of humidity in the range of 0-90 % RH. The capacitive response of the fabricated sensor was very accurate (+/- 2 %RH), highly repeatable (+/- 0.01%), very low hysteresis (+/- 0.3%), and less drift. Thus, it can be used for making hygrometer for commercial applications. The main advantage of the cross-capacitor is the single-dimensional accuracy. The sensitivity of the sensor for humidity without any hydrophilic film was found to be less. However, by depositing the thin sensing film on the inner surface of the quartz tube, the sensitivity was considerably improved.