A High-Stability Quartz Crystal Resonator Humidity Sensor Based on Tuning Capacitor

In this paper, a quartz crystal resonator (QCR) with high quality factor and a humidity-sensitive capacitor were combined to fabricate a high-sensitivity and good-stability humidity sensor. Through the humidity sensor, the perturbation of the humidity-sensitive capacitor which was caused by different humidity levels could be transformed into the deviation of the resonant frequency of the QCR. The expression between the resonant frequency of the sensor and relative humidity was derived. The test curve of the resonant frequency responses of the sensor versus relative humidity changes was measured and showed a similar trend to the theoretical result. In addition, the influences of different sensitive materials on the sensor performance have also been researched. And the humidity sensors exhibited a high sensitivity (116.2 Hz/% RH) and a good stability with 0.038-Hz standard deviation of the output frequency in the range of 11.13% RH-97.3% RH. The properties of humidity hysteresis as well as response and recovery have also been investigated.