Surface acoustic wave (SAW) dew point sensor: application to dew point hygrometry

Abstract

Among the various hygrometric techniques, the dew point method is considered as the most accurate. Its principles are based on cooling a surface until water vapour condensation. Then, the measurement of the dew point temperature allows the relative humidity to be determined. This classical method consists of using an optical dew detection (light diffusion) and an additional temperature sensor to measure the dew point temperature. In order to increase the accuracy of this type of hygrometer, surface acoustic wave (SAW) devices are very useful because of their dual ability to detect the dew deposition and to measure the temperature with great accuracy. In this paper, Rayleigh wave properties were investigated in order to study the influence of dew deposition. The experimental device was cooled by means of a Peltier element until dew deposition. Because of polarization of the Rayleigh wave, the dew position induces a substantial attenuation of the wave amplitude and a shift in the wave velocity (mass-loading effect). In order to combine the influence of the temperature and of the dew deposition, a thermocouple has been deposited on an LST-cut quartz plate. The frequency versus temperature response was used for the dew point detection. Preliminary results confirm the great SAW sensitivity to dew deposition. Moreover, because of the thermal sensitivity, the accuracy of the relative humidity measurement may be considerably improved.