SURFACE-ACOUSTIC-WAVE (SAW) DEW-POINT SENSOR - APPLICATION TO DEW-POINT HYGROMETRY

Among the various hygrometric techniques, the dew point method is cons idered as the most accurate. Its principles are based on cooling a sur face 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 di ffusion) and an additional temperature sensor to measure the dew point temperature. In order to increase the accuracy of this type of hygrom eter, surface acoustic wave (SAW) devices are very useful because of t heir dual ability to detect the dew deposition and to measure the temp erature with great accuracy. In this paper, Rayleigh wave properties w ere investigated in order to study the influence of dew deposition. Th e experimental device was cooled by means of a Peltier element until d ew deposition. Because of polarization of the Rayleigh wave, the dew d eposition induces a substantial attenuation of the wave amplitude and a shift in the wave velocity (mass-loading effect). In order to combin e the influence of the temperature and of the dew deposition, a thermo couple has been deposited on an LST-cut quartz plate. The frequency ve rsus temperature response was used for the dew point detection. Prelim inary results confirm the great SAW sensitivity to dew deposition. Mor eover, because of the thermal sensitivity, the accuracy of the relativ e humidity measurement may be considerably improved.