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R. Schnitzer, M. Gröschl, L. Spassov, K. Paskalev, E. Benes:
"Temperature sensitive quartz crystals for high precision dew point hygrometry";
in: "Proc. 2nd Congress of Alps-Adria Acoustics Association and 1st Congress of Acoustical Society of Croatia", Acoustical Society of Croatia, Zagreb/Croatia, 2005, ISBN: 953-95097-0-x, S. 248 - 255.

In several technical applications, e. g. lyophilization of food, humidity needs to be controlled. There are
numerous humidity sensor principles on the market, but they exhibit usually lack of absolute accuracy. One of the most
accurate humidity measurement possibilities is based on the so-called dew point principle. In this paper a humidity
sensor based on a bulk acoustic wave (BAW) quartz crystal resonator is investigated. The environmental air
temperature is accurately determined by the measuring of fundamental resonance frequency of the thermo-sensitive
quartz crystal. Then the thermo-sensitive quartz crystal plate is cooled down by a Peltier element until the dew point is
reached. The dew point temperature again is determined by measuring the fundamental resonance frequency of the
thermo-sensitive quartz crystal. The reaching of the dew point, that causes the condensation of a thin water layer on the
outer surface of the quartz crystal, is detected by the measurement of the dramatic increase of the crystal's resonance
damping factor d and a respective decrease of the quality factor Q. (d = 1/Q). The algorithm, how to find and to follow
the resonant frequency will be discussed in detail.
Key words: temperature measurement, dew point, hygrometer, admittance measurement, piezoelectric sensors

http://aleph.ub.tuwien.ac.at/F?base=tuw01&func=find-c&ccl_term=AC05938030