Humidity sensors based on QCM (quartz crystal microbalance) have been developed using an anodized alumina thin-film as a sensing material. An aluminum thin-film is deposited on a silver-coated quartz crystal by electron beam deposition and is anodized in an acid solution. The resultant alumina film is porous. The resonant frequency gradually decreases from 5 to 94%RH and steeply changes near 100%RH. This result shows that the sensor can be used as a dew-point sensor as well as a humidity sensor. The response times to the humidity changes from 27 to 59%RH and from 59 to 27%RH are less than 5 s. The sensor shows reversible response to some gases such as ethanol, toluene and acetone vapors in addition to airborne particles such as pollen.
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