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Michael Hoenk
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Office Telephone: (818) 354-1881
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Brief Instrument Description of the
JPL Standing Acoustic Wave (SAW) Hygrometer


Developed at the Jet Propulsion Laboratory (JPL), this hygrometer uses a custom made surface acoustic wave (SAW) device as a sensitive detector of condensation in lieu of the more traditional chilled mirror and light beam approach. The SAW device is a quartz crystaldesigned to support  high-frequency acoustic oscillations. Because these oscillations are quite sensitive to surface effects, condensation produces a measurable shift in the resonance frequency of the SAW device. A two-stage thermo-electric cooler electronically heats or cools the SAW device, while a platinum resistor (thermistor) is used to monitor the temperature. The SAW hygrometer measures dewpoint by establishing equilibrium between evaporation and condensation on the surface of the SAW device. Using a SAW device as a fast, high-sensitivity moisture sensor, a feedback controller measures the condensation on the sensor surface and responds by heating or cooling the sensor to maintain equilibrium. The equilibrium temperature under feedback control is a measure of dewpoint or frostpoint, depending on the phase of the condensed moisture.


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