Developed at NASA's Jet Propulsion Laboratory (JPL), the JPL Standing Acoustic Wave (SAW) hygrometer uses a custom-made 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 crystal designed 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.