IMPROVED DEW-POINT MEASUREMENTS BASED ON A SAW SENSOR

Optical dew point hygrometers have several drawbacks including high co st, frequent mirror contamination, instability under continuous use, a nd inability to detect the frost-point transition. In this work a nove l hybrid sensor, and a surface acoustic wave (SAW) oscillator were use d to compare optical and SAW dew point measurement techniques in the a reas of condensation sensitivity, surface contamination effects, frost point transition behavior and dew point measurement resolution. The S AW technique was found to better control condensation density than the optical technique, allowing for a resolution of 0.03 degrees C versus 0.2 degrees C for a commercial optical dew point hygrometer. The SAW technique was also found to be much less susceptible than the optical technique to surface contamination. The frost point transition was fou nd to cause significant measurement instability when controlling conde nsation density with SAW velocity or optical reflection voltage while SAW amplitude provided stable measurements. A Teflon AF coating provid ed accurate SAW dew point measurements at extremely low condensation d ensities, which may allow for improved measurements at very low relati ve humidities.