A microwave resonance dew-point hygrometer

Abstract

We report the first measurements of a quasi-spherical microwave resonator used as a dew-point
\nhygrometer. In conventional dew-point hygrometers, the condensation of water from
\nhumid gas flowing over a mirror is detected optically, and the mirror surface is then
\ntemperature-controlled to yield a stable condensed layer. In our experiments we flowed moist
\nair from a humidity generator through a quasi-spherical resonator and detected the onset of
\ncondensation by measuring the frequency ratio of selected microwave modes. We verified the
\nbasic operation of the device over the dew-point range 9.5–13.5 ◦C by comparison with
\ncalibrated chilled-mirror hygrometers. These tests indicate that the microwave method may
\nallow a quantitative estimation of the volume and thickness of the water layer which is
\ncondensed on the inner surface of the resonator. The experiments reported here are preliminary
\ndue to the limited time available for the work, but show the potential of the method for
\ndetecting not only water but a variety of other liquid or solid condensates. The robust all-metal
\nconstruction should make the device appropriate for use in industrial applications over a wide
\nrange of temperatures and pressures.