Investigation of a microwave differential cavity resonator device for the measurement of humidity in gases

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Abstract
A resonant cavity based microwave differential device has been developed as a sensor for the measurement of small quantities of water vapor in gases. The presence of a contaminant is assessed by induced variations in the relative permittivity due to a shift in the resonant frequency of the measuring resonator. The measured output signal is related to the difference in the reflection coefficients of the measuring resonator and the reference one. A simple modeling approach of the system shows the proportionality between the difference of the reflection coefficients of the cavities and the variation in the dielectric constant. The evaluation of the minimum detectable change in the permittivity is possible using the Clausius–Mossotti equation for a binary gas mixture at a given concentration and pressure. The detection threshold is then determined by taking into account the signal-to-noise ratio of the differential setup. The estimation of the limit of detection for some practical moisture contaminated gases yields values in the low ppm level at T=293 K and atmospheric pressure. Experimental data suggest a detection threshold of 6 ppm for moisture.