Paper Sheet Moisture Measurements by Microwave Phase Perturbation Techniques^*

Abstract

Moisture measurements have been made from the phase-perturbation response of a microwave quarter-wave coaxial cavity by a wct paper sheet. In one method the phase (φ) response is measured directly as a function of the moisture content (md) at a fixed frequency (f0) equal to the resonant frequency of the moisture detector. This single-frequency approach has two essentially linear response regions, at 0–10 per cent and 40–100 per cent, and a non-linear response from 10 to 40 per cent. The accuracies in the 0–10 per cent and 40–100 per cent ranges are respectively ±0.3 per cent and ±11/2 per cent. These results were determined from measurements on several samples. In a second method the phase-frequency slope dφ/df is shown to be a function of the moisture content (md) and the base frequency (f). This method has interesting wide linear response ranges (e.g. 0–90 per cent) depending on the choice of the base frequency. In a third method the phase-frequency slope (dφ/df) is used to optimize the base frequency at fH such that dφ/df rests at a maximum value. The value fH – f0 is also shown to be a measure of the moisture content (md).