Microwave microstrip ring resonator as a paper moisture sensor: study with different grammage

This paper reports use of a nondestructive, miniaturized, microwave microstrip ring resonator for moisture sensing application in papers. Advantages offered by this sensor over the prevailing sensors are room-temperature operation, response time in milliseconds, measurements unaffected by dusty environment and ionic conductivity of samples. Samples of eleven types were tested, with grammage (grams per square metre) and thickness varying from 21 to 70 g m(-2) and 24 to 80 mum respectively. The wet basis moisture normalization (with respect to instantaneous moisture content in the sample) was established to remove scatter and to bring out clearly grammage dependence, thus avoiding error in moisture prediction due to density variations, using only scalar measurements. A single equation for f(r) variations is realized in terms of grammage and normalized percentage moisture (M) which is valid for all types of tested paper. A model of the wet paper is suggested mainly based on water-dry paper interaction, also considering parameters like thickness and surface roughness, to explain trends of the sensitivity curves. The estimated %M-ww shows an error of +/-0.9% in the estimated value as compared to the actual value.