Ultrasensitive moisture content characterization of wood samples by a cylindrical cavity resonator

Ultrasensitive moisture content sensing in wood samples is demonstrated employing a modified cylindrical cavity resonator geometry. A vertical sensing channel is introduced at the middle section of the cavity for sample placement. Field homogeneity inside the sensing channel is improved by a thin hollow quartz inset. A relation between the moisture content of wood and the resonance frequency for the dominant TM₀₁₀-mode is extracted for the operation frequency range. Full-wave simulations are used to characterize and optimize the sensing technique. For experimental verification, a wood sample with 24% moisture content is allowed to dry in the cavity at room temperature for 24 h and its resonance frequency is monitored. The experiment is repeated for a sample which partially fills the sensing channel. Finally, the variation of the moisture content of a freshly cut tree branch is monitored in vitro. The experiments and simulations reveal that the proposed technique offers a very high sensitivity for characterization of the moisture content of wood samples of different radii, owing to the very high quality factor of the cylindrical cavity resonator. The technique is thus a promising candidate for the non-destructive monitoring of the moisture content of trees towards the prevention of forest fires and for the evaluation of products made from wood in industry.