A capacitive multi-wavelength sensor for moisture content gradient sensing in wood

Technical advancements in the field of capacitive sensing and the availability of multi-channel capacitance-to-digital converters have led the way for improved sensing capabilities in the field of wood moisture content. A four-channel, planar capacitance sensor was developed and calibrated on maple veneers. The calibration material was conditioned to equilibrium at three different relative humidity levels. Optimal penetration depths were determined statistically and analytically. A rule of mixture approach and a volumetric model were applied to derive the moisture content at distinct layers. Different levels of discrepancy of the fit were observed among and within the different electrodes and calibration datasets. The sensor was able to determine changes in moisture content and positive moisture content gradients, rather than averages, with the limitation of detecting negative moisture gradients.