Characterizing Moisture Content and Gradients in Pinus radiata Soft Wood Using Electrical Impedance Spectroscopy

The electrical impedance of Pinus radiata soft wood was monitored across the grain, as it dried in air, by injecting alternating currents of frequencies 6x10-3 to 105Hz into test pieces via stainless steel plates clamped to the squared ends. The voltage response was sensed via nails embedded along one side equidistant from the ends and from each other. Impedance models based on concentration polarization/diffusion effects as well as dielectric, conduction, and geometrical properties of the tracheidal, wall pit, and polarization regions were fitted to the impedance spectra. The frequency-dependent polarization/diffusion capacitance provided the most consistent correlation with moisture content. Changes in the dielectric properties indicated that substantial shrinkage in the pit aperture occurred as the drying process traversed the fiber saturation point. The dependence of model parameter values on moisture provide a basis for characterizing critical moisture gradients between the surface and the center of timber that can lead to cracking during drying.