Non-invasive method to measure bulk electrical resistivity of spruce wood - influence of moisture content and anatomical direction

The correlation between electrical resistance and moisture content (MC) of wood has been studied for many years and is the basis for an important method for determining wood MC in practice. This study presents a non-invasive set-up using plate electrodes with a guard ring covered with silver foam and gold leaves to measure the direct-current (DC) electrical resistivity of spruce wood. The design ensures good electrical contact at a pressure of 0.68 MPa, without impairing the wood surface. Measurements were conducted across various moisture contents up to the fiber saturation point (FSP) for the three anatomical directions at a temperature of 21 degrees C. The regression parameters between electrical resistivity and moisture content were determined with high coefficients of determination (R-2 = 0,97-0.98). The effect of MC on the electrical resistance was significantly more pronounced in the radial direction, and the electrical resistance in the axial direction was lower than in the radial and tangential direction. Those findings support the basic understanding of the electrical properties of spruce wood and provide a basis for non-invasive volume resistivity measurements of wood.