Resolution abilities and measuring depth of High-Frequency densitometry on wood samples

The High-Frequency (HF) densitometry measures relative density variations on wood samples utilizing the dielectric properties in wood. This method is based on the propagation of an electric stray field through the surface-near region of a wood sample. We studied experimentally the penetration depth and the differentiation abilities of the HF-densitometry method on wood samples. Two experimental approaches showed that penetration depth is related to the geometrical dimensions of the micro-electrode measuring system. Characteristic patterns of the HF-output signal were used to determine the resolution abilities of each HF-probe. Due to a very small integration area geometric structures of earlywood cells of Norway spruce were indicated by stepwise profiles in the HF-output signal pattern. With the High-Frequency densitometry it was possible to distinguish different cell structures due to their variations in wood density up to a resolution of 1 mu m. In addition it was possible to determine wood density variations at various resolution levels. Based on the respective resolution abilities and penetration depths of five different purpose-built HF-probes we show the optimal operating conditions for measurements on wood of this indirect densitometry method. (C) 2012 Elsevier Ltd. All rights reserved.