A Microwave Method for Measuring Moisture Content, Density, and Grain Angle of Wood

A Microwave Method for Measuring Moisture Content, Density, and Grain Angle of Wood William L. James, Physicist Forest Products Laboratory, Madison, WI You-Hsin Yen, Graduate Student Ray J. King, Professor of Electronic and Computer Engineering, University of Wisconsin, Madison, WI The attenuation, phase shift and depolarization of a polarized 4.81-gigahertz wave as it is transmitted through a wood specimen can provide estimates of the moisture content (MC), density, and grain angle of the specimen. Calibrations are empirical, and computations are complicated, with considerable interaction between parameters. Measured dielectric parameters, however, correlate uniquely and repeatably with MC over a range of MC at least as great as 100 percent of the dry weight of the wood. Grain angle can be deduced reliably when the specimen thickness and MC are great enough to introduce sufficient dielectric anisotropy to appreciably depolarize the incident wave. At present, the apparatus and techniques are very complex, but the principles developed by this research show the method to be feasible for nondestructive testing of lumber as it is produced.