Wood physical property measurements using microwaves

The work described in this thesis is the first part of a project aimed at developing an advanced lumber strength grading system using microwave measurements. The overall objective is to develop an improved practical system for estimating lumber strength. A microwave instrumentation system is described in this thesis that can measure wood grain angle, specific gravity, and moisture content. These three physical properties directly influence lumber strength. In the development of the current microwave instrumentation system, an advanced microwave sensor system was designed to measure elliptically polarized microwave fields. A simplified microwave theory is presented to describe the relationship between the measurements from the sensor and wood grain angle, specific gravity, and moisture content. The simplified theory is very successful in explaining the experimental observations, and provides valuable guidance in the determination of grain angle, specific gravity, and moisture content using the microwave measurements from the new sensor system. Starting from the simplified microwave theory, a simple but efficient model is developed for determining the grain angle using the microwave measurements from the newly developed microwave sensor. For data collected from one hundred samples of Douglas-fir and spruce, the model gave a coefficient of determination r² = 95%, and a standard error of 1.8 degrees for grain angles up to 30 degrees. Simple yet efficient models for evaluating specific gravity and moisture content are also developed. For specific gravity, the proposed evaluation model gives a coefficient of determination r² = 88%, and a standard error of 0.026. For moisture content, the proposed evaluation model gives a coefficient of determination of 85% and a standard error of 0.7% in MC. Detailed study shows that the current microwave instrumentation system and the developed evaluation models are equally effective for measurement environments such as sawmills where temperature changes seasonally over a substantial range. The current microwave instrumentation system developed during this thesis research can provide accurate grain angle, specific gravity, and moisture content in realtime regardless of environmental temperature, wood species, and wood structural characteristics such as annual ring direction, diving grain, and small thickness variation. Accurate knowledge of grain angle, specific gravity, and moisture content will make it possible to calculate lumber strength using mechanistic procedures. This will make lumber strength evaluation more accurate and reliable.Applied Science, Faculty ofMechanical Engineering, Department ofGraduat