| Summary: | Wood is a natural material with a wide range of physical properties. For effective use
of the material resource, it is important to be able to make reliable non-destructive
measurements of material properties. Of particular interest here are grain angle, moisture
content and density of wood. These are important for process control and for wood strength
estimation.
Microwaves provide an effective method for measuring wood properties. The
measurements are non-contact, high speed and safe. Their data content is also very rich, with
a total of eight independent measurements at each point. Thus, there is good opportunity for
reliable material property evaluation and for data consistency checking. The microwave
system described here measures the attenuation and phase change of a microwave beam as it
propagates through wood specimens. It makes these measurements for both parallel and
perpendicular polarizations. In this study, microwave measurements are made on 76 hemlock
and 150 Douglas fir pieces. The collected data are used to estimate the wood grain angle,
moisture content and density. For estimations, linear models whose coefficients are
calculated by applying least-squares method are constructed. Also, since temperature has a
significant influence on electrical properties, the temperature is taken into account as a
variable in the formulations.
A significant species dependence was observed in the results, with different moisture
and wood density sensitivities for hemlock and Douglas fir. In general, the results from hemlock were superior to those from Douglas fir. For hemlock and Douglas fir respectively,
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the standard error of estimation (SEE) of wet density was 4kg/m3 and l0kg/m3 (moisture
density range = 40-120 kg/m3 ). For dry wood density, the SEE was 40 and 65 kg/m3 (dry
wood density range = 300-650 kg/m3). For moisture content, the SEE was 1.0% and 2.0%
(moisture content range = 6-30%). For grain angles, the SEE was 0.9 and 2.5 degrees (grain
angle range = 0-180°). All these results indicate that the microwave method has substantial
practical potential as a method for estimating wood moisture density, moisture content and
grain angle.
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