Measurement of basis weight and moisture content of composite boards using microwaves

Economic benefits from real-time, on-line measurement of partial wet and dry basis weight/density and true bulk moisture content (MC) of solid wood and composites are exceptional. Such information is useful for front-end processing, in-process control, and output quality control. This paper describes the use of a two-parameter (attenuation and phase), single-frequency (10 GHz), noncontacting microwave transmission system to simultaneously measure the partial water and dry wood basis weights, the total basis weight, and the MC. Attention is focused on fiberboard composites, but the measurement techniques apply equally to other composites and to dimensioned wood. The objective of this work is to show, through extensive experiments, how the measured changes in attenuation and phase delay of microwave signals transmitted through the wood can be used to determine m sub d and m sub w continuously during on-line production of composite boards. Accordingly, a linear interpretive model is constructed in which the calibration constants are determined by a least squares regression to the ensemble of calibration data taken using wood samples of known m sub d and m sub w. These coefficients are nearly linear functions of the wood temperature (T) for MC less than 10 percent, which is the range of most interest for composites just emerging from the press. Once calibrated for a particular wood type and temperature, the model is used to find the values and MC from the measured data, independent of the wood thickness (t). If the thickness is also known, the corresponding partial and total densities and the real and imaginary parts of the complex dielectric constant can also be found. The microwave system is interfaced to a microprocessor for real- time data acquisition, processing, display, and production control.

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