Microwave Method for In-kiln Moisture Content Measurement

A technique based on microwave sensing technologies has been developed which provides a means of estimating the average moisture content across a stack of timber during kiln drying. The microwave sensor, which comprises of a linear array of slots excited from both ends, was developed and trialed under a range of standard commercial kiln drying schedules using 100mm x 50mm Pinus radiata sample boards. Discrete power measurements in the frequency range from 5.3 to 5.5 GHz were performed and then summed to produce a gradient response. This gradient response is a measurement of the frequency shift of the resonant peak of the array with respect to the moisture content of the material covering the slot. Using this measured gradient response a linear model was developed and then inverted to produce an estimate of the average moisture content across the stack. Analysis of the relationship between the estimated and actual average moisture contents showed a significant correlation . Moisture contents of less than approximately 15% (by dry basis) gave an error of less than 3% in moisture content for confidence levels of 99%. The error increased as moisture content increased and for moisture contents below 50% an error of 10% at the 99% confidence levels was found. Further work is being performed to enhance this technique.