Some General Characteristics of Capacitive In-Kiln Moisture Monitors

The technology of in-kiln monitoring of the moisture content of lumber loads using the capacitance method is not perfected. The development of the technology has been empirical and progress seems to have been impeded by a lack of understanding of some basic principles involved. A study at the Forest Products Laboratory helped clarify some of these basic relationships. Monitors respond to the average MC of the load but are strongly biased by the specimens in direct contact with the electrode. The monitor response is biased, but to a lesser degree, by surface conditions of the specimens in contact with the electrode. Monitors in typical kiln schedules are unresponsive to changes in average MC of the load until the average MC is reduced to 40 to 50 percent. Monitor readings increase with increasing temperature of the kiln load with a positive interaction with MC. Temperature effects were of moderate magnitude, so temperature corrections would be easy to apply. It appeared that temperature effects were independent of species. Monitor readings were higher in a metal kiln than in a masonry kiln. Differences appear to be in the same order as those resulting from an MC difference of about 5 percent. Among operating parameters of the monitors, the most important was frequency of the sensing signal. Frequencies of 0.7 to 30 kHz were used. Sensitivity of the monitors increased as the frequency was decreased. Electrode design affected the monitor readings significantly only at the lower frequencies and when the specimens had at least 20 percent MC. A detailed description of this study may be found in the April 1982 issue of Wood Science.

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