Sensor type and placement for measurement of internal lumber temperature in dry kilns

Studies have shown that internal temperature of lumber can be part of a lumber-drying control system. Tests were done to determine whether 24-gauge, exposed-junction thermocouples or 40-gauge, stainless-steel sheathed thermocouples (subminiature probes) would serve better to measure the internal temperature of lumber during conventional drying. Ponderosa pine sapwood was used because of its high initial moisture content and rapid and even rate of moisture loss during drying. Sensors were inserted in holes of 1/8-, 3/16-, and 1/4-inch diameter at six depths (1 to 6 in.) in the edge of the boards. Expanding foam and silicone caulk were used to insulate the sensors and seal the holes in the wood. Four tests were used to show temperature changes in lumber during steadily changing, slowly changing, and erratic and rapidly changing conditions of temperature and humidity. Both types of sensors performed adequately. The 24-gauge thermocouple would be preferred because of lower cost and because it has no exposed junction inside the kiln. Lumber internal temperature varied directly and positively with kiln temperature. Hole diameter had no apparent effect on sensed temperatures, but 1/8-inch (or smaller) holes should be used for best results. Holes 3 or 4 inches deep, or 2 inches deep in 2- by 4-inch lumber, gave the most satisfactory results. Silicone caulk proved more practical than expanding foam.

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