Predicting moisture content in ponderosa pine boards

Introduction The ability to predict or measure the percent moisture content, both before and during the drying process, has been a research goal for many years. Commercial moisture meters are not reliable at the higher moisture content levels and hence, cannot be used for this purpose. If the moisture content of green boards could be determined then they could be sorted into groups of similar wetness which should improve drying efficiency. Also, the drying efficiency would probably be improved if the wood moisture content during drying were known. Earlier research aimed at estimating the moisture content during drying has related the average percent moisture content to the wood's surface temperature and the elapsed drying time.' The following study evaluates additional wood characteristics and attempts to relate them to the average percent moisture content. Among the additional factors examined were: green wood hardness; surface temperature; percent sapwood and heartwood; green volume and weight; and elapsed drying time. Procedure Clear lengths of green ponderosa pine boards were obtained from various Missoula sawmills. The material was primarily flat sawn, cut from the outermost log surfaces, and, as a result, contained varying amounts of sapwood. Test speciments, 1 by 6 by 20 inches, were cut from these boards and stored under water until used. Four hardness tests were made on each face of the 6-inch board and the results of the eight tests were averaged. The tests were spaced over the board surfaces to assure a fair sampling. Except for minor differences in specimen size, shape, and roughness, the hardness test was the same as that described by ASTM. 2 In general, this test requires the embedding of a 0.444-inch diameter ball to a depth one-half its diameter, as determined by the tightening of a collar against the specimen. The hardness tests were made in a Tinius Olson testing machine. When all hardness tests were complete, the specimens were replaced under water. Prior to drying, the green volume of the specimen was determined by the water immersion method. The specimen was then weighed and the percent of sapwood and heartwood estimated and the data recorded. After coating the specimen ends with an asphalt paint, to prevent end drying, the specimen was placed in a metal frame which was attached to the specimen ends. The frame was suspended in an oven from a balance located on top of the oven (Figure 1). The oven was a forced air circulation type that had been modified by having its doors replaced by a plywood cover. A small door in the