Predicting Moisture Content in Douglas-Fir and Ponderosa Pine Boards

Clear lengths of green Douglas-fir and 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 specimens, 1- by 6- by 20 inches were cut from the sample boards. Four hardness tests were made on each face of the 6-inch board and the results of the eight tests were averaged. Prior to drying, the green volume of the specimen was determined by the water immersion method. Also, the specimen was weighed and the percent sapwood and heartwood estimated. After coating the specimen ends with an asphalt paint, 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 the oven top. The specimens were dried at 100?C (230?F). During the drying process, the specimen weight and surface temperature were recorded at 10-minute intervals. The ovendry weight of the specimen was used to calculate the initial and intermediate moisture content percentages and the specific gravity. Regression analyses were used to evaluate the relationships between percent moisture content and the following: green wood hardness; percent heartwood; green weight and volume; and elapsed drying time. For Douglas-fir, initial percent moisture content can be estimated fairly well using green weight and volume (R2 = 0.90). For estimating the percent moisture content during drying these same two independent variables along with elapsed drying time were the most efficient combination (R2 = 0.95). For ponderosa pine, the analyses showed that green volume, green weight, and hardness can be used to predict initial moisture content with great precision (R2 = 0.98). This estimated initial moisture content in conjunction with elapsed drying time can then be used to predict the percent moisture content during drying (R2 = 0.93).

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