Analysis of Methods for Determining the Coefficient of Moisture Diffusion in Wood

The internal moisture diffusion coefficient D for wood and the coefficient S of surface moisture emission are the factors which determine the moisture gradients that exist in wood during the drying process. The diffusion coefficient D appears in one of three general forms depending upon whether the moisture gradient in the wood is defined in terms of moisture concentration C (weight of water per unit volume of the wood) , moisture content M (weight of water in percent of the dry weight of the wood), or water vapor pressure h (water vapor pressure of the wood). The diffusion coefficient D can be determined by steady-state methods in which the moisture distribution in the wood remains constant with time, or by unsteady-state methods in which the moisture distribution changes with time. The steady-state methods are more time consuming experimentally but permit the use of relatively simple mathematical relationships between moisture distribution and the diffusion coefficient, even when the diffusion coefficient varies with the moisture concentration of the wood. The unsteady-state methods require less experimental time than the steady-state techniques but the mathematical relationships between moisture distribution, time, and the diffusion coefficient are more complex. Analytical mathematical solutions for the unsteady-state diffusion equation for wood of finite thicknesses are available only for the case where the diffusion coefficient D and the surface emission coefficient S are assumed to be constant with moisture concentration. A number of steady-state and unsteady-state methods for finding the coefficient D are described and analyzed with respect to their advantages and disadvantages. Some of these methods have not been applied to wood, but they show promise of considerable usefulness for future research.

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