EFFECTIVE PENETRATION DEPTH AND EFFECTIVE RESISTANCE IN MOISTURE TRANSFER

The concepts of "effective penetration depth" and "effective resistance" have appeared in the literature recently with reference to lumped models of moisture transfer. These quantities are shown to be equivalent. The effective depth for the one-sided case is shown to be square-root D/2-omega and for the two-sided case is 1/6 the material thickness, with corresponding effective resistances. Significantly, the effective penetration depth is shown to be not dependent upon the surface resistance of the boundary layer and any coatings. A simple lumped model used in defining the concepts gives very close agreement with the exact solution to the diffusion equation for the amplitude and phase of the mean moisture content for a periodic driving potential. This suggests that effective penetration depth and effective resistance can be regarded as universal physical quantities and not merely approximations or artifacts arising out of simplifications.