Permeability of Wood . . . Exemplified By Measurements on Redwood

To develop a theory of fluid flow through a material, it is possible either to measure the microscopic structure of the medium and calculate the rate of flow, or to measure flow rates for the macroscopic medium and establish a macroscopic theory by experiment. In this study, an experiment with redwood was performed to compare the flow rates of two gases differing in viscosity and molecular weight; to compare permeability in different anatomical directions; to determine the influence of extractives; and to compare the flow rate of a gas with the flow rate of a liquid. A permeability cell connected to a gas pressure cylinder on the entrance side and a flow meter on the exit side was used. Gas was supplied from the cylinder through a pressure-regulating valve and forced into the cell. Flow through the samples was measured with Manostat flow meters, and the pressure on the entrance side and the pressure drop across the samples were measured with manometers. In testing longitudinal versus lateral permeability, it was found that, as expected, permeability was far greater along the grain. Differences between radial and tangential values were relatively small. Permeability increased with the removal of extractives. No association was found between permeability along the grain and specific gravity. It was concluded that the permeability of wood should be expressed in terms of empirical values established by experiment. For practical purposes, Darcy’s law can be employed to describe permeability, if laminar streaming is assumed to be the only occurring form of flow. Flow rates of gases through wood remain constant as long as the capillary structure through which flow occurs is not changed. If the flow rate of one gas through a sample is known, the flow rate of another gas can be calculated. Redwood heartwood has extremely low permeability, particularly across the grain, and the presence of extractives contributes to this. The velocity with which a gas or liquid will flow through wood is largely dependent on the viscosity of the fluid. Though gas-flow measurements can give some indication of the resistance wood will offer to penetration of liquids, it is doubtful that the flow rate of a liquid through wood can be calculated from values of permeability to gas, primarily because of the so-called filter effect.

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