Longitudinal Permeability of Wood to Gases and Nonswelling Liquids

A number of samples of both heartwood and sapwood of eastern hemlock were selected in order to cover a very broad range of permeability. The permeability of these samples was then measured with nitrogen and helium gas at several mean pressures and with iso-octane and n-amyl alcohol. Molecules of nitrogen and helium are quite different, helium being monatomic and nitrogen diatomic. The mean free path of helium is much greater than that of nitrogen. Iso-octane and amyl alcohol were selected because neither of them swells wood to any extent, and the viscosities of the two liquids are quite different. The results demonstrate that the permeability of the wood is a characteristic of the wood and is independent of the fluid used to measure it as long as the fluid does not swell the wood. Permeability measurement with gases yields a “superficial gas permeability” which is somewhat higher titan the true permeability because of slip flow. The magnitude of the slip effect increases with the mean free path of the gas and with decreasing permeability, of the wood. The average effective pore size in wood can be estimated from gas flow measurements. The values obtained probably differ somewhat from the actual sizes, but are of the right order of magnitude. Relative differences in pore size between wood of similar structure determined in this manner should be reasonably accurate. This technique should therefore be useful in determining the effect of treatments on permeability; for example, in determining whether changes in permeability are due to changes in the size or number of openings.

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