Relationship Between Permeability of Green and Dry Eastern Hemlock

Describes the general methodology used to measure the nitrogen gas permeability of eastern hemlock at various moisture contents and the results obtained. Specimens of eastern hemlock about 25 mm square in cross section and 9 mm long in the fiber direction were used. The specimens were preconditioned to 4, 9, 15, and 18 percent moisture content before measurements were made. The nitrogen gas was passed through saturated solutions of the salts used to precondition the specimens. It was then passed through the specimens using a specially designed sample holding cell while the pressure difference across the specimen cell was measured. Gas flow rate was measured with precision rotameters. The results showed that for heartwood there was essentially a linear relationship between the nitrogen gas permeability and the previously measured water permeability of green specimens. Linear relationships were also shown to exist for the square root of the nitrogen permeability versus the moisture content and the square root of the ratio of gas/liquid permeability to moisture content. For sapwood, no relationship could be shown for the nitrogen gas permeability of specimens at 4 percent moisture content and their water permeability. It was concluded that sapwood undergoes a large reduction in permeability on drying. The effect on permeability of using alcohol to replace the water in sapwood specimens before drying was studied. It was shown that this then produces a consistent relationship between the liquid permeability of the green wood and the gas permeability after the wood dries.

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