Gaseous diffusion in porous media Part 1. - A non-steady state method

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Abstract
Laboratory equipment, capable of modification for use on soils in the field, was constructed to study diffusion in porous systems. A simple katharometer employs the exposed element of a thermal milliammeter to measure the instantaneous composition of hydrogen/air mixtures; the most convenient working condition is when 15 mW are dissipated in the hot wire, at which the thermocouple output changes from 0.5 mV in pure hydrogen to 2.5 mV in air. The geometry of the experimental system is simple enough to permit a standard solution of the non-steady state equation for interdiffusion of two gases to be applied, and, as a check on performance, straight tubes of slightly varying geometry were used to measure the diffusion coefficient of hydrogen into air, and its dependence on temperature. Reproducibility was very good and, within the limits of experimental error, the coefficient obtained was the same for 100% hydrogen into 100% air, and 60% air + 40% hydrogen into 100% air. Measurements at 12, 20, 28 and 36° C, fitted D = D0(T/273)n, with D0 = 0.651 cm2 s-1 at n.t.p. n = 1.715 Standard values in the literature are D0 = 0.611 (International Critical Tables), 0.634 and 0.661 (Smithsonian Physical Tables). The latter reference gives n = 1.75.