A technique for the measurement of gaseous diffusion in porous media

An apparatus composed of two well-mixed vessels connected by a tube was used to measure the effective diffusivity of a gas (sulphur hexafluoride, SF₆) in porous beds of sand, steel wool and glass beads. The general solution to Fick's second law for the apparatus was employed in the analysis. The ratio of the measured effective diffusivity of SF₆ to SF₆-air diffusivity was found to equal approximately the void fraction of the porous bed raised to the 1.43 power. This relationship agreed well with previously collected data and with the theoretical models of Millington (1959) and Marshall (1959). The effective diffusivity of wet porous materials was also measured and, at higher moisture contents, found to deviate strongly from that measured in dry materials. The effective diffusivity of SF₆ in sand with 20 per cent moisture (volume basis) was found to be about one-fourth as large as the value expected from the dry-bed experiments. No significant deviation was found for sand of 10 per cent moisture content or lower.