“…However, Knudsen flow in the modified substrates was verified by comparison of the selectivity of the silicatreated substrates for various gas pairs with the theoretical Knudsen selectivity. Also, for pure Knudsen flow a pressure normalized plot of flux vs mean pressure should give a straight line with a slope of zero for a low sorbing gas such as helium (Dullien, 1992;Uchytil et al, 1992).…”