| Abstract: | Little data is available on gas transmission through thick polymer samples. There is, therefore, a temptation to assume an inverse relationship between gas transmission rate and thickness and to calculate flow through thick samples using permeability data obtained on thin films. This is incorrect, as the calculation assumes steady flow, and with thick samples this state may not be reached for months or even years. Thus both diffusivity and permeability are needed to calculate the gas flow and the calculations are more complex than simple permeability calculations. Because of the long time scale, the effects of varying the thickness are difficult to demonstrate experimentally. We describe a computer simulation of the manometric method which compares the behavior of two materials with similar permeabilities but different diffusivities. Although thin samples behave identically, they differ increasingly with thickness. The pitfalls in using the gradient and time lag of the pressure curve to calculate permeability and diffusivity are also discussed. |
