The ability of a barrier to transmit gas is usually expressed as y (gamma), the permeability. This is a measure of the total flow through a barrier and may be defined as the ratio of the number of molecules which pass through the barrier to the number which would pass through the space if the barrier were not there. This total flow through a barrier is known to be a combination of several different types; namely, diffusive flow, film flow, and viscous flow. As was previously explained, the type of flow that is of greatest importance for a gaseous diffusion plant is diffusive flow. However, as film flow and viscous flow affect the separation through a barrier, they cannot be ignored.

Film flow refers to the transport of molecules under the influence of the force field between the molecules and the barrier surface. When such force fields are significant, a large number of molecules will not have sufficient velocity normal to the surface to escape from this field so that their motion will be confined to the barrier surface and barrier pore surface. Such molecules would pass through the barrier as a film on the surface of the pores.

Viscous flow occurs when molecules flow as a group in the manner of ordinary flow through a tube. Referring back to permeability, it is known that the flow through a barrier increases as the pressure increases. Also, as the pressure increases, so does the viscous component of flow. The relation between permeability and viscous flow can best be shown by the following equation.

In this equation where P_f equals the fore or high side pressure and P_b equals the back or low side pressure. equals the permeability, , when

S is called the slope factor and is inversely proportional to the viscosity of the gas.

In the above equation, the term S(P_f + P_b) is a measure of the viscous component of flow. Figure A-2 illustrates the variation of permeability with the slope factor and changes in pressure.

Figure A-2 Variation of Penneability with the Slope Factor and Change in Pressure

It should be noted that in practice, permeability is expressed in two different ways; design and actual. Design permeability is expressed as a ratio as previously stated. It is usually determined by testing the barrier with a non-toxic gas such as nitrogen. Actual permeability is expressed as a percentage of the design permeability and is determined from tests with the barrier in actual operation in the cascade. Its primary value is in determining the performance of a barrier after being in use for a period of time.