Diffusion through a slab

Case 3. Barrer (i4a) treated the problem of diffusion through a slab of thickness Z, when in addition to pure diffusion two other rate-controlling surface processes are important ... 

In the steady-state permeation method, the surfaces of a polymer film of thickness i = 2b are kept at constant gas pressures Pao an PAr- At steady state Eq. 4.75 applies, and the permeability P is found from this equation. If the solubility is known, then the diffusivity can be found from Eq. 4.71. For hollow cylinders and spheres, expressions similar to Eq. 4.74 hold. On the other hand, in the time lag method we deal with the unsteady-state permeation of a diffusant through a slab of thickness 2b. The surfaces of the slab are kept at concentrations Cao and at zero. The accumulated amount of gas which has passed through the slab in time t. 

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