Product resistance to vapour flow

[A] A typical profile for the increase in product resistance with the increased height of the dry layer; [B] A potential physical model for such a profile based on the product having a dry layer, that extends across the entire internal diameter of the vial, below which there is a shrunken dry layer with a shunt pathway between the dry layer and the glass wall (note that the shunt pathway might be instead be due to cracks in the dry layer without shrinkage); [C] illustration of the resistances to vapour flow associated with the narrowing pathways for vapour diffusion for the container and product. The first is associated with the vent in the stopper (R_s and the second is associated with the connected pores within the dry layer (R_p). The transit of water vapour experiences these two resistances is in sequence (i.e., one followed by the other) and hence it is possible to combine them into a single parameter, R_ps, which is the sum of R_s + R_p.