1) Permeation of water vapour through the plastic

2) Water vapor entering through the opening each time the lid is opened.

I’ve calculated the (1) using Fick’s law of diffusion. I found the diffusion coefficient for the plastic that my container is made from. I then use this together with the thickness of the material, exposed surface area and differences in water vapour concentrations to work out the mass flow of water into the container when it is closed.

I’m now trying to figure out the mass flow of water into the container each time the lid is opened (2). I have found a diffusion coefficient for water in air at my temperature range of interest. I also know the surface area of the opening. The problem is that the Fick’s law formula I am working with seems to require a “membrane thickness”. In my case I have no membrane – just an opening. Can this formula still be applied? Is there another formula I should be looking at? How can I go about calculating how much moisture makes its way into the container each time it is opened?