Rubber permeability of hydrogen to air

I am currently studying permeation of rubber film in a university practice group.
The problem is to evaluate the permeation of hydrogen gas filled in a meteorological balloon placed in the air, but how to build a mathematical model?
In particular, it is about how the penetration phenomenon is when the rubber balloon goes up and down.


The statement is as following: A weather balloon contains hydrogen gas floats in air with its buoyancy. As balloon flies higher pressure in balloon decreases and at the same time volume of balloon increases. But because of partial pressure difference between the inside balloon and the outside balloon hydrogen permeates through rubber membrane of balloon to outside air.


How to evaluate permeation model of this phenomenon?
And also when balloon descent from high altitude to ground how this model is made?


