That's it! Sorry If my explanation wasn't clear enough, but what I'm having trouble is getting the appropiate conditions in order for the gas to change pressure. FLUENT shows me that the temperature does indeed change, that's why I though using the ideal-gas model (in materials properties) could be an option.

The main purpose it's to study the temperature change in the film of the ballon (im implementing radiation models and the obvious convection both natural and forced). I'm heading to read more about the DEFINE_GRID_MOTION, however if I'm to do it that way where should I implement the UDF?

Also I should probably have to read about how the diameter really change time dependant but I think this could very well be the solution. Thank you for your time.

Best regards