Method for measuring the thickness of foil-containers

A method of checking the leaktightness of a sealed container which holds a pharmaceutical active substance formulation in a chamber provided inside said container, the sealed container also holding a gas (the second gas) comprising the steps of a) acting upon the sealed container with a first gas which differs from the second gas enclosed within the container such that the increase in the quantity of the first gas inside the chamber can be analysed and b) opening the container and removing some of the gas which is located inside the chamber for the active substance formulation, for the purpose of qualitative, quantitative or both analysis.