I work for a company which has an interesting problem. We store at cryogenic temperatures in vapour phase, not submerged in liquid. In order to protect the inner bag we place an over wrap over the inner bag then vacuum pack it, in a sterile environment. The bag is then placed into cryogenic storage. Our problem is that randomly one of the bags inflates when being removed from cryogenic storage. Theories that surround this include trapped air, liquid nitrogen getting into the bag, holes in the bag and a poor seal none of which I have been able to replicate as a control. My answer is that air would condense and contract when freezing and would return to its original state when thawed. How can liquid nitrogen get into the bag when it doesn't come into contact with the liquid nitrogen? Does anyone out there have any other theories as to what happens which might explain the expansion as the bag is thawed.