Thank you for your responses. I think that seepage might be a possibility and conducting short experiments might not be long enough to pick this up. It might be that the LN2 is taking hours or possibly even days to seep into levels that cause expansion which is why I haven't been able to...
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...