Vacuum bags inflating in Cryogenic Storage

[Jimbowen1072]

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.

 

[UltrafastPED]

If the bag expands when warmed, then something has changed from liquid (or solid) to the gas state during the warming process.

If you have access to a laboratory the gas content could be analyzed - there are many possible techniques such as mass spectroscopy, or a residual gas analyzer, etc.

Once you know the gas composition you can work on how it is entering the bags.

 

[Bandersnatch]

Hi Jimbowen1072,

If the bag or the seal is faulty, gaseous nitrogen, as well as any other gases present can and will seep into the vacuum bag while in storage, condensing on the inner walls and the overwrap. Once out of the storage, it evaporates inflating the bag.

 

[Jimbowen1072]

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 replicate the problem with short tests.