A specification failure worth borrowing from laboratory cryogenics.
Cryogenic storage is universally specified as minus 196 degrees Celsius. That figure is correct for exactly one configuration: samples immersed in liquid nitrogen, which boils at minus 195.8 at atmospheric pressure and holds the whole canister at its boiling point.
Move to vapour phase storage, where samples sit above the liquid in cold gas, and the single number stops being a number at all. Sample temperature becomes a function of height above the liquid, the liquid level, how often the lid is opened, and the vessel design.
Bottom of the canister sits near the liquid temperature. The top sits warmer. How much warmer is not a constant, it is a function of state that drifts.
Specify against the tail
The sample that matters is the one at the top of a canister in a vessel running low. Nothing about it looks different from the outside. Specify against the worst position in the distribution, not the mean or the best case.
Monitor, do not sample
The ESHRE recommendations on good practice in the IVF laboratory (Human Reproduction 41(8), 1245) expect continuous monitoring with alarms that detect and log out of range temperature or level. Continuous and logged, not sampled. A once-daily manual check is a point observation of a variable that moves between observations, which is the same reason nobody monitors production on a once-daily cron.
Find the leading indicator
The thing holding the temperature is a vacuum between two vessel walls, maintained by a getter with finite capacity. It degrades gradually, and the observable is nitrogen consumption. A rising trend at unchanged workload is the leading indicator. The visible symptom, frost on the outer shell, is a terminal one.
Full write up: https://cryolab.co.uk/liquid-nitrogen-storage-temperature/
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