Cryo freezers boast 2-3 week hold times below Tg (-135°C), which look great in brochures, and they’re real...under ideal conditions. But ideal conditions are not typically assumed when assessing failure modes and making emergency response plans. So what happens if my cryo freezer fails? How much time do I really have? What are the leading and lagging indicators? With several recent tragic and high profile incidents, there has been lots of information (and misinformation) surrounding LN2 based cryo storage. In an effort to clarify and help mitigate sample risk here’s a brief overview:
Cryo freezers are cold because of LN2. Specifically the vaporization, or boiling, of LN2 inside the freezer. How cold and for how long depends on the freezer performance and how much LN2 it holds. It’s analogous to a car’s range being set by its gas mileage and the size of its fuel tank.
So How Long Will My Cryo Freezer Really Last?
Simply put, as long as there is LN2 inside. Since LN2 is a consumed refrigerant, all cryo freezers must be refilled or they will eventually warm to room temperature. Leading cryo freezers can maintain -190°C with less than a 10°C range even with only a couple inches of LN2. But not all cryo freezers are created equal. Some have a much larger gradient where the temperature fluctuates with the level, spikes when the lid is opened and only stays below Tg for a couple of days. The only way for a industry-leading LN2 freezer to warm up is for it to run out of LN2. This can happen if there is not adequate LN2 supply on hand, if there’s an extended power loss, or if the freezer consumption rate suddenly increases beyond the planned supply capacity.
Then why are samples lost if all you have to do is keep LN2 in the freezer? Well, first you have to have the right freezer with sufficient LN2 supply and routine manual checks. Then if a situation develops, you have to be alerted with enough time to properly respond. Unfortunately, users are often notified too late, or not at all, and their response window is dramatically cut short or already closed.
When a Cryo Freezer Fails
Here is what should happen if a cryo freezer fails and there is a sudden increase in LN2 consumption:
An increase in consumption compared to the baseline average will trigger an LN2 usage alarm.
The freezer will fill more often as more LN2 is consumed. As long as the freezer is supplied with more LN2, it should stay in a high frequency fill cycle.
As the LN2 supply runs out, the freezer will not be able to complete a fill cycle and will trigger a fill time alarm.
When the LN2 supply is gone, the freezer level will drop and trigger a low level alarm to accompany the LN2 usage and fill time alarms.
As the freezer runs out of LN2, the storage temperature will begin to rise and trigger a high temperature alarm. The temperature will continue to warm to room temperature unless more LN2 is added.
Leading and Lagging Indicators
Independent temperature monitoring is a great and popular add-on for cryo freezers especially for traceability and record keeping. However, temperature is a lagging indicator of cryo freezer performance. It’s kind of like getting your low mileage alert when your car starts to slow down because it's already run out of gas. There are leading indicators that can signal a potential problem sooner and significantly widen the response window.
Notice the alerts in the above example that point to a potential problem. All 3 alarms prior to the temperature alarm are associated with the LN2 level and they may occur days before the storage temperature even changes. LN2 usage and LN2 level are the leading indicators of a change in cryo freezer performance.
Above we assumed the freezer had these features and worked properly to notify the user. The best cryo freezer performance is important, but to fully mitigate sample risk requires sufficient LN2 supply, smart control systems, remote alerts with independent monitoring and routine manual checks.