Storage temperature gets quoted as a single number, and it is not one. It is a number for liquid phase, a range for vapour phase, and a function of the vessel wall in both cases. Specifying storage on the single number is how laboratories end up with samples sitting warmer than they think.

The Actual Figure

Liquid nitrogen boils at minus 195.8 degrees Celsius at atmospheric pressure. Samples immersed in the liquid sit at that temperature, and they sit at it uniformly, because the boiling liquid holds the whole canister at its boiling point.

Minus 196 is the rounded version of the same number and it is what most documentation uses. Either is defensible. What is not defensible is treating minus 196 as the temperature of a vapour phase store.

Vapour Phase Is a Gradient, and the Gradient Is the Specification

In vapour phase storage, samples sit above the liquid in cold nitrogen gas. That removes direct contact with the liquid, and with it the cross contamination route that comes with the liquid. The ASRM and SART Practice Committees, writing on cryostorage in Fertility and Sterility (2020;114:486-491), note that liquid nitrogen is not sterile, which is the foundation of the vapour phase argument.

The cost is a vertical temperature gradient. The bottom of a canister sits close to the liquid temperature and the top sits warmer, and how much warmer depends on the vessel design, the liquid level and how often the lid is opened.

Which means the temperature to design around is the warmest position a sample can occupy, not the temperature at the bottom of the vessel. A sample inventory that fills the top of the canister in a vessel running at a low liquid level is the specific failure this gradient produces, and it produces it silently.

Why the Level Alarm Matters More in Vapour Phase

Because the gradient worsens as the liquid falls. In liquid phase, a falling level gives you a margin measured in the depth of liquid still covering the samples. In vapour phase there is no such margin: the samples are already above the liquid and every centimetre lost pushes the warm end of the gradient further down the canister.

The ESHRE recommendations on good practice in the IVF laboratory, published in Human Reproduction (41(8), 1245), expect cryostorage tanks to be continuously monitored with alarms that detect and log out of range temperature or LN2 level. Continuous and logged are both load bearing words there. A daily manual dipstick check tells you the level at one moment each day, which is not the same thing at all.

Where the Dewar Wall Comes In

Everything above depends on the vessel holding its temperature, and the vessel holds it with nothing more than a vacuum.

Open the wall of a cryogenic storage vessel and there is no insulation in it in the ordinary sense. No foam, no fibre. There is an inner vessel, an outer jacket, and an evacuated space between them. Heat cannot conduct or convect across a vacuum, which is why the arrangement works at all.

Inside that space are three things that decide how well it works and how long it keeps working. Multilayer insulation, wrapped in layers to interrupt radiant heat. Internal materials chosen and cleaned so that they outgas as little as possible. And a getter, a chemically active component that absorbs the stray gas molecules that inevitably accumulate, keeping the annulus evacuated over years.

None of that is visible on delivery and none of it appears on a comparison table. All of it decides your evaporation rate and your service life.

How the Wall Fails, and How You See It Coming

Two ways. Slowly, as the getter approaches the end of its capacity and the vacuum softens. Or suddenly, when a mechanical shock cracks a weld or fractures an internal support, admitting gas into the annulus.

The second one is the reason a vessel that has been caught by a trolley should be treated as suspect even when it looks undamaged. There is frequently no external evidence at all, and the vacuum then degrades quietly from that point.

You see either one coming in the same place: consumption. A steady rise in litres per week at unchanged workload is a vacuum question. On a pressurised vessel the pressure trend moves first, because pressure responds to the rate at which liquid is turning to gas while the level only responds once the liquid has gone. Frost on the outer shell is the last warning, not the first, and by then the vessel usually needs to come out of service.

What This Means When You Specify

Decide liquid or vapour phase deliberately and document the reason, because it is among the first things an inspector asks and the answer has to be justified rather than inherited. Specify the monitoring to match: level and temperature alarms with a remote output, and a relay to a building management system or autodialler that somebody actually answers out of hours.

Then track consumption from day one, because the wall you cannot see is reporting its condition through that number and through nothing else.

The vessel selection side of this is covered in Choosing a Liquid Nitrogen Storage Tank for an IVF Laboratory, and our CryoNest storage vessels and CryoBank series cover the range from laboratory to large steel bank.