Stand in your cryo store and do the sum. Length times width times height gives you the room in cubic metres. Now take the biggest vessel in it, in litres, and multiply by 683. That is how much nitrogen gas it holds, in litres, waiting to come out (BCGA Code of Practice 30). For a 4 by 5 by 3 metre room with one 175 litre vessel, the room is 60 cubic metres and the vessel is about 120 cubic metres of gas. If it all came out at once, oxygen would fall to roughly 7 per cent. You would not make it to the door, and you would not know why.

Liquid nitrogen tank safety comes down to four controls: ventilation and oxygen monitoring in the room, keeping the vessel neck free of ice so pressure cannot build, protective clothing for anyone decanting or retrieving samples, and level and temperature alarms with a tested response plan. None of them is expensive. All of them are skipped somewhere every week.

The gas you cannot smell

Nitrogen is what most of the air already is. Your body has no sensor for it, and no sensor for missing oxygen either. CP30 puts it flatly: asphyxia due to oxygen deficiency is often rapid with no prior warning to the victim. The same document records the HSE’s recommendation that workplace oxygen stay above 19.5 per cent. The University of St Andrews sets its own trigger points at 19.5 per cent for urgent investigation and 18 per cent for immediate evacuation, which is a sensible pair of numbers to copy into your own procedure.

Nobody’s vessel dumps 175 litres at once. But every vessel boils off continuously, every fill vents a surge, and every delivery dewar parked in the corner adds to the total. A room that is safe with one tank is not automatically safe with three.

What that means on a Tuesday afternoon:

• A fixed oxygen monitor with its alarm light on the outside of the door, so the warning reaches the person who has not yet walked in.
• Extract ventilation sized for every litre of liquid in the room, delivery dewars included.
• A rule that vessels travel in lifts alone. Send it up, take the stairs.
• Portable monitors clipped to anyone filling from a supply tank or decanting from a CryoCrest dispenser.

The coin sized plug that turns a dewar into a pressure vessel

Storage tanks are not pressure vessels. The lid is a loose insulating plug and the neck vents to atmosphere. That arrangement works until moisture from the room freezes in the neck. Ice builds, the vent closes, and the boil off has nowhere to go. CP30 describes the sequence: an ice plug can form a seal that prevents the release of gas, leading to a rise in internal pressure. The end point is a ruptured vessel.

The prevention is dull, which is why it lapses. Use only the stopper supplied with the vessel, never a tighter one that “keeps the cold in”. Look at the neck every time you open it. If frost is bridging, clear it with a warm tool while someone is standing there. And write into the training that a tank hissing louder than usual, or with a swollen neck, is a reason to leave the room, not a maintenance ticket.

Self pressurising supply tanks are built to hold pressure and carry relief valves and burst discs for the purpose. They are a different animal, with a different article.

Cold that does not feel cold at first

At about minus 196°C, liquid nitrogen freezes skin on contact, and CP30 warns that severe damage to skin may be caused by contact with liquid or cold gaseous nitrogen. A splash to the eye is the injury that ends careers. Aluminium canes and goblets lifted from a tank are cold enough to stick to bare fingers.

The cryogenic workwear that prevents this is basic: loose fitting insulated gloves that can be flicked off if liquid runs inside, a full face shield when decanting rather than glasses alone, an apron that liquid runs off, closed shoes. Loose gloves are not a comfort feature. A tight glove full of liquid nitrogen is a glove you cannot remove.

The clipboard that beats the alarm

A well run cryo room has a routine an outsider would find tedious.

• Level dipped daily. A 100 cm dipstick shows the liquid line as a frost mark and does not need a battery.
• Every fill logged: date, vessel, litres. A tank that starts asking for nitrogen more often is telling you its vacuum is going, weeks before any probe will.
• Alarms tested on a schedule. ESHRE’s 2026 good practice recommendations for the IVF laboratory expect cryostorage tanks to be continuously monitored with alarms that detect and log out of range temperature or LN2 level. ASRM’s 2020 cryostorage opinion goes further and recommends level probes in the liquid over temperature probes in the vapour, because level falls first.
• A response plan that names a person for 2 am, and says where the backup vessel is.

Large vessels in the CryoBank Series are listed with level, temperature and power alarms and a remote output. Smaller tanks usually need monitoring added, and if it is not in the purchase order it tends not to arrive.

Decanting: where the incidents cluster

Pouring is where splashes, spills and oxygen dips all happen at once. The single biggest improvement most laboratories can make is to stop tipping vessels. A foot pump dispensing dewar keeps the vessel upright and the operator’s hands away from the stream. The CryoCrest 35L and 50L use a manual aluminium foot pump and can be supplied on a tipping trolley and roller base so nobody is lifting.

Three rules for the pour: fill slowly into a warm receiving vessel, because the first litres boil violently; never fill above the rated line; never walk away from a filling vessel.

The document to have on the shelf

BCGA Code of Practice 30, revision 4 (2024), is the UK industry code for liquid nitrogen in open dewars and dewars fitted with liquid withdrawal devices operating below 0.5 bar. Earlier editions were scoped to vessels up to 50 litres; the current one is defined by pressure, not volume. If your safety file still quotes the 50 litre wording, it is out of date.

Go back to the sum at the top. If the answer for your room was below 19.5 per cent, the rest of this article is not optional reading.

If your cryo room needs a dispensing dewar, monitoring or protective workwear, the Cryolab team can advise on what fits the vessels you already run.