A gas is defined as cryogenic if it can be liquified at or below 123 K at atmospheric conditions. The unit measurement for the temperature of cryogenics is Kelvin (0K = -273 C).
A cryogenic gas has no colour and no odour, so it is very difficult to detect its presence (human senses do not warn!).
At CERN, the most commonly used cryogenic fluids are helium, argon and nitrogen:
Helium: is lighter than air (up) – at atmospheric pressure, it becomes liquid at temperatures lower than 4K.
Argon: it is heavier than air (down) – at atmospheric pressure, it becomes liquid at 87K.
Nitrogen: if cold, it is heavier than air (down) – at atmospheric pressure, it becomes liquid at 77K.
The main cryogenic hazards are:
- Cold surfaces and vapor;
- Oxygen Deficiency Hazard - ODH;
- Pressure release.
ODH is particularly dangerous, if we consider that one litre of cryogenic fluid expands to ~700 litres (0.7 m3) of gas at ambient temperature. The presence of these gas in the air decreases the amount of oxygen.
CERN Safety Rules
The CERN safety documents concerning the cryogenic hazards are the following
- Cryogenic Safety – Awareness (online)
- Cryogenic Safety – Fundamentals (classroom)
- Cryogenic Safety – Helium Transfer (classroom)
The classroom trainings should be followed up by people working with cryogenic liquids.
For further information about the cryogenic hazard, please contact:
- HSE Unit Expert – Gunnar Lindell