Chapter 5
Cryogenic Fluids
The two main cryogenic fluids used in Physics Dept. labs are liquid nitrogen and liquid
helium. The three principal hazards associated with these materials are the possibilities of asphyxiation, frostbite and explosion. Death by asphyxiation can occur if the liquids are allowed to boil off or are spilled in confined, poorly ventilated areas. When in the liquid or cold gas phase, they can cause severe frostbite to the eyes or skin. Do not touch frosted pipes or valves, particularly with wet skin. (Yes, the stories you have heard are true, your tongue will freeze to metal at liquid nitrogen temperature and have to be surgically removed. Shown here is a wet rag, frozen to a piece of metal at liquid nitrogen temperature, modeling what can happen to your tongue!) Protect your eyes with safety goggles or a face shield, and cover skin to prevent contact with the liquid or cold gas. Protective gloves that can be quickly and easily removed and long sleeves are recommended for arm protection. Wear cuffless trousers outside boots or over high-top shoes to shed spilled liquid. If accidental exposure occurs that causes an injury, a physician should be consulted immediately. Tissue suffering from the severity of frostbite that can be inflicted by cryogenic fluids may die, eventually leading to gangrene.
Liquid helium is cold enough to freeze air solid. If a helium container is left open to the air, even for a relatively short time, air may freeze and clog the line. Evaporation of helium gas in the container will cause the pressure to rise, leading to a possibly catastrophic result. Although rare, this is the most serious hazard associated with liquid helium. Liquid nitrogen is cold enough to condense liquid oxygen from the air. It can also freeze water vapor from the air into ice. Ice can clog tubes, leading to a pressure explosion. Concentrated liquid oxygen is an explosion hazard and can also greatly enhance the flammability of any nearby combustibles.
All glass dewars must be wrapped with tape to avoid flying glass if the dewar is broken. Do not dispense or transport cryogenic fluids in a container that can be easily broken or spilled; certain plastics can shatter easily when chilled to extremely low temperatures. If a non-standard dewar is to be used, that container must be within a container such as a wooden box to avoid a catastrophic spill. A large spill could expose an individual to frostbite and/or asphyxiation.
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