Department of Physics & Astronomy College of Science

Notes on Handling Liquid Nitrogen

1. Cryogenic Fluids: The two main cryogenic fluids used in the Department of Physics and Astronomy labs are liquid nitrogen (LN2) and liquid helium (LHe). 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, valves, or other metal parts that have been in contact with liquid nitrogen, particularly if you have 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.) “Generally, frostbite is accompanied with discoloration of the skin, along with burning and/or tingling sensations, partial or complete numbness, and possibly intense pain.” (Wikipedia) If you observe any of these symptoms, immediately remove the affected body parts from the LN2 transfer system and warm them, e.g. by contact with other body parts or with running water.
2. 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 cuff-less 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.
3. 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. One specific hazard is noteworthy. If you are using LN2 to cool a trap on a vacuum system and leave the system open to air, you will condense liquid oxygen. If your trap is vertical and the liquid oxygen can drip to warmer system parts below, there is no hazard. If the system is horizontal and the LN2 extends to the bottom of the trap, there is a scenario in which you leave the system open to air, condense liquid oxygen, seal the system (close the valve that between the system and the air, then remove the LN2 by simply allowing it to evaporate). In this case, the system is extremely likely to explode. Think about whether this is possible in any vacuum system you use. If it is, how will you assure that it doesn't happen to you?
4. LN2 may only be dispensed into dewars specifically designed for LN2 use. This excludes anything purchased at commercial stores such as Smith's, Home Depot, or designed for coffee. It must be an actual dewar. 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.
5. Issues have arisen concerning riding the elevator with LHe and LN2 containers, specifically in the event the elevator becomes stuck between floors. While this risk in not zero, a much greater concern is dropping and breaking a glass dewar of LN2 in an elevator. Be extra careful not to do this: this act will quickly generate a lot of gas in a confined space.
6. LN2 access shall automatically terminate upon the breach by the user of any terms hereof. Upon termination, departure, or graduation, the user’s access to the compressed gasses shall be disabled. The Department of Physics and Astronomy may also in its sole discretion, and at any time, discontinue providing access to LN2, without notice.