A cryogen or cryogenic liquid is defined by the National Institute for Standards and Technology (NIST) as any liquid with a boiling point below 93K (-180°C or -240°F) at 1 atmosphere of pressure. This definition includes liquid nitrogen (LN2), liquid argon (LAr), liquid helium (LHe), liquid hydrogen (LH2), and liquid oxygen (LO2), among others. This definition does not include liquid propane, liquid butane, liquid acetylene, or liquefied natural gas (methane).
Liquid helium, liquid hydrogen and liquid oxygen present additional hazards that liquid nitrogen and liquid argon do not. Liquid nitrogen is the most frequently used cryogen on campus.
The following table summarizes the physical properties of common cryogens:
| Cryogen | Boiling Point (1 atm) °C (°F) | Critical Pressure (psig) | Liquid Density (g/L) | Gas Density (27°C) (g/L) | Liquid-to-Gas Expansion Ratio | Type of Gas |
|---|---|---|---|---|---|---|
| Argon | -186(-303) | 710 | 1402 | 1.63 | 860 | Inert |
| Helium | -269(-452) | 34 | 125 | 0.16 | 780 | Inert |
| Hydrogen | -253(-423) | 188 | 71 | 0.082 | 865 | Flammable |
| Nitrogen | -196(-321) | 492 | 808 | 2.25 | 710 | Inert |
| Oxygen | -183(-297) | 736 | 1410 | 1.4 | 875 | Oxygen a |
| Methane | -161(-256) | 673 | 425 | 0.72 | 650 | Flammable |
a Although oxygen does not burn, it will support combustion. Oxygen-enriched atmospheres may lead to violent reactions, such as rapid combustion or explosions, with incompatible materials.
Before the beginning of any experiment or work related to cryogens, all personnel must be familiar with cryogens, equipment, and the system they will be using. It is recommended to create a Standard Operating Procedure (SOP) to include a description of the work being performed and have this available for all lab workers. Include procedural information regarding the use of equipment. The SOP should include the manufacturer’s instructions regarding the use of and controls for commercially obtained cryogen equipment.
Liquid cryogens have an extremely low temperature. Contact with the cryogenic liquids causes not only frostbite or cryogenic burns but can also damage materials like water pipes, flooring, sinks, and electrical cables.
Cryogens are stored in confined containers such as pressurized dewars and cylinders. Cryogenic liquid storage at warm temperature for an extended time can cause flash vaporization and produce extremely high pressure. Therefore, it is important to evaluate the pressure relief valve, venting valve, rupture disk and integrity of the container at least every month or before using it. Cryogenic fluids have large liquid-to-gas expansion ratios:
- LN2 is approximately 696 to 1, (by volume)
- LHe is approximately 757 to 1
- LAr is approximately 847 to 1
These ratios mean that any accidental release or overflow of these cryogenic liquids will quickly boil into gas and create an asphyxiation hazard by displacing the oxygen content of the surrounding area. In the case of liquid argon and liquid nitrogen, the gas generated from malfunctioning equipment or spills will be cold and denser than ambient air. Even well-ventilated lab spaces that have pits or other low-lying areas could have the oxygen displaced by this cold, dense gas. Large volumes of cryogen liquid used in small laboratory spaces or in poorly ventilated areas increase the asphyxiation hazard. Oxygen deficiency monitors should be used in these areas.
The extremely low temperatures associated with cryogenic liquids can easily condense moisture from the air and cause ice formation. This ice can cause components or systems to malfunction (e.g., can plug vent lines and impede valve operation) or can damage piping systems. Carbon steel, metals, plastics, porcelain, and many other materials increase their brittleness in contact with cryogen and become susceptible to failure or malfunction. Therefore, always consult with the most appropriate reference when selecting materials for cryogenic applications.
In many situations LN2 and LHe2 can condense the surrounding air. The concentration of oxygen in the condensed air will increase and lead to oxygen enriched conditions. The concentration of oxygen can reach as high as 80% and amplify flammability and explosion hazards. Air should be prevented from condensing into liquid nitrogen by using loose-fitting stoppers or covers that allow for the venting of nitrogen boil-off gas.
Using cryogens in high ionizing radiation fields can generate ozone or nitrogen oxides. These compounds may cause a potential explosion hazard when the cryogen condenses oxygen from the atmosphere. The best control measure is to minimize the accumulation of oxygen coming into contact with the cryogen and to keep containers free of hydrocarbon contamination.
Other cryogenic liquids present specific hazards in addition to the above concerns. Examples include:
- Liquid oxygen presents enhanced combustion hazards.
- Liquid hydrogen presents additional hazards of flammability and material embrittlement
- Liquid helium presents additional hazards of material embrittlement and solidification of air
Personal Protective Equipment (PPE) while working with cryogens:
Face Shield:
The primary function of a face shield is to protect the head and face from any splashes that may occur, especially during operations that are carried out at eye level (e.g. when topping up reservoirs on electron microscopes). A good face shield should have adjustments to fit many head sizes and should provide good coverage of the entire front of the head, including around the sides. The shield should have a sloped top surface to direct splashes away from the top of the head and to prevent splashes from getting behind the shield from above. Face shields, as per ANSI Z87.1, are highly recommended. Some models have replaceable clear shields so that when the shield becomes scratched or dirty, it can be replaced without purchasing new headgear. You must still wear safety glasses underneath the face shield.
Cryogen gloves:
Cryogen gloves are intended for handling very cold items and to protect against an accidental splash. They are not designed to be submerged in a cryogenic liquid. Cryogen gloves are usually made from a non-porous material and are different from heat resistant gloves. Never used heat resistant gloves to handle cryogen. Cryogen gloves are usually loose fitting and easy to remove quickly. Always inspect the cryogen gloves integrity before each use for rips, tears, or holes where cryogen could get inside.
Close-toed shoes:
Close-toed shoes (without mesh tops) are required when handling cryogens. Shoes should have an enclosed heel and entirely covered tops.
Additional PPE:
A lab coat is required to minimize skin contact. Ear plugs can be worn to protect the ear in case a cryogenic vial explodes. Wear full length pants.
Training
University of Texas at Austin students, faculty, staff, and affiliate researchers who work in labs containing cryogen materials must complete OH 241 Cryogen Safety training.
Transportation of cryogen container
Transportation of cryogenic liquids in elevators poses a potential asphyxiation and fire/explosion risk if workers become trapped in an elevator with a container of cryogen. Pressurized cryogen cylinders are strictly prohibited in passenger elevators. Always use a freight elevator to carry cryogenic containers and cylinders. In any situation, do not store or transport cryogen containing containers in the closed cabin or trunk of a vehicle. Cryogens should never be brought onto a bus or shuttle. Cryogenic liquid cylinders and dewars must be transported in the open bed of a truck or similar vehicle. Do not use a personal vehicle to transport any cryogen container.
Cryogen container transport between and within buildings
Small quantities of liquid nitrogen or liquid argon may be transported in and around buildings provided they are contained within an appropriate low pressure dewar (fitted with a lid) and carried by their handles. Do not transport cryogenic liquids in open dewars. Do not attempt to carry additional items while holding the dewar, such as books, beverages, samples or tools.
Larger, low-pressure dewars may come with casters/wheels attached to the bottom. Otherwise, the dewar may need to be secured to a dolly or cart. The dewar should be kept as low to the ground as possible when using a dolly or cart for transportation. Do not transport low-pressure dewars on the top shelf of a hand cart. Check the entire route ahead of time to ensure that it is free of any obstructions or obstacles that may cause a cart or the dewar to tip.
Heavy cryogenic liquid cylinders must be transported with an appropriate cart designed for the specific cylinder model. Never roll cryogenic liquid cylinders. Appropriate PPE is still required when transporting cryogenic liquids around buildings. Before transporting cryogenic liquid cylinders, the user must ensure that all process valves (gas supply, liquid supply, pressure builder, and vent) are closed.