Cryogenic Liquid Safety

Properties of common cryogens are listed below.  The common requirements for handling all of these have already been discussed.  Now, cryogen specific hazards will be listed.

COMMON CRYOGENS
CRYOGEN
BOILING POINT
DENSITY
  °C °F K g/L
argon -185.85 -302.53 87.30 1.784
helium -268.93 -452.07 4.22 0.179
hydrogen -252.87 -423.17 20.28 0.090
nitrogen -195.79 -320.33 77.36 1.251
oxygen -182.95 -297.31 90.20 1.429

2. Physical Property Considerations
  • Density
    • The density of air is 1.293 g/L
    • Oxygen and Argon will tend to accumulate at the floor.
    • Helium and Hydrogen will accumulate at the ceiling
    • Nitrogen is very close the the density of air, but might have a very slight tendency to accumulate at the ceiling.

  • Boiling point
    • Oxygen has the highest boiling point of 90.2 K
    • All of the other cryogens can condense oxygen out of the air.
    • All dewars left uncovered will slowly accumulate oxygen.  This can become a problem if an open dewar is continually topped off as nitrogen evaporates, allowing the oxygen concentration to continually increase.
    • Keep dewars covered (but vented) as much as possible to prevent the accumulation of oxygen. 
3. Liquid Oxygen
  • Most people don't use liquid oxygen as a cryogen.  Since it tends to accumulate in other cryogens, it is possible to be handling it without knowing it.  It is therefore prudent to consider liquid oxygen to be present in all operations.
 
    • Keep dewars and equipment clean, especially from organic contamination. Almost pure oxygen can condense in empty containers that are exposed to air and cooled by cryogens. Some of the organic materials that can react violently with oxygen when ignited by a spark or even a mechanical shock are oil, grease, asphalt, kerosene, cloth, tar, and dirt that may contain oil or grease.
    • Some materials may detonate if exposed to liquid oxygen and an ignition source.
4. Liquid Hydrogen
  • Hydrogen is highly flammable and it's use requires specialized training.  Since the use of liquid hydrogen is rare, it will not be covered here.