Cryogenics
Cryogenics, study and use of materials at very low temperatures. The upper limit of cryogenic temperatures has not been agreed on, but the National Institute of Standards and Technology has suggested that the term cryogenics be applied to all temperatures below -150° C (-238° F or 123° above absolute zero on the Kelvin scale). Some scientists regard the normal boiling point of oxygen (-183° C or -297° F), as the upper limit (see Absolute Zero ). Cryogenic temperatures are achieved either by the rapid evaporation of volatile liquids or by the expansion of gases confined initially at pressures of 150 to 200 atmospheres. The expansion may be simple, that is, through a valve to a region of lower pressure, or it may occur in the cylinder of a reciprocating engine, with the gas driving the piston of the engine. The second method is more efficient but is also more difficult to apply. See Heat .