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INTRODUCTION
Cryogens are substances (liquids or fluids) usually at very extreme low temperatures (below –150 °C, –238 °F or 123 K). Cryogenic technology can be defined as the study of the production of low temperature fluids, measurements at low temperatures, and behavior of these materials at low temperature and how to produce them. It is also concerned with the application of low temperature processes and techniques
Low temperature is the temperature obtained by liquefaction of gases whose critical temperatures is below terrestrial temperature. The common gases, which can be liquefied, are: Nitrogen, oxygen, air, hydrogen and helium.
Cryogenic fluids are developed at extremely low temperatures. These are generally fluids which at atmospheric pressure has a boiling point below 25oc . Cryogenic fluids are kept in metallic thermos flasks called Dewar’s flasks. These flasks are never sealed, otherwise there would be a danger or a risk of explosion due to pressure build up as the liquids inside evaporates to form gasses.