In physics, cryogenics is the study of the production and behaviour of materials at very low temperatures. It is not well-defined at what point on the temperature scale refrigeration ends and cryogenics begins, but scientists assume it starts at or below −150 °C (123 K; −238 °F). The U.S. National Institute of Standards and Technology has chosen to consider the field of cryogenics as that involving temperatures below −180 °C or −292.00 °F or 93.15 K. This is a logical dividing line, since the normal boiling points of the so-called permanent gases (such as helium, hydrogen, neon, nitrogen, oxygen, and normal air) lie below −180 °C while the Freon refrigerants, hydrogen sulfide, and other common refrigerants have boiling points above −180 °C. (above −150 °C, −238 °F or 123 K). A person who studies elements that have been subjected to extremely cold temperatures is called a cryogenicist. Cryogenicists use the Kelvin or Rankine temperature scales.

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