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Cryogenics
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<p>In <a href="/facts/Physics/a7aH2y08">physics</a>, cryogenics is the production and behaviour of materials at very low <a href="/facts/Temperature/QyGe4gmj">temperatures</a>. </p><p>The 13th <a href="/facts/International_Institute_of_Refrigeration/GgWs8MxH">International Institute of Refrigeration</a>'s (IIR) International Congress of Refrigeration (held in Washington, DC in 1971) endorsed a universal definition of "cryogenics" and "cryogenic" by accepting a threshold of 120 K (−153 °C) to distinguish these terms from conventional refrigeration. This is a logical dividing line, since the normal <a href="/facts/Boiling_point/E2BK9QoS">boiling points</a> of the so-called permanent <a href="/facts/Gas/hVVAoodV">gases</a> (such as <a href="/facts/Helium/yRul93TG">helium</a>, <a href="/facts/Hydrogen/BoYHjl0J">hydrogen</a>, <a href="/facts/Neon/5bF1FwGL">neon</a>, <a href="/facts/Nitrogen/Nf1QpGDz">nitrogen</a>, <a href="/facts/Oxygen/acyn6o8r">oxygen</a>, and normal <a href="/facts/Air/hQ95Jznz">air</a>) lie below 120 K, while the <a href="/facts/Freon/JkfAjIb9">Freon</a> refrigerants, <a href="/facts/Hydrocarbon/Dcg95Ju5">hydrocarbons</a>, and other common refrigerants have boiling points above 120 K. </p><p>Discovery of <a href="/facts/Superconducting/pqQAlSzJ">superconducting</a> materials with critical temperatures significantly above the boiling point of nitrogen has provided new interest in reliable, low-cost methods of producing high-temperature cryogenic refrigeration. The term "high temperature cryogenic" describes temperatures ranging from above the boiling point of liquid nitrogen, −195.79 °C (77.36 K; −320.42 °F), up to −50 °C (223 K; −58 °F). The discovery of superconductive properties is first attributed to <a href="/facts/Heike_Kamerlingh_Onnes/bJk48dFD">Heike Kamerlingh Onnes</a> on July 10, 1908, after they were able to reach a temperature of 2 K. These first superconductive properties were observed in mercury at a temperature of 4.2 K. </p><p>Cryogenicists use the <a href="/facts/Kelvin/E3trzC4C">Kelvin</a> or <a href="/facts/Rankine_scale/ya5sNd2C">Rankine</a> temperature scale, both of which measure from <a href="/facts/Absolute_zero/U3UyPl2K">absolute zero</a>, rather than more usual scales such as <a href="/facts/Celsius/okAEu1CA">Celsius</a> which measures from the freezing point of water at sea level or <a href="/facts/Fahrenheit/2TVmyJod">Fahrenheit</a> which measures from the freezing point of a particular brine solution at sea level. </p>