![]() It is the basis of an absolute temperature scale, called the Kelvin scale, whose unit, called a Kelvin (K) rather than a degree, is the same size as the Celsius degree. The existence of absolute zero was first pointed out in 1848 by British mathematician and physicist William Thomson, Lord Kelvin (1824 –1907), and is now known to be –459 ☏ ( –273 ☌). Absolute zero represents the lowest attainable temperature and is associated with the complete absence of atomic and molecular motion. The temperature of a sample, whether it be a gas, liquid, or solid, is a measure of the energy it contains, energy that is present in the form of vibrating atoms and moving molecules. (Absolute zero is defined as the lowest temperature possible, a state where no heat energy exists.) Consequently, the term cryogenic applies to temperatures from approximately –148 ☏( –100 ☌) down to absolute zero. The origin of cryogenics as a scientific discipline coincided with the discovery by nineteenth century scientists, that the permanent gases can be liquefied at exceedingly low temperatures. ![]() Among others, they include oxygen (O), nitrogen (N), hydrogen (H), and helium (He). More specifically, a low-temperature environment is termed a cryogenic environment when the temperature range is below the point at which permanent gases begin to liquefy. The word cryogenics comes from the Greek word kryos, meaning ”cold “ combined with a shortened form of the English verb ”to generate, “it has come to mean the generation of temperatures well below those of normal human experience. Laser cooling and Bose-Einstein condensateĬryogenics is the science of producing and studying low-temperature environments. Since it is a constant-enthalpy process, it can be used to experimentally measure the lines of constant enthalpy (isenthalps) on the ( p, T ) is typically expressed in ☌/ bar (SI units: K/ Pa) and depends on the type of gas and on the temperature and pressure of the gas before expansion.Methods of producing cryogenic temperatures The throttling due to the flow resistance in supply lines, heat exchangers, regenerators, and other components of (thermal) machines is a source of losses that limits their performance. Throttling is a fundamentally irreversible process. In hydraulics, the warming effect from Joule–Thomson throttling can be used to find internally leaking valves as these will produce heat which can be detected by thermocouple or thermal-imaging camera. The gas-cooling throttling process is commonly exploited in refrigeration processes such as liquefiers in air separation industrial process. Most liquids such as hydraulic oils will be warmed by the Joule–Thomson throttling process. At room temperature, all gases except hydrogen, helium, and neon cool upon expansion by the Joule–Thomson process when being throttled through an orifice these three gases experience the same effect but only at lower temperatures. This procedure is called a throttling process or Joule–Thomson process. ![]() In thermodynamics, the Joule–Thomson effect (also known as the Joule–Kelvin effect or Kelvin–Joule effect) describes the temperature change of a real gas or liquid (as differentiated from an ideal gas) when it is forced through a valve or porous plug while keeping it insulated so that no heat is exchanged with the environment. For the concept in computing, see rate limiting.
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