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Triple point
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== Triple-point cells == Triple-point cells are used in the [[calibration]] of [[thermometer]]s. For exacting work, triple-point cells are typically filled with a highly pure chemical substance such as hydrogen, argon, mercury, or water (depending on the desired temperature). The purity of these substances can be such that only one part in a million is a contaminant, called "six nines" because it is 99.9999% pure. A specific [[isotope|isotopic]] composition (for water, [[Vienna Standard Mean Ocean Water|VSMOW]]) is used because variations in isotopic composition cause small changes in the triple point. Triple-point cells are so effective at achieving highly precise, reproducible temperatures, that an international calibration standard for thermometers called [[International Temperature Scale of 1990|ITSβ90]] relies upon triple-point cells of [[hydrogen]], [[neon]], [[oxygen]], [[argon]], [[mercury (element)|mercury]], and [[water (molecule)|water]] for delineating six of its defined temperature points.
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