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Quantity |
Thermodynamic temperature |
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Unit |
kelvin |
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Symbol |
K |
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Definition |
The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. |
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Comments |
| The above definition was adopted in 1953, although the symbol "K" as opposed to " oK" was not adopted until 1968. |
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| The relationship between a temperature T in Kelvin and a temperature t in degrees Celsius, is t/° C = T/K -273.15. |
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| According to BIPM guidelines, direct measurements of the thermodynamic temperature can only be made with one of a small number of so-called primary thermometers. The definition of a primary thermometer is a thermometer whose equation of state can be written down without the introduction of unknown, and possibly temperature-dependent, constants. Examples of such thermometers are the constant-volume gas thermometer, the acoustic gas thermometer, the spectral and total radiation thermometers, and the electronic noise thermometer. |
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| Primary thermometers are accurate to a few mK at temperatures up to 373 K. The use of such accurate thermometers is time-consuming and difficult. For practical use, secondary thermometers, such as the platinum resistance thermometer, are used. |
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| The temperature scale currently in use is the International Temperature Scale of 1990 (ITS-90), which measures from 0.65 K upwards. The scale is based on a defined set of thermodynamic fixed points (that are reproducible) and a defined set of interpolations between them. These interpolations are defined on the helium vapour-pressure equations from 0.65 K to 5 K, interpolating constant-volume gas thermometers from 3 K to 24.5561 K, platinum resistance thermometers from 13.8033 K to 1234.93 K, and the Planck radiation law at higher temperatures. |
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| It can be seen that in several ranges there are two definitions of thermodynamic temperature. Within these ranges each definition is decreed to be equally valid. |
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References |
The International System of Units , Bureau International des Poids et Mesures, 7 th edition, 1998 |
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