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Kelvin is a way of measuring temperature.

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Kelvin scale is a thermodynamic (absolute) temperature scale where absolute zero-the lowest possible temperature where nothing could be colder and no heat energy remains in a substance-is defined as zero kelvin (0 K). The unit increment of the Kelvin scale is the kelvin (symbol: K), which is the SI unit of temperature and is one of the seven SI base units.

kelvin and its scale are defined by two points by international agreement. Image: William Thomson, 1st Baron Kelvin.

The Kelvin scale is named after British physicist and engineer William Thomson, 1st Baron Kelvin

Definition of Kelvin.

The unit 'kelvin' and its scale are defined by two points by international agreement: absolute zero, and the triple point of specially prepared (VSMOW) water. This new definition also precisely relates the Kelvin scale to the Celsius scale. Absolute zero-the temperature at which nothing could be colder and no heat energy remains in a substance-is defined as being precisely 0 K and -273.15 ºC. The triple point of water is defined as being precisely 273.16 K and 0.01 ºC. This definition does three things: 1) it fixes the magnitude of the kelvin unit as being precisely 1 part in 273.16 parts the difference between absolute zero and the triple point of water; 2) it establishes that one kelvin has precisely the same magnitude as a one-degree increment on the Celsius scale; and 3) it establishes the difference between the two scales' null points as being precisely 273.15 kelvins (0 K = -273.15 ºC and 273.16 K = 0.01 ºC). Temperatures in Kelvin can be converted to other units per the table at top right.

Some key temperatures relating temperatures on the Kelvin and Celsius scales are shown in the below table.

Kelvin Celsius Fahrenheit
Absolute zero

(precisely, by definition)

0 K -273.15 ºC Â459.67 ºF
Melting point of ice 273.15 K 0 ºC 32 ºF
Water's triple point

(precisely, by definition)

273.16 K 0.01 ºC 32.018 ºF
Water's boiling pointA 373.1339 K 99.9839 ºC 211.9710 ºF

A For Vienna Standard Mean Ocean water at one standard atmosphere (101.325 kPa) when calibrated solely per the two-point definition of thermodynamic temperature. Older definitions of the Celsius scale once defined the boiling point of water under one standard atmosphere as being precisely 100 ÂºC. However, the current definition results in a boiling point that is actually 16.1 mK less. For more about the actual boiling point of water, see VSMOW water in temperature measurement.

SI prefixed forms of kelvin

SI prefixes are often employed to denote decimal multiples and submultiples of the kelvin. The most commonly used factors of kelvin are listed below

Submultiples Multiples
Factor Name Symbol Factor Name Symbol
10Â3 millikelvin mK 103 kilokelvin kK
10Â6 microkelvin ÂK 106 megakelvin MK
10Â9 nanokelvin nK 109 gigakelvin GK
10Â12 picokelvin pK 1012 terakelvin TK

Kelvin: Typographical and usage conventions. Uppercase/lowercase and plural form usage.

When reference is made to the unit kelvin (either a specific temperature or a temperature interval), kelvin is always spelled with a lowercase k unless it is the first word in a sentence. When reference is made to the Kelvin scale, use an uppercase K.

Until the 13th General Conference on Weights and Measures (CGPM) in 1967-1968, this unit was called a degree just as other temperature degrees are, distinguished from other degrees with the adjective 'Kelvin' in 'degrees Kelvin,' or often as 'degrees absolute' (which was more ambiguous, since it could also refer to degrees Rankine). When the units were degrees, the plural was formed by adding an 's' to degree and like other adjectives in English, the adjective identifying the scale was unchanged in the plural. After the name change, the plural of kelvin is 'kelvins.'[3] When reference is made to the 'Kelvin scale,' the word 'kelvin'-which is normally a noun-functions adjectivally to modify the noun 'scale' (like 'Georgia peach') and is capitalized.

Kelvin temperatures and intervals:

Conversion fromtoFormula
Kelvin Celsius ºC = K - 273.15
Celsius Kelvin K = ºC + 273.15
Kelvin Fahrenheit ºF = K - 1.8  459.67
Fahrenheit Kelvin K = (ºF + 459.67) / 1.8
Kelvin electronvolts K / 11,604.5 = eV
electronvolts Kelvin eV - 11,604.5 = K
Note that for temperature intervals rather than temperature readings,
1 K = 1 ºC and 1 K = 1.8 ºF
Comparisons among various temperature scales
Conversion calculator for units of temperature.

Because the kelvin is an individual unit of measure, it is particularly well-suited for expressing temperature intervals: differences between temperatures or their uncertainties (e.g. 'Agar exhibited a melting point hysteresis of 25 kelvins,' and 'The uncertainty was 10 millikelvins'). Of course, the kelvin is also used to express specific temperatures along its scale (e.g. 'Gallium melts at 302.9146 kelvin'). Note that to avoid confusion, especially when temperatures and uncertainties both appear in the same document, the singular form of kelvin (no 's' at the end) is preferred when referring to temperatures; the plural form is preferred when referring to intervals.

One disadvantage of the kelvin is that intervals and specific temperatures on the Kelvin scale both utilize the exact same symbol (e.g. 'Agar exhibited a melting point hysteresis of 25 K,' and 'The triple point of Hydrogen is 13.8033 K'). Thus, wherever ambiguity might arise due to the dual use of the symbol K within a document, it is preferable to use the symbol for denoting temperatures and to express the intervals using the full unit name in its plural form, kelvins, (e.g. 'The helium temperature was 650 mKÂ and our standard deviation in this set of experiments was 15 millikelvins.')

Kelvin formatting and typestyle for the K symbol.

The kelvin symbol is always a roman (non-italic) capital K since the lowercase version is the SI prefix for 1 - 103. The admonition against italicizing the symbol K applies to all SI unit symbols; only symbols for variables and constants (e.g. P = pressure, and c = 299,792,458 m/s) are italicized in scientific and engineering papers. As with most other SI unit symbols (angle symbols, e.g. 45º 3 4Â, are the exception,) there is a space between the numeric value and the kelvin symbol (e.g. '99.987 K'). For more information on conventions used in technical writing, see the informative SI Unit rules and style conventions by the NIST.

The special Unicode kelvin sign.

Unicode, which is an industry standard designed to allow text and symbols from all of the writing systems of the world to be consistently represented and manipulated by computers, includes a special 'kelvin sign' at U+212A. One types K when encoding this special kelvin character in a Web page. Its appearance is similar to an ordinary uppercase K. To better see the difference between the two, below in maroon text is the kelvin character followed immediately by a simple uppercase K:

When viewed on computers that properly support Unicode, the above line appears as follows (size may vary):

Depending on the operating system, Web browser, and the default font, the 'K' in the Unicode character may be narrower and slightly taller than a plain uppercase K; precisely the opposite may be true on other platforms. However, there will usually be a discernible difference between the two. If the computer being used to view a particular Web page doesn't support the Unicode kelvin sign character (-), it may be canonically decomposedbrowser into U+004B (uppercase K) and the two would appear identical. In still other computers, the kelvin symbol is mapped incorrectly and produces an odd character.

Accordingly, for Web use, it is better to use the simple uppercase K to represent the kelvin symbol so it can be properly viewed by the widest possible audience.

Why technical articles use a mix of Kelvin and Celsius scales.

In science (especially) and in engineering, the Celsius and Kelvin scales are often used simultaneously in the same article (e.g. 'Âits measured value was 0.01023 ºC with an uncertainty of 70 ÂKÂ') This issue had been addressed by both Resolution 3 of the 13th CGPM (1967/68) which stated that 'a temperature interval may also be expressed in degrees Celsius' and by Resolution 7 of the 9th CGPM (1948) which stated 'To indicate a temperature interval or difference, rather than a temperature, the word Âdegree' in full, or the abbreviation Âdeg' must be used.' Yet the practice of simultaneously using both 'ºC' and 'K' remains widespread throughout the technical world as the use of prefixed forms of 'degrees C' (such as ' deg C') to express a temperature interval has not been well-adopted.

This practice of using both 'ºC' and 'K' is usually avoided in non-technical articles intended for the general public where both the kelvin and its symbol, K, are not well recognized and could be confusing.

History of the Kelvin scale.

Below are some historic milestones in the development of the Kelvin scale and its unit increment, the kelvin. For more on the history of thermodynamic temperature, see Thermodynamic temperature: History of thermodynamic temperature.

  • 1848: William Thomson, (1824 - 1907) also known as 'Lord Kelvin,' wrote in his paper, On an Absolute Thermometric Scale, of the need for a scale whereby 'infinite cold' (absolute zero) was the scale's null point, and which used the degree Celsius for its unit increment. Thomson calculated that absolute zero was equivalent to -273 ºC on the air-thermometers of the time. This absolute scale is known today as the Kelvin thermodynamic temperature scale. It's noteworthy that Thomson's value of '-273' was actually derived from 0.00366, which was the accepted expansion coefficient of gas per degree Celsius relative to the ice point. The inverse of -0.00366 expressed to four significant digits is -273.2 ºC which is remarkably close to the true value of -273.15 ºC. .
  • 1954: Resolution 3 of the 10th CGPM (Conférence Générale des Poids et Mesures, also known as the General Conference on Weights and Measures) gave the Kelvin scale its modern definition by choosing the triple point of water as its second defining point and assigned it a temperature of precisely 273.16 kelvin (what was actually written 273.16 'degrees Kelvin' at the time). .
  • 1967/1968: Resolution 3 of the 13th CGPM created the unit increment of thermodynamic temperature (as distinct from the scale) and gave it the name 'kelvin', symbol K, instead of 'degree Kelvin', symbol ºK. In so doing, feeling it useful to explicitly define the magnitude of this new unit increment, 13th CGPM also decided in Resolution 4 that 'The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.'.
  • 2005: The CIPM (Comité International des Poids et Mesures, also known as the International Committee for Weights and Measures) affirmed that for the purposes of delineating the temperature of the triple point of water, the definition of the Kelvin thermodynamic temperature scale would refer to water having an isotopic composition defined as being precisely equal to the nominal specification of VSMOW water. .

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