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Kelvin

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Kelvin temperature conversion formulas
Conversion from to Formula
kelvins degrees Celsius °C = K - 273.15
degrees Celsius kelvins K = °C + 273.15
kelvins degrees Fahrenheit °F = K × 1.8 − 459.67
degrees Fahrenheit kelvins K = (°F + 459.67) / 1.8
kelvins electronvolts K / 11,604.5 = eV
electronvolts kelvins eV × 11,604.5 = K
Note that for temperature intervals rather than temperature readings,
1 K = 1 °C and 1 K = 1.8 °F
Comparisons among various temperature scales
Conversion calculator for units of temperature

The kelvin (symbol: K) is the SI unit of temperature and is one of the seven SI base units. The kelvin is a unit measure of temperature on the thermodynamic (absolute) temperature scale.

The unit “kelvin” and its scale are, by international agreement, defined by two points: absolute zero, and the triple point of specially prepared ocean water. Absolute zero is defined as being precisely 0 K and –273.15 °C. Absolute zero is where all kinetic motion in the particles comprising matter ceases and they are at complete rest in the “classic” (non-quantum mechanical) sense. At absolute zero, matter contains no heat energy. Also, the triple point of water is defined as being precisely 273.16 K and 0.01 °C. 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 °C and 273.16 K = 0.01 °C). Temperatures in kelvin can be converted to other units per the table at top right.

Some key temperatures relating kelvin values to other scales are shown in the below table.

Point Kelvin Celsius Fahrenheit
Absolute zero

(precisely by definition)

0 K –273.15 °C −459.67 °F
Water's freezing point 273.15 °K 0 °C 32 °F
Water’s triple point

(precisely by definition)

273.16 K 0.01 °C 32.018 °F
Water’s boiling pointTemplate:Sup 373.1339 K 99.9839 °C 211.9710 °F

Template:Sup For Vienna Standard Mean Ocean Water at one standard atmosphere (101.325 kPa) when calibrated solely per the two-point definition of thermodynamic temperature.

The kelvin is named after the British physicist and engineer William Thomson, 1st Baron Kelvin; his barony was in turn named after the River Kelvin, which runs through the grounds of the University of Glasgow.

Contents

Typographical conventions

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 is more ambiguous, since it can 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".[Webster's 11th Collegiate; NIST SP 811]

Still, the kelvin scale and the individual unit comprising the scale share a common name. Resolution 3 of the 13th CGPM (1967/68) declared that “…the unit of thermodynamic temperature and the unit of temperature interval are one and the same unit [and] ought to be denoted by a single name and single symbol.” [1] Thus, 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”).

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. 45° 3′ 4″, 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.

Unicode includes the "kelvin sign" at U+212A (in your browser it looks like K). However, the "kelvin sign" is canonically decomposed into U+004B, thereby seen as a (preexisting) encoding mistake, and it is better to use U+004B (K) directly.

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. “…its measured value was 0.01023 °C with an uncertainty of 70 µK…”). This issue had been addressed by Resolution 3 of the 13th CGPM (1967/68), which stated that “a temperature interval may also be expressed in degrees Celsius” [1] yet this practice of using both “°C” and “K” remains widespread throughout the technical world.Template:Cite-needed

The kelvin is particularly well-suited for expressing differences between temperatures and their uncertainties (e.g. “Agar exhibited a melting point hysteresis of 25 kelvins,” and “The error was 10 millikelvins”). Of course, the kelvin is also used to express specific temperatures along its absolute scale (e.g. “The triple point of hydrogen is 13.8033 kelvin”). Note however, that when referring to a particular temperature on the kelvin scale, the singular form for kelvin (no “s” at the end) is used; when referring to uncertainties and differentials, the plural form is used.Template:Cite-needed

Differentials, uncertainties, and specific temperatures all utilize the 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 exist, the full unit name in its plural form, kelvins, is preferred when expressing uncertainties and differentials.

This practice of using both “°C” 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.

Comparison of temperature scales

Comparison of temperature scales
Comment Kelvin Celsius Fahrenheit Rankine Delisle Newton Réaumur Rømer
Absolute zero 0 −273.15 −459.67 0 559.725 −90.14 −218.52 −135.90
Coldest recorded surface temperature on Earth
(Vostok, Antarctica - July 21, 1983)
184 −89 −128.2 331.47 283.5 −29.37 −71.2 −39.225
Fahrenheit's ice/salt mixture 255.37 −17.78 0 459.67 176.67 −5.87 −14.22 −1.83
Water freezes (at standard pressure) 273.15 0 32 491.67 150 0 0 7.5
Average surface temperature on Earth 288 15 59 518.67 127.5 4.95 12 15.375
Average human body temperature ¹ 309.95 36.8 98.24 557.91 94.8 12.144 29.44 26.82
Hottest recorded surface temperature on Earth
(Al 'Aziziyah, Libya - September 13, 1922)
331 58 136.4 596.07 63 19.14 46.4 37.95
Water boils (at standard pressure) 373.15 100 212 671.67 0 33 80 60
Titanium melts 1941 1668 3034 3494 −2352 550 1334 883
The surface of the Sun 5800 5526 9980 10440 −8140 1823 4421 2909

¹ Normal human body temperature is 36.8 °C ±0.7 °C, or 98.2 °F ±1.3 °F. The commonly given value 98.6 °F is simply the exact conversion of the nineteenth-century German standard of 37 °C. Since it does not list an acceptable range, it could therefore be said to have excess (invalid) precision. Here's a list of various measurements.
Some numbers in this table have been rounded off.


See also

References

Template:TemperatureScales

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