Kelvin (K) Unit Definition

The kelvin is the base unit of temperature in the International System of Units (SI), having the unit symbol K. It is named after the Belfast-born, Glasgow University engineer and physicist William Thomson, 1st Baron Kelvin (1824–1907).

The kelvin is now defined by fixing the numerical value of the Boltzmann constant k to 1.380649×10⁻²³ J⋅K⁻¹. This unit is equal to kg⋅m²⋅s⁻²⋅K⁻¹, where the kilogram, meter and second are defined in terms of the Planck constant, the speed of light, and the duration of the caesium-133 ground-state hyperfine transition. Thus, this definition depends only on universal constants, and not on any physical artifacts as practiced previously, such as the International Prototype of the Kilogram, whose mass diverged over time from the original value. One kelvin is equal to a change in the thermodynamic temperature T that results in a change of thermal energy kT by 1.380649×10⁻²³ J.

The Kelvin scale fulfills Thomson's requirements as an absolute thermodynamic temperature scale. It uses absolute zero as its null point or in other terms zero degree mark. However, unlike the degree Fahrenheit and degree Celsius, the kelvin is not referred to or written as a degree. The kelvin is the primary unit of temperature measurement in the physical sciences but is often used in conjunction with the degree Celsius, which has the same magnitude.

In 1848, William Thomson, who was later created Lord Kelvin, wrote in his paper, On an Absolute Thermometric Scale, of the need for a scale whereby infinite cold or absolute zero was the scale's null point, and which used the degree Celsius for its unit increment. Kelvin calculated that absolute zero was equivalent to −273 °C on the air thermometers of the time. This absolute scale is known today as the Kelvin thermodynamic temperature scale. Kelvin's value of -273 was the negative reciprocal of 0.00366. This is the accepted expansion coefficient of gas per degree Celsius relative to the ice point, giving a remarkable consistency to the currently accepted value.

In 1954, Resolution 3 of the 10th General Conference on Weights and Measures (CGPM) gave the Kelvin scale its modern definition by designating the triple point of water as its second defining point and assigned its temperature to exactly 273.16 kelvins.

In 1968, Resolution 3 of the 13th CGPM renamed the unit increment of thermodynamic temperature kelvin, symbol K, replacing degree Kelvin, symbol °K. Furthermore, feeling it useful to more explicitly define the magnitude of the unit increment, the 13th CGPM also held in Resolution 4 that "The kelvin, unit of thermodynamic temperature, is equal to the fraction ¹⁄_273.16 of the thermodynamic temperature of the triple point of water."

In 2005 the CIPM sought to redefine the kelvin along with the other SI units using a new methodology. In particular, the committee proposed redefining the kelvin such that Boltzmann constant takes the exact value 1.3806505×10^{−23 J}⁄_K. The committee had hoped that the program would be completed in time for its adoption by the CGPM at its 2011 meeting, but at the 2011 meeting the decision was postponed to the 2014 meeting when it would be considered as part of a larger program. The redefinition was further postponed in 2014, pending more accurate measurements of Boltzmann's constant in terms of the current definition, but was finally adopted at the 26th CGPM in late 2018, with a value of k = 1.380649×10^{−23 J}⁄_K.

From a scientific point of view, the main advantage is that this will allow measurements at very low and very high temperatures to be made more accurately, as the techniques used depend on the Boltzmann constant. It also has the advantage of being independent of any particular substance. From a practical point of view, the redefinition will pass unnoticed; water will still freeze at 273.15 K (0 °C), and the triple point of water will continue to be a commonly used laboratory reference temperature.

The difference is that, before the redefinition, the triple point of water was exact and the Boltzmann constant had a measured value of 1.38064903(51)×10^{−23 J}⁄_K, with a relative standard uncertainty of 3.7×10⁻⁷. Afterward, the Boltzmann constant is exact and the uncertainty is transferred to the triple point of water, which is now 273.1600(1) K.

On 16 November 2018, a new definition was adopted, in terms of a fixed value of the Boltzmann constant. With this change the triple point of water became an empirically determined value of approximately 273.16 kelvin. For legal metrology purposes, the new definition officially came into force on 20 May 2019, the 144th anniversary of the Metre Convention.

• Absolute zero (exactly): 0 K

• Boiling point of liquid nitrogen: 77.35 K

• Sublimation point of dry ice: 195.15 K

• Intersection of Celsius and Fahrenheit scales: 233.15 K

• Melting point of H2O (purified ice): 273.1499 K

• Room temperature (NIST standard): 293.15 K

• Normal human body temperature (average): 310.15 K

• Waters boiling point at 1 atm (101.325 kPa) (approximate: see Boiling point): 373.1339 K

Here are the conversion formulas used to convert both from and to kelvin:

• Degrees Celsius or Centigrade (°C)
• Degrees Delisle (°De)
• Degrees Fahrenheit (°F)
• Degrees Newton (°N)
• Degrees Rankine (°Ra)
• Degrees Réaumur (°Ré)
• Degrees Rømer (°Rø)
• Kelvin (K)

• Degrees Celsius or Centigrade (°C)
• Degrees Fahrenheit (°F)

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“Kelvin.” Wikipedia, Wikimedia Foundation, 7 Apr. 2020, en.wikipedia.org/wiki/Kelvin.