Temperature Sensors

Kelvin degree

Kelvin degree

The Kelvin is the temperature unit of the International System. The Kelvin is one of the seven basic units of temperature. Its symbol in the international system of units is K. The Kelvin scale is a thermodynamic (absolute) temperature scale where the absolute zero, the theoretical absence of energy, is zero (0 K). This unit is named after the British physicist, mathematician and engineer William Thomson (1824 - 1907), who was later named Lord Kelvin. Lord Kelvin wrote about the need for a "thermodynamic temperature scale".

To express the temperature difference or the interval, the use of kelvins instead of degrees centigrade helps to avoid situations in which these quantities are confused with the centigrade temperature. The Kelvin is the unit of temperature suitable for use in derived units, such as W / (m · K) or to have a prefix, such as milli in mK.

Units of measurement of the Kelvin scale

Unlike degrees Fahrenheit and Celsius degrees, kelvin is not a "degree", nor should it be written with the degree symbol. The correct name of the unit is Kelvin (started with lowercase). Sometimes you can see written as Kelvin degree, or with the symbol ° K by analogy with the degree Celsius, but it is not correct. Their units are kelvins and they express with a single K.

The Kelvin was defined in 1954 as Kelvin grade in the 10th General Conference of Weights and Measures. By decision taken in 1967 at the 13th CGPM the name becomes the current Kelvin.

Definition of kelvin

The kelvin is defined based on 2 factors,

  • Zero Kelvin correspond to the absolute zero, which is the minimum temperature that can be achieved, and corresponds to the lack of movement at the molecular level. This temperature equals -273.15 degrees Celsius, and at -459.67 degrees Fahrenheit
  • A kelvin is exactly 1 / 273.16 of the thermodynamic temperature of the triple point of water. A difference of one kelvin is equivalent to one degree Celsius, currently, the Celsius scale is defined from the kelvin and the Celsius degree is a unit derived from the international system of units.
  • In 2007, the 23rd CGPM adopted a resolution in which it clarified that the water to which the Kelvin definition refers must have a specific isotropic composition.

    Practical uses of the kelvin

    The Kelvin degree is used in many aspects of physics. However, we present two applications in which the kelvin has a special importance.

    Color temperature

    Kelvin is often used to measure the color temperature of light sources. The color temperature is based on the principle that a black body radiator emits light whose color depends on the temperature of the radiator. Black bodies with temperatures below about 4000 K seem reddish, while those above about 7500 K appear bluish.

    Color temperature is important in the image projection and photography fields, where a color temperature of approximately 5600 K is required to match the "daylight" film emulsions. In astronomy, the stellar classification of stars and their place in the Hertzsprung-Russell diagram are based, in part, on their surface temperature, known as effective temperature. The photosphere of the Sun, for example, has an effective temperature of 5778 K.

    Digital cameras and photographic software often use the color temperature in K in the editing and configuration menus. The simple guide is that the higher the color temperature, the whiter or blue the image will be. The reduction in color temperature will give an image more dominated by reddish, "warmer" colors.

    Kelvin as a measure of noise

    In electronics, the kelvin is used as an indicator of how noisy a circuit is in relation to the final noise floor, that is, the noise temperature. The so-called Johnson-Nyquist noise of discrete resistors and capacitors is a type of thermal noise derived from the Boltzmann constant and can be used to determine the noise temperature of a circuit that uses Friis formulas for noise.

    History of the Kelvin scale

    The Kelvin scale and its unit have had different development milestones throughout history:

    1848: William Thomson, first Baron of Kelvin wrote in his diary On the absolute thermometric scale, of the necessity of a scale where "the infinite cold" (absolute zero) was the null point of the ladder, and that he used the degree Celsius for its increase in unity. This absolute scale is known today as the Kelvin thermodynamic temperature scale. It is notable that Thomson's value of -273.15 actually obtained 0.00366, which was the accepted coefficient of expansion of the gas per degree Celsius relative to the freezing point. The inverse of -0.00366 expressed with five significant digits is -273.22 ° C which is extraordinarily at the edge of the true value of -273.15 ° C.

    1954: Resolution 3 of the 10th CGPM gave the Kelvin scale its modern definition, designating the triple point of water as its second point that defines and assigns its temperature to exactly 273.16 degrees Kelvin.

    1967/1968: Resolution 3 of the 13th CGPM renamed the increase in the thermodynamic temperature as kelvin, symbol K, which replaced absolute degree, symbol K. Moreover, it is useful to define more explicitly the magnitude of the increase, the 13th CGPM also affirms at 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.

    2005: The International Committee of Weights and Measures (CIPM), a CGPM committee, stated that for the purposes of delineating the temperature of the triple point of water, the definition of the thermodynamic temperature scale Kelvin would refer to water with a Isotope composition specified in the Vienna Standard Mean Ocean Water (VSMOW).

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    Last review: August 28, 2017