The Kelvin scale is defined by a specific relationship between the pressure of a gas and the temperature. This says that "the pressure of the gas is directly proportional to the temperature in Kelvin". This means that Kelvin is an absolute temperature scale, and scientists use this scale more than any other.
The kelvin is a base SI unit of measurement, since 2018 defined by setting the fixed numerical value of theBoltzmann constantk to 1.380649×10−23 J⋅K−1.[1]
The temperature of thetriple point of water is a hundredth of a degreeCelsius above the freezing point, or 0.01 °C. The coldest possible temperature is calledabsolute zero and is equal to -273.15 degrees Celsius, or zero kelvin (0 K). By writing temperatures in kelvins one does not need to usenegative numbers.
Theabsolute temperature scale was designed so that a change in temperature of 1 kelvin is equal to a change of 1 degree Celsius. This means that it is easy to convert a temperature from degrees Celsius to kelvin.
To change a temperature in degrees Celsius into Kelvins you must add 273.15 units. For example, 0 degrees Celsius (0 °C), which is the temperature at whichwaterfreezes, is 273.15 kelvins (273.15 K).
To change a temperature in Kelvins into degrees Celsius you must subtract 273.15 units. For example, 310 kelvins is 36.85 degrees Celsius, which is roughly the normal temperature of ahuman body.
It is important to notice that the name of this unit is simplykelvin (with a lowercase initial), not "degree Kelvin". In English, it undergoes normalpluralinflection askelvins. For example, the boiling point of liquid nitrogen is 77 kelvins.
In everyday use, the kelvin is most commonly used to measure very low or very high temperatures, such as the temperature of liquid nitrogen or the temperature of a light bulb filament.
Therankine (or degree Rankine) is a related absolute temperature scale, but based on the degreeFahrenheit rather than the degree Celsius.
