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A measure of the average kinetic energy of the particles in a sample of matter; may be expressed either in kelvin (symbol K) or in degree Celsius (symbol °C).

The thermodynamic temperature (quantity symbol: T) is one of the base quantities in the International System of Quantities (ISQ), on which the International System of Units (SI) is based. Thermodynamic temperature can be measured with a primary thermometer, examples of which are constant volume gas thermometers, acoustic thermometers, or total radiation thermometers.

The unit of thermodynamic temperature is the kelvin (unit symbol: K), which is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

The Celsius temperature (quantity symbol: t or ϑ) is defined as t = T − T0, where T is thermodynamic temperature (see above) and T0 := 273.15 K. The thermodynamic temperature T0 is exactly 0.01 K below the thermodynamic temperature of the triple point of water.

The unit name degree Celsius (unit symbol: °C) is a special name for the kelvin for use in stating values of Celsius temperature: 1 °C := 1 K

The units of thermodynamic and Celsius temperature difference or change are identical. Such differences or changes may be expressed either in kelvin (unit symbol K) or in degree Celsius (unit symbol °C).

The unit degree Rankine (unit symbol °R) is a non-SI unit of thermodynamic temperature T:
1 °R := 5/9 K.
The use of this unit is deprecated.

The unit degree Fahrenheit (unit symbol °F) is a non-SI unit of Fahrenheit temperature tF:
tF/°F := (9/5) (t/°C) + 32 = (9/5) (T/K) − 459.67
The use of this unit is deprecated.

It should be noted that the symbol K (for kelvin), the symbol °C (for degree Celsius), the symbol °R (for degree Rankine), and the symbol °F (for degree Fahrenheit) shall be preceded by a space when one expresses values of temperature.

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