What is the base SI unit for thermodynamic temperature?

The base SI unit for thermodynamic temperature is Kelvin. The Kelvin scale is an absolute temperature scale that starts at absolute zero, the point at which all molecular motion ceases. This unit is used universally in scientific contexts.

Kelvin is defined based on the triple point of water, which occurs at exactly 273.16 K. This provides a reliable and reproducible reference point. The significance of the Kelvin scale is that it is directly related to thermodynamic principles, which is essential for calculations involving heat and energy.

Other temperature scales like Celsius and Fahrenheit are not SI units; instead, they are derived from the Kelvin scale. Celsius is defined relative to the melting point of ice and the boiling point of water, while Fahrenheit is primarily used in the United States and is based on different fixed points. Rankine is another temperature scale, primarily used in engineering systems, specifically thermodynamics, but it is also not an SI unit. It is simply a version of the Fahrenheit scale adjusted to absolute zero, making it less commonly used in international scientific contexts compared to Kelvin.