1. International Temperature Scale of 1990 (ITS-90)

For historical reasons, it is still common practice to express temperature in degrees Celsius, symbol °C. The Celsius temperature unit is equal to the kelvin unit by definition. A temperature difference or interval can be expressed in kelvins as well as in degrees Celsius. The numerical value of the Celsius temperature expressed in degrees Celsius is linked to the numerical value of the thermodynamic temperature expressed in kelvins by the relationship: t (in °C) = T (in K) - 273.15 K.

In this article, only the specifications of the ITS-90 will be described, as they are necessary for understanding the origin of the influencing parameters affecting the calibration of a standard probe at fixed...