Calibration uncertainty at fixed points
Uncertainties in calibration of long stem platinum resistance thermometer according to the ITS-90.

The results of calibration of the standard probe according to ITS-90 will be presented in the form of a table corresponding to the temperature of each fixed point (T₉₀) a reduced resistance W (T₉₀) and an associated uncertainty expressed in temperature.

To calculate this uncertainty, we need to develop a mathematical model of the measurement that will allow us to introduce all the influencing parameters identified.

Let's consider the determination of the W(PF) presented by the standard probe at a given fixed point. The measurement carried out at the triple point of water can be modelled by the following expression :