On the propagation of the uncertainty at the triple point of water, associated with the calibration of SPRTs according to the ITS-90: A case study

The overall uncertainty between the fixed points underlying the primary calibration of standard platinum resistance thermometers (SPRTs) may be described by terms, each of which is associated with the uncertainty at a given fixed point T-j, within the sub-range covered by the calibration. Each term is calculated, using the appropriate deviation function Delta W(T), by imposing an uncertainty of value zero at the other fixed points. Deviation functions with a non-zero value of the deviation Delta W(T-0) at the triple point of water, T-0, are not defined in the ITS-90, since, nominally, Delta W(T-0) = 0. For this reason the propagation of the uncertainty at T = T-0 is usually accounted for only indirectly through its contribution to the uncertainties delta W(T-j), where j not equal 0. By slightly extending the current mathematical scheme within the formalism provided by the ITS-90 it is shown that the uncertainty delta W(T-0) at the triple point of water is also directly propagated.