Low uncertainty thermodynamic temperature above the silver point using relative primary radiometry.

The mise-en-pratique for the definition of the kelvin has given the possibility of relative primary thermometry with uncertainties competitive with the ITS-90 above the silver freezing point. The mise-en-pratique does not constrain users to any particular experimental method to realise the kelvin, so while there is supporting documentation it may not be obvious to everyone how to actually establish SI traceable temperatures. NPL has established thermodynamic temperature above the silver point using high-temperature fixed-points. This approach provides access to thermodynamic temperature with lower uncertainties than can be routinely achieved through absolute primary radiometry. We demonstrate direct traceability to the kelvin and that this is more easily and robustly achieved than its equivalent ITS-90 approximation. [ABSTRACT FROM AUTHOR]