Challenges of primary thermometry above 300 K

Historically, the realization of the base unit kelvin was exclusively possible via International Temperature Scales (ITS). The new definition of the kelvin and the accompanying Mise en Pratique of the kelvin (MeP-K) enables the alternative to use primary thermometry for the realization of the kelvin. During the last years, considerable improvements in primary thermometry in the low-temperature range have been achieved. Thereby, above 300 K, still a lot of work is necessary to be competitive with the existing ITS realization. In this contribution, the interplay between primary thermometry, ITS and scale carriers (platinum resistance thermometers, thermocouples) in a historical perspective is sketched. After a short introduction to primary thermometry in general and the established techniques within the MeP-K, the focus will lie on the challenges connected with thermodynamic temperature measurement above 300 K. The advantages and disadvantages of the different techniques will be discussed briefly, and a summary will be given showing that thermodynamic temperature T, TITS and scale carriers are highly connected and improvement in one field relies on improvement in the others. The background of the discussion will be the deficiencies of the existing ITS and thus the requirements for the temperature-measurement community. This article is part of the Theo Murphy meeting issue 'The redefined kelvin: progress and prospects'.