The transition from ITS-90 to primary thermometry above 1235 K

Area of Science:

• Metrology
• Thermodynamics
• Physics

Background:

• The international system of units (SI) redefined the kelvin (K) based on fundamental constants, requiring practical realization methods (mise en pratique - MeP).
• Primary thermometry techniques provide thermodynamic temperature (T), complementing existing practical scales like the International Temperature Scale of 1990 (ITS-90).

Purpose of the Study:

• To describe and compare primary methods for measuring thermodynamic temperature (T) against ITS-90 (T90) above the silver point (1234.93 K).
• To review absolute and relative primary radiometric thermometry, assessing their uncertainties and feasibility for realizing the redefined kelvin.

Main Methods:

• Radiometric thermometry utilizing Planck's radiation law in absolute and relative forms.
• Comparison of primary thermometry measurements with ITS-90 (T90) reference points.
• Feasibility and uncertainty analysis of different primary thermometry approaches.

Main Results:

• Primary radiometric thermometry is a viable method for realizing thermodynamic temperature (T) above the silver point.
• Both absolute and relative radiometric methods offer different advantages and disadvantages regarding uncertainty and practicality.
• Examples from the Centro Español de Metrología illustrate the application and comparison of these methods.

Conclusions:

• The mise en pratique for the kelvin (MeP-K) enables accurate thermodynamic temperature (T) realization above the silver point using primary radiometric thermometry.
• Primary thermometry provides a more fundamental measurement of temperature (T) compared to the practical ITS-90 scale (T90).
• Understanding the performance of these primary methods is crucial for disseminating the redefined kelvin accurately.