Low uncertainty Boltzmann constant determinations and the kelvin redefinition

Area of Science:

• Metrology and fundamental physics.
• Thermodynamics and temperature scales.

Background:

• The 25th General Conference on Weights and Measures (CGPM) approved a path for redefining base units of the International System of Units (SI).
• Key base units including the kilogram, ampere, kelvin, and mole are slated for redefinition based on fixed numerical values of fundamental constants.

Purpose of the Study:

• To outline the planned redefinition of SI base units, focusing on the kelvin.
• To report on the progress and perspectives of measuring the Boltzmann constant for the new kelvin definition.
• To discuss the implications of the revised SI, particularly for temperature measurements.

Main Methods:

• The kelvin will be defined by the Boltzmann constant (k), where 1 kelvin equals the change in thermodynamic temperature resulting in a thermal energy change of kT by 1.380 65×10⁻²³ J.
• Primary thermometers like acoustic gas thermometers, dielectric constant gas thermometers, noise thermometers, and Doppler broadening techniques are used to determine the Boltzmann constant.

Main Results:

• Progress in measurements using various primary thermometry techniques is reported.
• Necessary conditions for the redefinition are detailed, ensuring accuracy and consistency.

Conclusions:

• The redefinition represents a significant step towards a more stable and universally applicable SI.
• The new definition of the kelvin based on the Boltzmann constant will impact thermodynamic temperature measurements.