Refractive-index gas thermometry.

Area of Science:

• Thermodynamics and Metrology
• Gas Thermometry
• Optical Physics

Background:

• Primary refractive-index gas thermometry (RIGT) is a key technique for high-precision thermometry.
• Advancements in resonator design and measurement techniques have improved RIGT capabilities.
• Accurate temperature measurements are crucial for fundamental physics and metrology.

Purpose of the Study:

• To review the principles and techniques of primary refractive-index gas thermometry.
• To present implementations of RIGT using microwave and optical frequencies.
• To provide uncertainty budgets for various RIGT configurations.

Main Methods:

• Absolute primary RIGT utilizing microwave measurements with helium-filled quasispherical resonators.
• RIGT near ambient temperature using argon-filled cylindrical microwave resonators.
• Optical frequency RIGT for Boltzmann constant measurements.

Main Results:

• Implementation of absolute primary RIGT at neon, oxygen, argon, and water triple points with uncertainties from 9.1 × 10⁻⁶ to 3.5 × 10⁻⁵.
• Relative RIGT measurements near ambient temperature achieved uncertainties between 3.8 × 10⁻⁵ and 4.6 × 10⁻⁵.
• A Boltzmann constant measurement using optical RIGT yielded a relative standard uncertainty of 1.2 × 10⁻⁵.

Conclusions:

• Primary RIGT, particularly microwave and optical implementations, provides highly accurate temperature measurements.
• The technique is applicable across a range of temperatures, from cryogenic to room temperature.
• Further development in optical RIGT shows promise for even greater precision in metrology.