Patrick Rourke, Christof Gaiser, Roberto M. Gavioso, Michael R. Moldover, Laurent Pitre, Robin Underwood
The principles and techniques of primary refractive-index gas thermometry (RIGT) are reviewed. Absolute primary RIGT using microwave measurements of helium-filled quasi-spherical resonators has been implemented at the temperatures of the triple points of neon, oxygen, argon and water, with relative standard uncertainties ranging from 9.1 x 10-6 to 3.5 x 10-5, and a sample uncertainty budget for this approach is provided. Researchers are now adapting cylindrical microwave resonators for RIGT at high temperatures and conducting relative RIGT measurements on isobars at low temperatures. RIGT at optical frequencies is progressing.