Century-Stable Accurate Cryogenic-Temperature Fixed Points: Problems Solved and Problems To Be Solved

摘要

It is now more than a of a quarter century since the first cryogenic triple-point cell was sealed. The cell still exists at IMGC, with most of the more than 150 others produced by IMGC, and reproduces the same temperature value of the triple point within about +- 0.15 mK. Some other cells are not as reproducible, for problems that were studied: most of these problems have now been solved, some have been understood but not solved yet, and a few others are still not understood well enough. The paper discusses the main problems solved since the '70s, the problems still caused by the cell design or by the enclosed substance, and the problem of a definition of the transition temperature that could remove the ambiguities present in the ITS-90 current definition that have arisen as the experimental uncertainty has decreased by about one order of magnitude.