Realisation, dissemination, and comparison of the ITS-90 and the PLTS-2000 below 1 K at PTB

The International Temperature Scale of 1990 (ITS-901 and the Provisional Low Temperature Scale (PLTS-2000)2 are the internationally agreed temperature scales in the low temperature range. At temperatures between 0.65 K and 5 K, the ITS-90 is defined by polynomials relating the vapour pressure pv of 4He and 3He to the temperature T90, whereas the PLTS-2000 is defined by a polynomial relating the 3He megting pressure pm to the temperature T2000 in the range from 0.9 mK to 1 K. Here, we describe the methods and the cryogenic facilities used for the realisation and dissemination of the ITS-90 and the PLTS-2000 as well as the level of accuracy, which is achieved at PTB. We focus on how the pressures are measured traceably to the national pressure standard. Using this experimental basis, we plan to compare directly the vapour and megting pressure of 3He in the overlapping temperature range from 0.65 K to 1 K.