The provisional low temperature scale from 0.9 mK to 1 K, PLTS-2000

An internationally-accepted ultra-loin temperature,scale is needed to provide the basis for reliable thermometry in the temperature range in which commercial dilution refrigerators operate, and in experiments investigating the properties of He-3 and other condensed matter. Several laboratories haze developed He-3 melting-pressure scales, but there are substantial differences even between the most recent of them. These amount to about 0.3 % of T near 500 mK, rising to about 6 % of T at 0.9 mK. In 1996 a collaboration was initiated between low, temperature physicists in national laboratories and elsewhere to derive an equation for the melting pressure of He-3 which could be accepted for international case from 1 K to 0.9 mK, the Neel temperature of solid He-3. After an open workshop in Leiden in 1998, discussions took place to see if thermodynamic calculation of He-3 melting pressures could re-solve the differences. In.January 2000 the authors (apart from ALR and C,5) met at NIST and Mere able to reach a compromise on the Provisional Loco Temperature Scale, PLTS-2000. Its 1-sigma uncertainty is estimated to be 0.3 % of T (up to a maximum of 0.5 mK), but this rises to about 2 % of T at 0.9 mK. The provisional status recognizes that the PLTS-2000 is a compromise, rather than a true consensus, but it is likely to be some years before it can be replaced by a more accurate scale. The scale was announced at the Quantum Fluids and Solids Conference in Minnesota, USA, in,Tune 2000, and was formally adopted by the Comity International des Poids et Mesures in October 2000.