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Consistent Low-Temperature Scale Adopted

The International Committee for Weights and Measures (CIPM) has adopted the PLTS-2000 as a low-temperature scale. For the first time the CIPM now has an internationally valid and consistent temperature scale for thermometry in the millikelvin range. The new scale utilises the melting pressure of helium-3 for temperature measurement and is based, among other things, on measurements performed by PTB.

Cryostat insert of PTB used to generate very low temperatures. The nuclear demagnetisation stage at the lower end consists of about 2 kg of high-purity copper. The initial magnetic induction is 8 T. Final temperatures of 65 µK can be generated and long-term temperatures of 300 µK can be maintained.

With defined temperature fixed points, standard thermometers and interpolation procedures, the International Temperature Scale of 1990 (ITS-90) enabled consistent temperature measurements to be performed down to the ITS-90 lower limit of 0,65 K. For temperatures below that the situation was confusing until now. Scientists could choose among several scales leading to different temperature values, in particular, below 6 mK. This unsatisfactory situation has now been resolved by adopting a low-temperature scale for the range from 0,9 mK to 1 K.

The new scale is based on the melting-pressure curve of 3He. The new scale specifies the relation between the melting-pressure p and the temperature T by a polynomial function p(T).

The specification of p(T) is based on precise measurements using primary thermometers such as noise and nuclear-orientation thermometers. These devices rely on fundamental physical relations and are irrespective of material properties. The measurements were performed at the US metrology institute NIST, at the University of Florida and at PTB in Berlin (cf. PTBnews 97.1). At tempeatures above approximately 0,3 K the new scale is identical with the PTB scale. In the lower temperature range the measured values are not yet in perfect agreement, and a suitable average has been taken in order to minimize deviations from the ideal thermodynamic temperature scale. This explains why the new scale is named "Provisional Low Temperature Scale of 2000" (PLTS-2000).

In spite of the uncertainty remaining, the adoption of a unified scale and its world-wide application represent a great step forward because measurements will be immediately comparable in future. In case further investigations should give rise to slight corrections of the scale, older measurement results can be readily converted into more precise values.

Contact at PTB:

M. Kühne,
fax: +49 (0) 30 3481-490