PTB News 2/2007 (1.2 MB) PTB News 2/2007, English edition, Issue September 2007In the low-temperature range between 0.65 K and 1 K, at present two overlapping, but different International Temperature Scales are available. The new PTB scale for the temperature interval from 0.65 K to 3.2 K (in short: PTB-2006), which was completed in 2006, now shows how the two scales can be combined in future to form one improved International Temperature Scale.
he temperature range below 1 K is of interest for basic research investigations and for metrological applications such as the use of highly sensitive detectors and bolometers. For traceable temperature measurements, manufacturers and users of cryotechnical facilities have currently two temperature scales at their disposal: the Provisional Low-Temperature Scale PLTS-2000 and the International Temperature Scale ITS-90, whose overlapping range extends from 0.65 K to 1 K. Close to 0.65 K, however, the scales deviate from each other considerably with decreasing temperature. It is known that in the overlapping range, the ITS 90 – in contrast to the PLTS-2000 – shows clear deviations from the thermodynamic temperature scale. It is the task of the PTB-2006 to overcome this inacceptable state.
For the temperature interval from 0.65 K to 3.2 K, the ITS-90 prescribes a special polynomial by means of which the temperature T90 must be determined from the measured vapour pressure of 3He. By analogy, the PLTS-2000 uses the melting pressure of 3He for temperature determination. In the ideal case, both the vapour pressure and the melting pressure of the 3He would have to be measured at the same temperature to achieve having the two scales coincide. As such a direct comparison has so far not been possible due to cryotechnical reasons, an indirect comparison via very stable rhodium-iron resistance thermometers was selected for the establishment of the PTB-2006. In several partial steps, a data set has been determined for the temperature dependence of the 3He vapour pressure which covers the total range from 0.65 K to 3.2 K. This data set, as well as an adapted new 3He vapour pressure polynomial, have recently been published.
Thanks to the PTB-2006, a new 3He vapour pressure scale is now available which is, on the whole, consistent with the thermodynamic temperature scale, passes – above 2 K – into the ITS-90, and – below 1 K – into the PLTS-2000 and which, compared to the ITS-90, shows only one third of the uncertainty between 0.65 K and 1 K.