Investigation of low-temperature fixed points by an international star intercomparison of sealed triple-point cells

Abstract

An overview of the results of an international star intercomparison of low-temperature fixed points is given. Between 1997 and 2005, 68 sealed triple-point cells (STPCs) of the twelve laboratories represented by the authors were investigated at PTB. The STPCs are used to realize the triple points of hydrogen, neon, oxygen and argon as defining fixed points of the International Temperature Scale of 1990, ITS-90. The melting curves (MCs) of all STPCs have been measured on the same experimental equipment, adhering strictly to a single measurement program. This protocol enables separation of the effects influencing the MCs and direct comparison of the thermal behaviour of the STPCs, which are quite different with respect to design, age, gas source and filling technology. In the paper, special emphasis is given to the spread of the liquidus-point temperatures and to the uncertainty of their determination. Connections between the star intercomparison and completed and ongoing international activities are also discussed.

The ITS-90 after definition of neon isotopic reference composition: extent of the isotopic effect tested on previous inter-comparison results

“…Isotopic studies were undertaken, initially on e-H 2 (HD in H 2 ) [5,6] and H 2 O (D 2 and 18 O 2 ) [7][8][9][10], and later also on Ne [4,[11][12][13][14][15].…”

“…In addition, some data are also traceable to the first International Inter-comparison of sealed cells performed in 1978-84 [34] or also ensures traceability for several results of the 1997-2005 Star Inter-comparison [12]. Legend: R cs = static thermal resistance; t = cell dynamic time constant; T tp = triple point temperature.…”

“…1 In Table 5 the results of the isotopic corrections for the Star Inter-comparison are reported [12]. For important specific conditions concerning the way the data of each laboratory were obtained, see the OSI associated with this paper.…”

“…Should one assume that the sample in question is from the same bottle that was used for the cell participating to the 1978-84 Inter-comparison [34] and the more recent Star Inter-comparison [12], an isotopic correction of À0.29 mK would apply, leading to a difference of À0.01 mK (#1) and À0.07 mK (#2), respectively.…”

“…The results of the CCT-K2.x can also be compared in Figure 4 with the results of the largest direct comparison of samples in sealed cells made after the Int84 [34]: the Star Inter-comparison [12], whose data are compared in Table 5 using the data of Table 2. Figure 4 (right part) makes self-evident the improvement of the Star data (right part) with respect to the K2.x data (left part).…”