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VSL's revision of the Technical Annex for the International Temperature Scale of 1990 approved by the CCT of the BIPM

VSL's revision of the Technical Annex for the International Temperature Scale of 1990 approved by the CCT of the BIPM

21-09-2017

On June 1st and 2nd, 2017, the 28th meeting of the Consultative Committee for Thermometry (CCT) took place at the BIPM in Sevres. The CCT is one of the ten Consultative Committees (CCs) of the CIPM and brings together the world's experts in the field of thermometry, humidity and thermophysical quantities, with the task of advising the CIPM on all scientific advances that directly influence metrology in the covered field. VSL is one of the 23 members of the CCT and Andrea Peruzzi attended the CCT meeting as VSL delegate.

During the meeting, the CCT approved a revision of the Technical Annex for the International Temperature Scale of 1990 (ITS-90), prepared in the previous months by Andrea Peruzzi and Prof. Harro Meijer, Director of the Energy and Sustainability Research Institute (ESRIG) at the University of Groningen.

The Technical Annex for the ITS-90 contains the specifications beyond the definitions in the text of the ITS-90 that are deemed essential for the unambiguous realization of the ITS-90. Specifications within the Technical Annex are considered mandatory extensions of the text of the ITS-90.

The revision of the Technical Annex for the ITS-90 was considered necessary by the CCT after the publication of new more accurate isotopic correction coefficients and algorithms for the Triple Point of Water (TPW) in two Metrologia papers [1, 2] in 2015.

The two Metrologia papers contain the results of a joint research conducted by VSL and ESRIG (Univ. Groningen). This project combined two unique capabilities available in the Netherlands: the ESRIG capability of preparing and measuring the isotopic composition of water samples of the desired oxygen and hydrogen isotope content and the VSL capability of manufacturing and measuring the triple point of water cells. Using the water samples prepared and measured by the ESRIG, VSL manufactured 10 triple point of water cells, each one containing water of different isotopic composition (different content of 16O, 17O, 18O, 1H and 2H isotopes) and finally measured the triple point temperatures realized by each manufactured cell.

The analysis of the results allowed VSL and ESRIG to determine new (more accurate) isotopic correction coefficients and to propose a new (more accurate) correction algorithm. The new correction coefficients and algorithm have now been formally approved by the CCT and included in the revised Technical Annex for the ITS-90.

A document to explain to the CCT wider community the need of new correction coefficients and algorithm was prepared by Andrea Peruzzi and Prof. Harro Meijer and was published on the BIPM website [3].

This was one of the few cases in which a small national metrology institute like VSL had a relevant impact on a very fundamental issue that affects the foundation itself of thermometry (the isotopic composition of water).

 

Isotopologue

TPW temperature /K

1H216O

273.1587

1H217O

273.31

1H218O

273.46

2H216O

276.97

Table 1: Water is in fact a mixture of 4 different molecules (isotopologues), each one of them having a different TPW temperature. Only for a specified relative concentration of the 4 isotopologues (Vienna Standard Mean Ocean Water), the TPW temperature is exactly 273.16 K. The TPW temperature realized by a given water sample is calculated from its isotopic composition, by applying an algorithm that includes the isotopic correction coefficients.

Figure 1: Manufacture of a triple point of water cell at VSL: the glass blower seals off the cell from the distillation system.

References
[1] Faghihi V., Peruzzi A., Aerts-Bijma A.T., Jansen H.G., Spriensma J.J., van Geel J. and Meijer H.A.J. "Accurate experimental determination of the isotope effects on the triple point temperature of water. I. Dependence on the 2H abundance "Metrologia 52 (2015) 819-826
[2] Faghihi V., Kozicki M., Aerts-Bijma A.T., Jansen H.G., Spriensma J.J., Peruzzi A. and Meijer H.A.J. "Accurate experimental determination of the isotope effects on the triple point temperature of water. I. Dependence on the 18O and  17O abundances "Metrologia 52 (2015) 827-834
[3] A. Peruzzi and H.A.J. Meijer, CCT Document CCT/17-06, BIPM, Paris (2017) http://www.bipm.org/cc/CCT/Restricted/28/New_Isotopic_Correction_Coeffic...