Future of the international temperature scale in a mixed dissemination environment.

The International Temperature Scale of 1990 (ITS-90) has successfully underpinned thermometry worldwide for more than three decades. But times are changing. Primary thermometry technology has improved and continues to advance. The Consultative Committee for Thermometry (CCT) has encouraged National Metrology Institutes (NMIs) to directly disseminate thermodynamic temperature T according to the new definition of the kelvin. The CCT has also published a mise en pratique containing endorsed methods of doing so. It anticipates that T dissemination will gradually replace ITS-90 temperature T90 dissemination. But the path from universal T90 dissemination to universal T dissemination will be long and difficult, especially in the important middle part of the scale. Under the guidance of the CCT, some NMIs will begin to offer T calibrations, while many others continue to disseminate T90. Thermometers carrying T calibrations will coexist with those carrying T90 calibrations out in the world. This global Mixed Dissemination Environment (MDE) will fluctuate over many decades as different primary thermometry capabilities come online and evolve. The problem for users is that T and T90 are not equivalent: the difference between them is more than 10 times larger than ITS-90 calibration uncertainties over most of the long-stem standard platinum resistance thermometer (LSPRT) range. In the MDE, even users unconcerned with thermodynamic accuracy will need to keep track of which thermometers give T and which give T90, as these change over time in a patchwork fashion. This paper explores the future of the International Temperature Scale in the context of the coming MDE. It proposes simplifying the disjointed T90 to T transition by introducing a new temperature scale (ITS-XX) with temperatures TXX that are equal to T within uncertainties. NMIs everywhere could implement the new scale on day one with existing equipment by updating tables of SPRT reference function coefficients and fixed point values. Then further moves to direct T dissemination by individual NMIs would be transparent to the users. Concerns about tightening mercury regulations would also be resolved and scale realization uncertainties reduced. A new scale would be costly to adopt, but the minimal ITS-XX variant proposed here represents the path of least net disruption through the coming MDE. These changes could be implemented in the next 10 years, transforming the ITS-90 into the accurate, reproducible, flexible scale its creators intended to make in the first place. [ABSTRACT FROM AUTHOR]