NIST Temperature Scale Database (SRD 60), Version 3.0

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We provide here both the primary (defined) fixed points on the International Temperature Scale of 1990 (ITS-90), see Ref. 1, and also recommended secondary fixed point phase transition temperatures for use as companion fixed points on ITS-90.

International Temperature Scale Fixed Points

ITS-90 is defined by the temperatures of phase equilibrium transition temperatures of pure substances – several vapor-pressure/temperature relations and 15 fixed points, with the latter being highly accurate melting (MP), freezing (FP), and triple (TP) points of atoms and three molecules.
  • At the lowest temperatures on the scale (about 0.65 K to 24.6 K), the temperature scale is defined by vapor-pressure/temperature relations and triple points of 3He, 4He, and H2.
  • From 24.6 K to the freezing point of Cu at 1357.77 K (1084.62 ° C), the temperature scale is defined by the phase transition temperatures of Ne(TP), O2(TP), Ar(TP), Hg(TP), H2O(TP), Ga(MP), In(FP), Sn(FP) Zn(FP), Al(FP), Ag(FP), Au(FP), and Cu(FP).
  • Above the freezing point of Cu, ITS-90 is defined by the Planck radiation law using the freezing points of Ag, Au, or Cu as reference points.

ITS-90 is a “practical” temperature scale, an equipment calibration standard, for realization and dissemination of temperatures that approximates thermodynamic temperatures through “best fit” splined interpolation functions to the fixed points. Although the fixed points are defined exactly, they are just approximations, albeit highly accurate, of the thermodynamic temperatures. The differences between the ITS-90 and the true thermodynamic temperatures are very small ranging from about (0.2 to 3) mK at very low temperatures to about (10 to 15) mK at intermediate temperatures to about (30 to 50) mK at the highest temperatures below the freezing point of Cu.

Secondary Fixed Points

Companion to the defined points on the ITS-90 temperature scale are recommended secondary fixed points. These are not part of the official ITS-90 scale but are nevertheless important for realization and dissemination of temperatures. We include here many recommended secondary temperature reference transition temperatures. Many of these fixed points come from the work by Bedford et al . (Ref. 2).

We also include temperature reference points from the work of Burgess (Ref. 3) for eutectic and peritectic phase-transition temperatures of metal-carbon (e.g., Co-C) and metal carbide-carbon compounds (e.g., Fe3C-C). 18 of these are likely to be used as primary or secondary fixed points on a revised International Temperature Scale. All these phase-transition temperatures are at high temperatures (1400 K to 3000 K) above the highest fixed point, the freezing point of copper (1357.77 K), on the ITS-90 temperature scale. Eight of the phase-transition temperatures have very low uncertainties (0.12 K to 0.27 K) that likely will become primary fixed points on the temperature scale, and ten of the phase-transition temperatures have somewhat higher uncertainties (0.4 K to 1.5 K) that may become secondary fixed points. For completeness, we have also made recommendations for all other metal-carbon and metal carbide-carbon systems with phase-transition temperatures having high uncertainties (3 K to 40 K) and, thus, not suitable for fixed points on the temperature scale.

The table for secondary reference points is ordered by phase transition temperature from lowest to highest. Many of these temperature reference points (about 60) below about 1100 K (827 °C) have low expanded uncertainties, which we define here to be U(Ttrs)Ttrs(K) < 0.2 %, with about 45 of these with expanded uncertainties of less than 0.01 K. About 7 of the others having 0.01 K < U(Ttrs) < 0.02 K and another about 7 having 0.02 K < U(Ttrs) < 0.2 K. These transitions, however, are unlikely to become defined fixed points on the temperature scale because the current fixed points on ITS-90 are likely more than sufficient.

Some of these high-temperature reference points (about 25) above about 1100 K (827 °C) also have low expanded uncertainties [U(Ttrs)/Ttrs(K) < 0.1 % and U(Ttrs) < 2 K]. About 7 of these have very low expanded uncertainties [U(Ttrs)/Ttrs(K) < 0.02 % and U(Ttrs) < 0.4 K] and likely will be used as primary fixed points for realizing the new definition of the kelvin and to replace ITS-90 for measurements at these high temperatures determined using relative radiometric primary thermometry (which now uses Planck radiation law above the freezing point of Cu), see Refs. 4-6. Another about 9 of these have relatively low expanded uncertainties for high temperature points [U(Ttrs)/Ttrs(K) < 0.1 % and U(Ttrs) < 2 K] and possibly will be used as secondary reference points on the temperature scale. We also include here just for completeness about 25 other high-temperature transition temperatures that have relatively high uncertainties [U(Ttrs)/Ttrs(K) = (0.04 to 1) % and U(Ttrs) = (2 to 20) K].

References

  1. H. Preston-Thomas, "The International Temperature Scale of 1990 (ITS-90)," Metrologia 27, 3-10 (1990). DOI:
  2. R. E. Bedford, G. Bonnier, H. Maas and F. Pavese, "Recommended values of temperature on the International Temperature Scale of 1990 for a selected set of secondary reference points," Metrologia 33, 133-154 (1996). DOI:
  3. D. R. Burgess, Jr., “Binary metal-carbon phase-transition temperatures," NIST Technical Note 2278, (2023). DOI:
  4. Consultative Committee for Thermometry, "Mise-en-pratique for the definition of the kelvin in the SI," Bureau International des Poids et Mesures (BIPM), SI Brochure - Appendix 2 (2019).
  5. G. Machin, M. Sadli, J. Pearce, J. Engert and R. M. Gavioso, "Towards realising the redefined kelvin," Measurement 201, 9 (2022). DOI:
  6. M. Sadli, F. Bourson, D. Lowe, K. Anhalt, D. Taubert, M. J. Martin, J. M. Mantilla, F. Girard, M. Florio, C. Gozonunde, H. Nasibli, L. Knazovicka, N. Sasajima, X. Lu, O. Kozlova, S. Briaudeau and G. Machin, "Realizing the redefined kelvin: thermodynamic temperatures of Fe-C, Pd-C, Ru-C and WC-C for the mise-en-pratique of the kelvin up to 3020 K," AIP Conf. Proc. (to be published) (2024).