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2. Principles of the International Temperature Scale of 1990 (ITS90)The ITS90 extends upwards from 0.65 K to the highest temperature practicably measurable in terms of the Planck radiation law using monochromatic radiation. The ITS90 comprises a number of ranges and subranges throughout each of which temperatures T_{90 }are defined. Several of these ranges or subranges overlap, and where such overlapping occurs, differing definitions of T_{90 }exist: these differing definitions have equal status. For measurements of the very highest precision there may be detectable numerical differences between measurements made at the same temperature but in accordance with differing definitions. Similarly, even using one definition, at a temperature between defining fixed points two acceptable interpolating instruments (e.g. resistance thermometers) may give detectably differing numerical values of T_{90}. In virtually all cases these differences are of negligible practical importance and are at the minimum level consistent with a scale of no more than reasonable complexity: for further information on this point, see "Supplementary Information for the ITS90" (BIPM1990). The ITS90 has been constructed in such a way that, throughout its range, for any given temperature the numerical value of T_{90} is a close approximation to the numerical value of T according to best estimates at the time the scale was adopted. By comparison with direct measurements of thermodynamic temperatures, measurements of T_{90 } are more easily made, are more precise and are highly reproducible. There are significant numerical differences between the values of T_{90} and the corresponding values of T_{68} measured on the International Practical Temperature Scale of 1968 (IPTS68), see Fig. 1 and Table 6. Similarly there were differences between the IPTS68 and the International Practical Temperature Scale of 1948 (IPTS48), and between the International Temperature Scale of 1948 (ITS48) and the International Temperature Scale of 1927 (ITS27). See the Appendix and, for more detailed information, "Supplementary Information for the ITS90". FIG. 1. The differences (t_{90}  t_{68}) as a function of Celsius temperature Continues: 3 Definition of the International Temperature Scale of 1990

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