Mangum, B.W.; Pfeiffer, E.R.; Strouse, G.F. (1991, ASQC) National Institute of Standards and Technology
This abstract is an edited version of the author's original.
The precision platinum resistance thermometer (SPRT) is the standard instrument of the International Temperature Scale of 1990 (ITS-90) in the temperature range from 13.8033 K to 1234.93 K. These thermometers are calibrated at specified sets of the defining fixed points of the scale and are used for interpolating between the fixed points according to specified reference and deviation functions.
SPRTs are constructed of real materials and, consequently, do not behave in a perfectly ideal manner. The criteria for specifying the quality of SPRTs requires that the platinum element of the SPRT be pure platinum and that the element be constructed in a strain-free manner. The temperature behavior of SPRTs varies slightly from thermometer to thermometer, the SPRTs indicating different temperatures at any given point intermediate to the defining fixed-point temperatures at which they were calibrated. This is known as the non-uniqueness of the scale. Also, the ITS-90 allows SPRTs to be calibrated over a variety of partly overlapping temperature ranges using different sets of defining fixed points and associated deviation functions; consequently, any given SPRT calibrated over the different subranges will indicate different values of temperature for a given resistance. This is known as subrange inconsistency.
The authors examine both non-uniqueness of scale and subrange inconsistency and include many charts to illustrate their findings. They conclude that, as a general rule, one should not extrapolate any of the ITS-90 deviation functions beyond their range of application.