The Discontinuity in the First Derivative of the ITS-90 at the Triple Point of Water

This paper investigates the discontinuity in the derivative [d W /d T 90 ] TPW of the ITS-90 at the triple point of water, using data for over 40 calibrated standard platinum resistance thermometers (SPRTs). It finds that the discontinuity is in most cases somewhere between 0 and −6 parts in 10 5 , in relative terms, but that the higher numerical values are obtained for ‘less ideal’ SPRTs (those with lower temperature coefficients of resistance), and also for sub-ranges not extending beyond the indium point. These results are investigated vis-à-vis the long-standing observation that the ITS-90 reference values W r (Ga) and W r (Hg) are not completely consistent with data for W (Ga) and W (Hg) for real SPRTs. It discusses what may be done in a future scale to ensure continuity in the first derivative, and it concludes with a comment about the acceptance criteria for SPRTs in the scale.