Procedure for the impurity-related correction at the indium fixed-point

Abstract

Determining the influence of impurities on the fixed-point temperatures of the ITS-90 requires the completion of several tasks. In this paper, the progress made at Physikalisch-Technische Bundesanstalt (PTB) and BAM Federal Institute for Materials Research and Testing is presented and remaining questions are discussed. The projected characterization procedure at PTB, which is based on the established SIE method (sum of the individual estimates), using a new indium fixed-point cell is described as an example. This procedure includes an SI-traceable chemical analysis of the material in the fixed-point cell with sufficiently low uncertainties, the individual experimental determination of the influence of the quantified impurities on the fixed-point temperature, and the establishment of direct links to the phase-transition temperatures of the national standard and of an assumed material of ideal purity. A characteristic difference to the common practice is the chemical analysis of the fixed-point metal being done after determining the cell's freezing temperature. This allows for the detection and consideration of contamination and purification effects due to the filling process, or due to the contact with the carbon crucible and other parts of the fixed-point cell. A chemical analysis of an indium fixed-point was carried out by BAM with relative measurement uncertainties below 30 % which have not been previously achieved. The results provide evidence for the precipitation of some impurities, which is apparently inconsistent with the corresponding binary phase diagrams, but was explained in a recent publication. Implications for the use of the SIE method shall be described briefly at the end. \ua9 2011 Springer Science+Business Media, LLC.Peer reviewed: YesNRC publication: Ye