Effect of residual impurities in the ITS-90 freezing point of aluminium: a systematic evaluation of contemporary correction methodologies

Rodrigo Da Silva

Research output: ThesisDoctoral Thesis

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The International Temperature Scale of 1990 (ITS-90) defines that the calibration of standard platinum resistance thermometers (SPRTs) is based upon the use of temperature fixed points. Since residual impurities (even below parts-per-million levels) present in the fixed point cells influence their realisation temperature (in the order of a few millikelvins), the fixed point material employed in the cell must be ≥ 99.9999 % pure. Impurities usually constitute the most substantial contribution to the uncertainty of primary SPRT calibrations. With a view to tackle this matter, the Consultative Committee for Thermometry (CCT) of the Bureau International of Weights and Measures (BIPM) has recommended the use of a specific correction methodology (the sum of individual estimates, SIE) but other methods have also emerged, each being advocated by a particular National Metrology Institute.
The study reported in this thesis aims at investigating the application of seven available correction methodologies to the freezing point of aluminium (660.323 °C) to identify the most consistent methods together with any difficulties related to their implementation. In order to achieve this, a suite of five aluminium fixed point cells have been constructed according to a rigorous protocol, each cell using metal samples sourced from a different supplier. Glow discharge mass spectrometry (GDMS) assays have been obtained from three independent laboratories. Besides, for each cell constructed, a set of long duration freezing curves have been measured under nominally identical conditions. They provided the basis for the calculations of the correction methodologies investigated. The most consistent corrections were achieved with a hybrid method that combines the SIE and the overall maximum estimate (OME): the hybrid SIE/modified OME method. Furthermore, the correction methodology based on the fitting of a Scheil solidification model to the measured freezing curves was found to be highly consistent, provided certain constraints are applied.
Original languageEnglish
Awarding Institution
  • Royal Holloway, University of London
  • Casey, Andrew, Supervisor
  • Machin, Graham, Supervisor, External person
  • Pearce, Jonathan V., Supervisor, External person
Thesis sponsors
Award date1 Aug 2017
Publication statusUnpublished - 2017


  • Standard platinum resistance thermometer
  • ITS-90
  • fixed point
  • aluminium
  • impurity
  • uncertainty
  • calibration
  • SPRT
  • thermometry

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