Evaluation of Uncertainty Associated with the Avogadro Constant

The Avogadro constant is the link between the macroscopic measure of the amount of substance and the number of atoms in the substance. Its unit is atoms per mole. It is also the link between the macroscopic mass (kg) and the atomic mass unit (u). The evaluation of the Avogadro constant and its associated uncertainty is important because it may enable a redefinition of the kg, if it can be measured with sufficiently small uncertainty. Silicon was chosen as the material to build objects to measure because the production of single crystals is understood very well. Several spheres from different starting materials have been produced and their physical properties (density) and their chemical properties (molar mass) have been measured in different laboratories around the world. Uncertainty budgets have been developed for all these measurements. The last task was to combine all these interlaboratory results to calculate the molar volume of Silicon. This is the volume of one mole of Silicon, and it is assumed to be independent of the isotopic composition of Silicon. We discuss various methods to combine the interlaboratory results. A weighted mean was used as the combined value. In this talk the final measurement function will be presented together with the consistency checks which have been used to prove that the combined value is consistent with all contributory measurements. It turned out that the correlation between measurements was very important for ensuring that the final uncertainty realistically represents the state of knowledge about the Avogadro constant.

Speaker Bio: Ruediger Kessel is an electronic and data systems engineer and currently a guest researcher in the Mathematical and Computational Sciences Division (MCSD) of the Information Technology Laboratory (ITL) of the National Institute of Standards and Technology (NIST). He has a Ph.D. in sciences from the Analytical Chemistry Department of the University of Antwerps, Belgium. He is the developer of a standard software tool named GUM Workbench which is used to evaluate uncertainty of measurement. He is also the owner of the company Metrodata GmbH, Germany which sells the software tool. His current interests include evaluation of uncertainty in physical and chemical measurements, modeling of measurements and software development.

Presentation Slides: PDF

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