Accuracy of Temperature Measurements of the Avogadro-Project

For the determination of the Avogadro constant via the XRCD-method the ratio of two temperature-dependent quantities, the unit cell volume and the sphere volume, need to be measured with the smallest possible uncertainties. Since both quantities show the same temperature dependency, only the reproducibility of the temperature measurements has to be taken into account. This paper summarizes the test measurements made for determining the accuracy of the temperature measurements of the standard platinum resistance thermometers which are used in the measurements of the silicon-lattice parameter at the INRIM and PTB, and the volume, at the NMIJ and PTB.