Flowchart for the extraordinary calibrations prior to the redefinition of the kilogram. (BIPM proposal)

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1 Flowchart for the extraordinary calibrations prior to the redefinition of the kilogram (BIPM proposal) Pauline Barat Michael Stock Alain Picard Richard Davis 1

2 CIPM informed of intention to use IPK Document CIPM/12-44 (CCM/13-07) Protocol for the BIPM Calibration Campaign in anticipation of the kilogram redefinition: use of the international kilogram (IPK) Security issues are discussed in a separate document which was examined by the "Bureau" of the CIPM CIPM gave tentative approval, subject to advice from the CCM on timing, participation and other issues that have an impact on the redefinition of the kilogram and the mass programmes of NMIs. BIPM has made a detailed flowchart that raises all major issues, in order to open discussions within the WGSI-kg.

3 There is a yellow decision point after every major phase, to allow for discussion of unexpected problems. We suggest a support group in which these discussions can be held. This needs to be discussed. (BIPM recommendation) Selection of NMI participants Final Protocol approval Timetable for mass measurements Start date approved? Phase 1: BIPM preparatory measurements Selection of 2 BIPM working standards as reference standards during phase 1 Comparisons among the IPK, its 6 official copies, [25], [73] and the 2 reference standards prior to any cleaning-washing operations. 1st cleaning-washing of IPK + 2 official copies + [25] and [73] 2nd cleaning-washing of IPK + 2 official copies + [25] and [73] 3rd cleaning-washing necessary? Depends on change from 1st to 2nd cleaning? Phase 1 Selection of 2 official copies as reference standards for the direct extraordinary calibration in air of national standards Who are implicated: CCM, NMI participants, BIPM, CIPM? Phase 2A: Extraordinary calibration in air Direct calibration in air of national prototypes and stainless steel standards with respect to the 2 selected official copies (with before, in the middle and after comparisons among the IPK, its 6 official copies, [25], [73] and the 2 reference standards of phase 1 ) Agreement reached? What are the issues to address? Phase 2B: Extraordinary calibration in vacuum Indirect calibration in vacuum of standards with respect to the BIPM air-vacuum transfer standards (BIPM air-vacuum transfer standards calibrated in air with respect to BIPM working standards) Phase 2

4 Phase 2A: Extraordinary calibration in air Direct calibration in air of national prototypes and stainless steel standards with respect to the 2 selected official copies (with before, in the middle and after comparisons among the IPK, its 6 official copies, [25], [73] and the 2 reference standards of phase 1 ) Phase 2B: Extraordinary calibration in vacuum Indirect calibration in vacuum of standards with respect to the BIPM air-vacuum transfer standards (BIPM air-vacuum transfer standards calibrated in air with respect to BIPM working standards) Phase 2 Phase 3: Stability of IPK and its official copies Comparisons among the IPK, its 6 official copies, [25], [73] and the 8 BIPM working standards We suggest a review by the CCM or a CCM WG, where they can comment on the report. (BIPM recommendation) Phase 3 Final report of the BIPM Calibration Campaign in anticipation of the kilogram redefinition using the IPK Report End of the BIPM Calibration Campaign in anticipation of the kilogram redefinition using the IPK 4

5 Selection of NMI participants Final Protocol approval Timetable for mass measurements decided based on options chosen below Question 1 Which are the NMIs involved? 1A - Participating NMIs are ONLY those strongly involved in the redefinition of the kilogram (BIPM recommendation) 1B - Should additional NMIs be offered a chance to participate. Which ones? And why? Question 2 Which and how many standards per NMI should be calibrated? 2A - Maximum of 2 standards having nominal mass of 1 kg. (BIPM recommendation) Pt-Ir or high-quality stainless steel mass standards (in the shape of cylinder approximately height = diameter; OIML shape accepted for stainless steel), and silicon spheres. 2B - Other kinds of standards that should be calibrated. Which ones (ERMS?)? And why? Question 3 For which standards should an indirect comparison be carried out in vacuum? 3A - Only silicon spheres involved in the Avogadro Project (AVO28-S5 & AVO28-S8) after cleaning according to WGAC protocol. 3B - 3C - Silicon spheres involved in the Avogadro Project (after cleaning) and 3 standards belonging to the ERMS stored in vacuum (no cleaning). (BIPM recommendation) Additional standards that should be measured in vacuum. Which ones? And why? Question 4 For calibration only done in air, should the option to clean or not to clean the standards at the BIPM be proposed? 4A - cleaning for Pt-Ir and stainless steel mass standards. Only for silicon spheres involved in the Avogadro Project. (BIPM recommendation) 4B - 4C - At the request of the NMI, the cleaning process may be possible. As for the 3rd periodic verification of national prototypes of the kilogram, the mass of all standards should be determined in air before and after cleaning. 5

6 Conclusion and BIPM recommendations BIPM recommends creation of a support group to assist in making major decisions in a timely way. BIPM recommends not to proceed to cleaning of national standards, whether they be in platinum-iridium or in stainless steel. Silicon spheres involved in the Avogadro Project will be cleaned according to WGAC protocol. BIPM recommends a direct calibration in air for all artefacts involved in this extraordinary calibration. BIPM recommends an indirect calibration in vacuum limited to silicon spheres involved in the Avogadro Project and the standards belonging to the ERMS stored in vacuum. normal calibration service will be offered to NMIs of Member states (except for participating NMIs) during the extraordinary Calibration Campaign. Based on the BIPM recommendations, the BIPM Calibration Campaign in anticipation of the kilogram redefinition, using IPK, will last 13 months (incl. 1 month for unforeseen delays). 6