On the redefinition of the kilogram. Dr Philippe RICHARD

Transcription

1 On the redefinition of the kilogram Dr Philippe RICHARD

2 Agenda 1. Recent history of IPK and new SI 2. The CCM conditions 3. The joint CCM and CCU roadmap 4. The mise en pratique of the definition of the kg 5. The WB and XRCD routes 6. Traceability wrt to IPK 7. Consistency of results 8. Validation (pilot study) 9. Raising awareness 10. Conclusions METAS

3 Present definition "The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram." (CGPM, 1901)... immediately after cleaning and washing by a specified method (mise en pratique, CIPM 1989). IPK METAS

4 Present BIPM cleaning and washing method Wash with ethanol + ether + chamois leather Rinse with steam 1 kg METAS

5 Present hierarchy of mass scale IPK rarely used (every 50 years) Physical properties no better than those of national prototypes NMIJ METAS NMI n n 50 METAS

6 The swiss [No 38] and the japanese [No 6] prototypes of the kilogram METAS

7 Why change the present definition? Advantages of the present definition Consistency of mass measurements worldwide Simple and concise Very easy to understand Disadvantages (= reasons to change) Verification of the prototypes only every 50 years Drift of prototypes between verifications IPK is not an absolute reference, the long-term drift of IPK is unknown Risk (damage, loss) Local, unique Electrical units in a conventional system METAS

8 Some orders of magnitude 0.2 µg Classical mass metrology at 1 kg (comparison of two identical objects) 2.0 µg BIPM uncertainty (3 rd periodic verification) 3.5 µg BIPM uncertainty (extraordinary calibration) 19 µg Realization with the best watt balance 20 µg Best realization with the XRCD experiment 29 µg CMC NMIJ (mass standards) 290 µg METAS watt balance I (2011) < 20 µg METAS watt balance II (2017) METAS

9 Uncertainty versus stability Before redefinition Realization with an uncertainty of about 3 µg But estimated drift up to 50 µg in 100 years (may be more) After redefinition Realization with an uncertainty of 10 µg to 20 µg But long term drift reduced to zero METAS

10 Redefinition of the kilogram in 2018 METAS

11 General Conferences and the kilogram 1st CGPM (1889) The Prototype of the kilogram adopted by the CIPM. This prototype shall henceforth be considered as the unit of mass 3rd CGPM (1901) The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram 21st CGPM (1999) Resolution 7: On the definition of the kilogram 24th CGPM (2011) Resolution 1: On the possible future revision of the International System of Units (SI) 25th CGPM (2014) Resolution 1: On the future revision of the SI 26th CGPM (2018) METAS

12 CCM conditions (before redefinition) R1 Consistency Three independent experiments (XRCD & WB) with consistent results with u rel < R2 Uncertainty at least one result with u rel < R3 Traceability Extraordinary calibration with BIPM R4 Validation Validation of the mise en pratique according to the CIPM-MRA METAS

13 Joint CCM and CCU roadmap CCM CCU Convocation CGPM Metrologia special edition Mise en pratique (mep kg) 15 CCM mep kg (with references for the mep) approved BIPM extraordinary calibration with IPK Initial pilot study (kg) (preparation, measurements & report) CCU Traceability CCM to IPK R3 updated mep kg 16 approved CCM CCM R4 Consultation / announcement BIPM pool established XRCD and available WB publish consistent results and characterized European know and NewKILO projects CCM R1& CCU CIPM Pool linked to experiments CODATA Closing date (final LSA) for new data NMI directors meetings Convocation CGPM Conditions from CCM Recommendation G1 (2013) METAS

14 Mise en pratique of the definition of the kilogram Redefinition of the kilogram Select a constant measured in terms of the IPK Formulate a definition of the kilogram based on this constant Write the mise en pratique of the definition In the case of the Planck constant, the constants that define the second and the metre are also needed. The mise en pratique recommends primary methods of practical realization of the mass unit. METAS

15 The mise en pratique of the definition of the kilogram Provisionally approved by the CCM in February 2015 Available on the CCM website ( Final approval in 2017 Special issue of Metrologia «Realization, maintenance and dissemination of the new kilogram» planned for 2017 to provide essential references cited in the mise en pratique METAS

16 The Watt Balance route 1. Determine the numerical value of h in the present SI other SI base units (s, m) present definition of the kilogram IPK Planck constant, h 2. Redefine the kilogram by fixing this numerical value Planck constant, h fixed new definition of the kilogram 3. Use the fixed Planck constant to realize the kilogram (measurement duration: about 1 month) METAS

17 WB Principle (1): static phase / weighing mode mg I d B = mg I U j = (QHE & JVS) R

18 WB Principle (2): dynamic phase / velocity mode U = v d B ind METAS

19 WB Principle (3): combination of modes Only if G m = G e GB (, ) mg U = = UI = I v mgv static dynamic electrical power mechanical power METAS

20 WB Principle (4): combination of modes Using the expressions from quantum physics (QHE & JVS) h U= C U = C n f 2e J J J J J h R= CH RH = CH n 2 H e = f J1 J2 m C h C = f gv C n C n n 4C J1 J1 J2 J2 H H METAS

21 An example METAS

22 The projects around the world Coming soon... KRISS (Korea); MSL (New Zealand), UME (Turkey)

23 The XRCD route (Avogadro collaboration) METAS

24 Counting the number of atoms in a silicon crystal sphere N = A 8 M Si sphere 3 m 220 sphere d V METAS

25 The XRCD route Before redefinition (stable parameters) Molar mass (isotopic composition) Crystal perfection (impurity content) Lattice parameter Measured only once (duration about 6 month) After redefinition (changing parameters) Volume Surface layer characterization Measured for each primary realization (duration: about 1 month) METAS

26 Traceabillity with respect to IPK CCM Recommendation (2010) On the use of the international prototype of the kilogram to confirm the traceability of the BIPM prototypes Comparison of IPK and BIPM official copies (2014) Metrologia 52 (2015) Re-calibration of NMI Pt/Ir prototype ( ) (only NMIs with available primary methods) METAS

27 Mass evolution of the BIPM official copies METAS

28 Extraordinary calibrations Repeatibility of weighings of the prototypes 0.5 µg IPK and six official copies behaved consistently during cleaning and washing Mass change of the official copies with respect to the IPK since the 3rd Periodic Verification ( ) on average by only -2 µg The mass increase of the official copies with respect to the IPK is stopped! The NMI transfer standards were recalibrated with respect to the IPK in 2015 METAS

29 Consistency and uncertainty (latest results) METAS

30 Consistency analysis First statistical evaluation of consistency was presented at the CCM in February 2015 CODATA 2014 adjustment is claiming consistency CCM will keep this under review as further results become available until mid 2017 and inform the CIPM 2 CCM is expecting a χχ obs well within the high-density region of the PDF CCM will have to decide about consistency at its next meeting in 2017 METAS

31 Validation (pilot study) Check the validation of procedures for the future realization and dissemination of the kilogram as described in the mise en pratique. The primary methods should be compared as directly as possible using real data acquired during the pilot study Link the BIPM ensemble of reference mass standards to primary realizations Limited to NMIs capable of realizing the mass unit with low uncertainty METAS

32 Validation (pilot study) Comparison of the realization of the kilogram: Determination of the mass values of a set of transfer standards in vacuum based on a numerical value for h, stated in the protocol Comparison of the dissemination of the kilogram: Determination of the mass values of a set of transfer standards in air, traceable to the primary realization Measurements in 2016 The results should be available by the next CCM meeting in 2017 METAS

33 Raising awareness Can I get traceability to the SI kilogram using a watt balance? X Z Y arxiv: v1 METAS

34 Raising awareness Some major stakeholders never heard of the Planck constant and are still suspicious that it can be used successfully to define the kilogram The question of whether every NMI now needs a watt balance or a 28 Si sphere instead of a national prototype still arises METAS

35 Raising awareness Concerns electricity also, due to discontinuity between 1990 conventional units and SI units in the new SI Information of the scientific community and the general public is to be intensified between now and 2018 Official campaign from CCU as a CIPM task group According to the roadmap, consultations and information should lay open questions to rest by 2018 METAS

36 CCM conditions: present situation R1 Consistency R2 Uncertainty R3 Traceability Data published recently include three independent experimental results with relative uncertainties < 50 µg/kg, the total set of data is very close to consistency, further experimental results expected Data published recently include two experimental results with relative uncertainty < 20 µg/kg, further experimental results expected Fully established R4 Validation on-going (measurements in 2016) METAS

37 Future definition of the kilogram The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be when expressed in the unit J s, which is equal to kg m 2 s 1, where the metre and the second are defined in terms of c and ν Cs. METAS 37

38 Conclusions The CCM (and the CCU) continue their planning and hard work to be ready for 2018 according to the joint roadmap No significant deviation on the timeline of the joint roadmap is expected The 26th CGPM will take place in 2018 Excellent recent results and very good reasons to be optimistic for an important change of the SI in 2018 METAS

39 Conclusions After 2018, the IPK will not any more have a special status among the standards in use at the BIPM METAS

40 Be ready for 2018! Thank you very much