Report on the. CCM Workshop on the mise en pratique of the new definition of the kilogram. BIPM, Sèvres, November 2012

Transcription

1 MeP_2012/-16 Report on the CCM Workshop on the mise en pratique of the new definition of the kilogram. BIPM, Sèvres, November 2012 The agenda of the workshop, as well as most of the presentations given, are available on the BIPM website, where the interested reader can find more detailed information. Richard Davis, former director of the mass Department of the BIPM and chairman of the workshop, declared the workshop open and Michael Kühne, BIPM director through 2012, welcomed the participants, stressing the importance of the workshop in view of the redefinition of the kilogram. In this respect, he recalled that the CGPM had decided that a mise en pratique be in place before a unit could be redefined. The chairman, after recalling the motivations that led to the organization of the workshop, essentially the opportunity to revive and broaden the debate on the draft mise en pratique that had been prepared, described the particular situation of the mass community in this delicate moment. The position of CCM President had been vacant since June 2012, due to the resignation of Mitsuru Tanaka, and only in October 2012 had the presidency been conferred on Philippe Richard, co-organizer of the workshop in his capacity as chairman of the CCM Working Group on changes to the SI kilogram (WGSI-kg), who he congratulated. In addition, the illness of Alain Picard, director of the BIPM Mass Department, made necessary the appointment of an interim CCM Executive Secretary to assist in the work, the interim being Richard Davis himself. He then read a welcome message from Alain Picard to the attendees. Subsequently, Philippe Richard (METAS) made a brief introduction, in which he explained the need for changes in the CCM governance and in the structure of its Working Groups, also as a consequence of his election as President. He then, after summarizing the history of the mise en pratique since the earliest draft, prepared in June 2010 by members of the WGSI-kg, illustrated the objective of the workshop and the conditions for its success, while pointing out that it would not have authority to take decisions on behalf of the CCM. Horst Bettin (PTB), chair of the CCM Working Group on the Avogadro Constant (WGAC), gave an overview of the status of the Avogadro project, concerned with the determination of the Avogadro constant by means of an X-ray crystal density (XRCD) experiment. He first recalled the definition of the Avogadro constant N A and showed how, with the XRCD experiment, the Planck constant h can also be determined through the molar Plank constant hn A, which is presently known with a relative uncertainty u(hn A )/(hn A ) = 7 x Therefore, the XRCD experiment is a candidate for a realization of the kilogram after its new definition, based on a fixed numerical value of the Planck constant. After describing the principle of the experiment, he went through the various quantities involved, discussed their relative uncertainties and showed that the experiment can realize the kilogram with a standard relative uncertainty of 1.5 x 10 8, which is the target for He then remarked that, except for the absolute measurement of the lattice parameter of silicon, which has been determined so far by only one experiment (but a second independent measurement will start soon), the other quantities are measured independently by many laboratories, thus ensuring a certain redundancy. He then considered the status of the XRCD experiment after the redefinition of the kilogram. He also went through an analysis of the time needed to realize the kilogram by means of a Si sphere, previously characterized in terms of molar mass, lattice parameter, volume etc., concluding that 1

2 the only operation required for the realization (surface conditioning and analysis) would take no longer than a week. He also claimed that a spherical crystal made of natural Si, instead of the isotopically-enriched 28 Si used for XCRD method, would be appropriate to monitor the International Prototype of the kilogram, IPK. In the subsequent discussion, a parallel was drawn between monitoring the mass of the IPK in terms of a Si sphere and monitoring the Earth rotation rate in terms of the hyperfine transition period of the cæsium 133 atom. There followed a series of presentations on watt balances. Jon Pratt (NIST) gave updates on the NIST-3 (operating) and NIST-4 (under development) watt balances. The team working on the balances is totally new. The experiment with NIST-3 is blind, in the sense that the test mass in the balance has been assigned a fictitious value which is slightly altered with respect to its true value, known only to the NIST mass department. This should improve the independence of the future value for the Planck constant with respect to the previous estimate. Among the distinctive features, he mentioned the unique possibility to alter both intensity and profile of the magnetic field (generated by a superconducting coil), which helps detecting possible systematic effects. He also mentioned the improvements, mostly on the electronics, that occurred since the 2012 CPEM. A new estimate for the Planck constant should be available by the end of Within this deadline, a comparison made by exchanging test masses with NRC (Canada) is also envisaged, in order to help resolve the discrepancy between the two existing estimates of h. He then commented that the experiments intended to realize the kilogram should be different from those originally developed to measure the Planck constant. In this respect, considering that whatever method that links mass to the Planck constant is accepted, he defended the idea that the value at which the mass scale is related to the Planck constant could be significantly smaller than one kilogram. For example, a realization of mass values much smaller than 1 µg could be greatly advantageous. In the subsequent discussion, there was general agreement that the present draft of the mise en pratique does not preclude realizations at any scale. Richard Davis asked whether there are already potential customers for nano-masses. The reply was that there are none at present, but high-accuracy devices of this kind could be produced at low cost and thus attract customers. Further comments were that there would be anyway a need for validation of these devices, for example through comparison at the suitable, say, submilligram level. The next presentation was made by Henri Baumann (METAS), who introduced the METAS watt balance MARK II, or BWM II. The balance, which has a target relative uncertainty of 1 x 10 8, is being developed in close co-operation with the École Polytechnique Fédérale de Lausanne, as concerns the translation stage, the CERN for the magnetic circuit and Mettler-Toledo for the weighing system. He went through the technical features and specifications of the balance, among which a capacity of 1.8 kg, a weighing range of ± 4 g, a standard deviation of 0.3 µg, as concerns the weighing system. As to the translation stage and the magnetic circuit, many adjustments are possible, in order to optimize the alignment of the various components. According to the timeline, the balance is expected to be fully operational by the end of 2013, and the target uncertainty should be achieved by Dave Inglis (NRC) then presented the NRC watt balance, summarizing its earlier history. The balance was purchased from the NPL late in 2008 and produced a first estimate of the Planck constant in September 2011 with a relative standard uncertainty equal to , subsequently reduced to The 2

3 estimate is in acceptable agreement with the XRCD value, whereas it is discrepant with the NIST value by (relative). He showed that the dispersion in the mass estimate of a 500 g Si standard, with a range previously larger than 100 µg, is now about 50 µg. It is expected that NRC will be ready to contribute in 2013 both to the BIPM and to their own ensembles of artefact mass standards, with mass values derived from their watt balance, with a relative uncertainty of The NRC contribution is expected to continue for the foreseeable future. The following speaker, Gérard Genevès (LNE), illustrated the LNE watt balance by giving a brief sketch of its features. It is an equal-arm balance with a mass exchanger able to move both horizontally and vertically, for the weighing stage. The balance beam is locked during the moving stage, when velocity is controlled and measured using three interferometers. All the apparatus is contained in a large airtight enclosure. Preliminary measurements, carried out in summer 2012 by alternating dynamic and static phases, gave values for h consistent within 6 x The next steps, in the framework of the European project know, will concern improvements in many areas, such as velocity control and measurement, extension to 1 kg from the current value of 500 g and, above all, the ability to perform the experiment in vacuum, which is expected in Further studies are being carried out, within the European project NewKILO, on suitable materials for mass standards, the study of surface properties, the definition of a LNE ensemble of mass standards and the phenomena occurring in the vacuum-to-air transfer. The position of the LNE concerning the time of re-definition is that there is no urgency, as the current number of realizations is considered not sufficient. In addition, National Metrology Institutes (NMIs) can better motivate requests for funds in a framework of determination of a fundamental constant. Finally, the mise en pratique is not yet finalized and its robustness has to be evaluated, which would take some time. Concerning dissemination, the LNE is in favour of preserving the universality of the definition. Therefore, the reference value of a key comparison (KCRV) among laboratories realizing the kilogram in a primary way should be adopted worldwide. As concerns financial support, European projects will ensure funding for the next three years. National support is also granted for some time, at least until the time of re-definition. Longer-term developments will depend upon strategy in research and development. In the subsequent discussion it was remarked that moving to measurements under vacuum as soon as possible might be advantageous. It was also clarified that the main systematic errors are currently due to alignment, vibrations and velocity measurement. The BIPM watt balance was presented by Hao Fang (BIPM), who explained why it is advisable that the BIPM develops a watt balance. As an international organization, the BIPM can ensure, on a cost-shared basis, long-tem, sustainable operation of the balance. A unique feature of the balance is that, in addition to the traditional two-stage functioning, a simultaneous mode is possible, in which weighing is carried out while the balance is moving. In this way some systematic effects are minimized. The experiment is table-sized and operates at room temperature. It started in 2005, became operational in 2009 and provided a first value for h in 2010, with a necessarily large uncertainty. The current relative standard deviation is about 1.4 x 10 6, although consideration of systematic components raises the value to a few parts in An interesting option to remove unwanted resistive voltage arising in simultaneous operation is by using a bifilar coil. This enables three different modes of operation, two by using the coils individually à la BIPM, i.e., simultaneously and using both coils jointly without the superimposition of both voltages, thus obtaining three distinct values. The balance is being moved to a new laboratory and several improvements are underway. It is anticipated that uncertainty will be about 5 x 10 7 within 2015 and below 1 x 10 7 in These estimates were declared to be conservative by the BIPM director, Michael Kühne. 3

4 Qing He (NIM) then introduced the NIM joule balance, an equal-arm balance in which energies, rather than powers, are compared. The current uncertainty is 25 x 10 6, due mostly to thermal effects in the (superconducting) coil system. The target uncertainty is 2 x Various planned improvements were described in order to achieve the target. Chris Sutton (MSL) reported on the progress of the MSL project, on which two people are working half-time and ten more are contributing. Core funding is not guaranteed after 2013, whereas MSL funding would continue. The target uncertainty, to be reached in 2014, is 1 x The conceptualization and design stages are close to completion, and some parts have been purchased or developed. The aim is a table-top sized experiment, in which the weighing mode is by means of two interconnected, gas-operated pressure balances. In the dynamic mode the coil is oscillated at low frequency (f < 1 Hz) and small amplitude (1 mm). The merit factor Q of oscillation has been measured in collaboration with NIST. Various developments and tests are under way. Trials suggest a weighing uncertainty of 15 µg less. In the subsequent discussion, it was pointed out that the piston design has advantages in terms of self centring, so that alignment problems are comparatively minor. The experiment will be carried out under vacuum. Jinhee Kim (KRISS) presented the KRISS project, resulting from ten years of internal discussion. The project was eventually started in April 2012, funded with 1 million dollars for the first three years. First operation of the balance is expected by December 2014 and the target uncertainty, anticipated for 2017, is 2 x The strategy is to build a simple and reliable balance, by using, for example, the load cell of a commercial instrument. It was stressed that the balance is intended as an instrument to realize the kilogram after the re-definition, rather than to determine h prior to it. Enrico Massa (INRIM) then reported on the JRP, European joint research program, know, aimed generally at meeting the requirements established by the CCM Recommendation G1 (2010). The JRP, in which all the European NMIs involved in the redefinition of the kilogram co-operate, is constituted of five work packages (WPs), of which three are technical and concern the watt balance experiment, the 28 Si XRCD experiment and the resolution of discrepancies, respectively. The final report should become available by autumn During the discussion some data were judged too optimistic. The different status of partners and collaborators was explained, and some clarifications were made on the connection with CCM activity. Stuart Davidson (NPL) reported on another relevant JRP, NewKILO, aimed at implementing the kilogram redefinition and facilitating maintenance and traceability following redefinition. The JRP is structured in five work packages. WP1 concerns the development of new artefacts suitable to provide maintenance and dissemination to a redefined kilogram. WP2 deals with procedures, apparatuses and techniques for the mass transfer from vacuum to atmosphere. WP3 studies surface effects and dynamic changes occurring during transfer. WP4 concerns storage, cleaning and transport methods, and their effects on the stability of standards. Finally, WP5 deals with the uncertainty inherent in the mise en pratique and its propagation through the dissemination process. Details on the status of the various WPs were also given. Richard Davis asked whether there is a way to evaluate progress. Indeed, a report is requested every six months. Chris Sutton asked whether an ensemble of artefacts is planned, which is not the case. Carl Williams (NIST) declared that, being that NIST is working on similar issues, they would be happy to interact. 4

5 The following presentation, by Jon Pratt, was concerned with resolving the discrepancy between the estimates provided by the NIST and NRC watt balance experiments. According to recent comparisons, the NIST and NRC electrical standards agree, as well as length standards and measurements of acceleration due to gravity. As to time standards, the NRC experiment is traceable to primary Cs standards, whereas NIST is GPS-based. Mass standards will be exchanged, although in both cases they are traceable to the IPK. NRC uses a Si standard (diamagnetic) calibrated at the NPL in vacuum, NIST the Pt-Ir Prototype No. 85 (paramagnetic). Jon Pratt discussed also the conceptual difference between comparing estimates of the Planck constant in terms of the IPK, prior to the redefinition, and comparing mass standards in terms of h, after the redefinition. In this respect, a new value for h obtained with NIST-3 will be available in spring Subsequently, mass comparisons will be carried out based on a common value for h agreed upon between NIST and NRC, and the results should be available in autumn It was pointed out that NIST-3 and NIST-4 will both be operational at the same time, at least for a period. Roman Schwartz (PTB), CIML vice-president, reported from the OIML. In Resolution 25 of its 46 th meeting, in 2011, the CIML encouraged submission of comments to an ad-hoc OIML working group on changes to the SI. Some comments were received. Among them, a position paper was also received from CECIP, the European weighing industry association, representing more than 700 manufacturers and involving a turnover of more than 3 billion euros per year. In the paper, concerns are expressed that a premature redefinition could jeopardize the current worldwide uniformity and equivalence of mass measurements. Roman Schwartz then mentioned Resolution 1 of the 47 th CIML meeting, in 2012, stating the OIML position on the changes to the SI. The position is of broad approval, with a recommendation to strictly respect the conditions set by the CCM concerning the redefinition of the mass unit. In addition, support is given to the CGPM intention to further improve formulations of the definitions in order to make them more easily understandable. A paper on this topic is anticipated to appear in the January 2013 issue of the OIML Bulletin. During the subsequent discussion, Michael Kühne added that the CECIP paper, which had been sent directly to the BIPM, was then forwarded to the CIPM and that a reply is under preparation by the CIPM itself. Carl Williams (NIST) commented that no significant reaction had been received from North-American balance manufacturers. In addition, he said that an effort should be made in order to persuade users that there is no reason to fear any practical consequences of the kilogram redefinition. Zeina Jabbour (NIST) noted that there is a difficulty in explaining the Planck constant to a non-specialist audience. Terry Quinn (director emeritus of the BIPM) defended the present proposed definition and recommended that people be invited to the laboratories and shown what is happening. Alan Steele (NRC) acknowledged that there is a difficulty in understanding what the mass of a kilogram is. Walter Bich (INRIM) commented that the problem is that action, as a quantity, is largely unknown not only to the general public but also to many people working in technical fields, contrary to mass, or force, or pressure etc. The discussion went on about possible ways of educating users. According to Carl Williams, there is little mysterious in the basic equation governing the watt balance, as it is a simple equality between two last-century well-known physical laws. Cécile Goyon (BIPM), described how mass calibrations are carried out at the BIPM under the current definition. She recalled many features concerning the manufacturing of prototypes, their specifications (for example, the maximum permissible error on mass is ± 1 mg) and the cleaning-washing procedure. Concerning the last of these, she compared the typical mass evolution of cleaned and non-cleaned prototypes. She then went through the traceability chain of the BIPM working standards, the calibration procedure and the comparison design; she also analyzed the various uncertainty components. The standard uncertainties associated with the mass estimates of a 1 kg Pt-Ir and stainless steel standards, calibrated by 5

6 the BIPM, are 7 μg and 14 μg, respectively. She eventually described the available facilities and gave an overview of the evolution in the number of issued certificates. Horst Bettin asked whether there is an explanation for the instability observed in the mass of some prototypes. It seems that this is not the case. There was also curiosity about the recent increase in the number of issued certificates. Concerning instabilities, Patrick Abbott (NIST) mentioned the case of prototype No. 9 and Richard Green (NRC) suggested that this prototype might be especially interesting to study. Terry Quinn commented that long-term stability is based on the assumption that cleaning resets the mass to its original value, and recalled that it was only on the occasion of the third verification that any statements could be made about long-term drift. Carl Williams raised the issue of the environment effect. Alan Steele highlighted the advantages of a pool of artefacts over a single standard in terms of robustness against instabilities. Estefanía de Mirandés (BIPM) presented a general method to reproduce the mass values assigned to the BIPM working standards from 1889 to The algorithm uses as input the available mass differences among 18 BIPM prototypes, obtained in various different periods through 136 series of mass comparisons, each series giving differences among 2 to 11 prototypes. The main finding is that the historic mass values could be confirmed within a few micrograms. In one particular case a difference of 60 micrograms was observed, which is explained by the cleaning of the IPK prior to the Third Verification; cleaning studies of the IPK are not included in the model. Uncertainties were also discussed. Michael Borys (PTB), in his capacity as chairman of Task Group 1 on Mass metrology under vacuum of the CCM WG Mass standards, reported on the TG activity, after reviewing terms of reference and membership. He started with an update on the comparison of sorption artefacts, organized among members of the TG to evaluate the global ability to transfer mass values from vacuum to air in a meaningful way. Preliminary results of the comparison, still in progress, show appreciable dispersion in the values of the sorption coefficients provided by participants. The residual pressure at which vacuum measurements were performed might play a role in this behaviour. Another relevant activity was the determination of the mass of 28 Si spheres in vacuum, carried out as a joint effort between the PTB, the NMIJ and the BIPM. The agreement between participants was in the order of 1 x A third activity was a comparison among mass standards used on the NPL and NIST watt balances, in which also the BIPM participated. The comparison yielded useful information about the stability of mass standards made of different materials (excellent agreement and stability of Si mass standards, reasonable stability and agreement of stainless steel standards, anomalous results for Au-Cu kilogram). He then showed activities carried out individually by other NMI members of the TG. The LNE (CNAM) is working on surface analysis by thermal desorption (TDS), as well as on cleaning and storage of standards. In this respect, first LNE results indicate that the material behaviour seems to be more stable in nitrogen than in argon. BIPM is working on a storage network for its pool of artefacts. METAS is working on the combination of XPS and mass measurements as a means for studying different cleaning techniques and storage environments. Interesting results have been published by Peter Fuchs et al. in Metrologia (2012). The NPL is working on the effects of vacuum-to-air transfer, in particular on the influence of residual pressure, which seems to cause significant sorption hysteresis if higher than 0.1 Pa. The NIST mass and force group continues work on magnet-suspension balance (one pan in air; one pan in vacuum; the pans connected by magnetic coupling) used to transfer the kilogram realization from the watt balance to ambient atmosphere. This project is declared to have top priority among others, which include provision of a mass-in-vacuum system for comparison purposes with magnetic coupling. PTB is working on 28 Si spheres as concerns both a redetermination of N A and the evaluation of the stability of mass values. Connections with EMRP projects were also shown. Peter Fuchs (METAS) gave 6

7 complementary information on mercury contamination and its removal. Contamination seems to be much higher than previously believed. For Pt-Ir standards, mass increase is 19 to 26 μg in the first 20 minutes. Subsequently, mercury continues to increase up to about 180 μg. Polishing of Pt-Ir seems to activate the surface. Removal of mercury by H 2 plasma is explained by formation of di-hydrides. Lars Nielsen (DFM), chairman of Task Group 2 on Uncertainty components due to traceability to the international prototype of the kilogram" of the CCM WG Mass standards, reported on the TG activity, after reviewing the terms of reference. A mathematical model was developed to infer the evolution of masses of prototypes over time. The solutions are given by a least-squares adjustment. Results were shown for some prototypes, including the IPK. Lessons learned include the importance of modelling, of regular comparisons with a reference, and of more frequent comparisons without a reference. Cleaning should be minimized, as this procedure does not reset mass to its prior value. On day two, Richard Davis opened the session and Philippe Richard, after thanking again the audience, gave the agenda of the day, focused on the discussion of the mise en pratique as drafted by the WGSI-kg. He went through the document and its various sections. The second presentation of the day was by Ian Robinson (NPL), Chair of the CCEM Working Group on Electrical Methods to Monitor the Stability of the Kilogram, and was concerned, as far as possible, with the consensus position. The WG encourages both direct dissemination from national watt balances and scaling of mass measurements using watt balances. According to the view of the WG, five to six experiments should be active simultaneously. It was remarked that the mise en pratique is crucial as, depending on its formulation, a laboratory can be encouraged to start (or continue) an activity or discouraged from doing so. In this respect, since a watt balance can work at any nominal mass value, this possibility should be promoted in the mise en pratique, so that uncertainties could improve for nominal values away from the unit. Local ensembles are viewed with favour, as well as the ensemble world, although no ensemble should play a pre-eminent role. Dave Inglis asked how many groups contributed to the WG position and how much of it instead reflected the views of individuals. The reply was that there was necessarily a small number (three or four) of responses, and that the position expressed in part reflected also views of individuals. Carl Williams strongly defended the possibility of realizing mass at a level different from the kilogram. Alan Steele, while appreciating the good balance of Ian Robinson s presentation in reporting individual and group views, clarified the point of the different scales of operation of watt balances in connection with the strategy of laboratories. A laboratory allowed to directly disseminate from its watt balance, whatever the mass value of the realization may be, might be encouraged to pursue its experiment, whereas a mere contribution to an internationally agreed value might not be sufficient to persuade management to continue (or start) funding. Philippe Richard recalled that it is not the responsibility of the mise en pratique to establish the number of watt balances, but the CCM might formulate a suitable condition in a recommendation. A long discussion took place on the role of local versus collective realizations. Alan Steele defended the right to disseminate locally, while contributing to the collective value. Michael Kühne recalled that the uncertainty of a local realization is expected to be larger than that of a collective one. Philippe Richard wondered whether some form of common financing could represent a viable way to encourage laboratories. The debate moved to the way in which an international value should be agreed. Chris Sutton expressed the view that a normal traceability model could be used rather than traceability to the KCRV and was 7

8 supported in his views by Alan Steele and Carl Williams. Martin Milton (director designate of the BIPM 1 ) thought that the world ensemble is an ill-defined concept. Carl Williams wondered whether the two 28 Si spheres currently in existence will be part of the BIPM ensemble. Walter Bich expressed the view that local dissemination is by no means incompatible with a worldwide reference value represented by a virtual average of the BIPM ensemble. Alan Steele cautioned that this might discourage further realizations, so that only in an initial period this practice might be advisable. Michael Kühne reminded the workshop that the mise en pratique is not a permanent document, and could well be modified according to the evolving needs. Alan Steele insisted that it is not likely that the principles of a mise en pratique will be changed in five or ten years. Terry Quinn also defended the BIPM ensemble. Walter Bich said that as the BIPM is an international bureau, its ensemble cannot be other than international. Michael Borys and Chris Sutton reminded the workshop that rules already exist for CIPM comparisons, which should be appropriate and sufficient for realizing the SI unit of mass. Alan Steele agreed, saying that correcting for the individual bias with respect to the KCRV is not the only way to ensure worldwide uniformity, as all the necessary tools are given in the Mutual Recognition Arrangement (MRA). Kenichi Fuji (NMIJ) agreed. Richard Davis read a modification to the mise en pratique, proposed by Carl Williams, which better clarifies the possibility of local dissemination at nominal mass values other than 1 kg. There was general agreement on the proposal. He also asked for further contributions. Terry Quinn encouraged the CCM to produce a good mise en pratique so as to reassure users before redefinition. Roman Schwartz asked that comments be available well in advance of the February 2013 CCM meeting. Horst Bettin gave the position of the CCM WG on the Avogadro constant (WGAC) on the draft mise en pratique. He reviewed the members of the cooperation, the equations governing the XRCD experiment after redefinition, and re-stated some considerations about redundancy in the measurement of the various input quantities, noting that a possible further way, being explored nowadays, is to determine the molar Planck constant, hn A, by atom interferometry. He made some comments on wording, specifically on the use of the term primary and Carl Williams profited from this point to ask that wording of the mise en pratique be widely shared and agreed. Horst Bettin also formulated a criticism concerning a sentence in subclause 2.3 of the mise en pratique according to which the CCM examines the claims made for each primary realization, which is considered unnecessary. He then presented an example aimed at demonstrating that the current conditions on consistency imposed by the CCM recommendation on the various determinations of h (or N A ) are not sufficiently robust. This example raised a debate, in which Alan Steele agreed in principle on the risk inherent in the present conditions. However, he remarked that, since there will be no further experiments on N A (or h) after the redefinition, the case presented is purely theoretical. Horst Bettin replied that the date of the redefinition depends upon the risk we are prepared to accept. Michael Kühne also discussed the risks of changes in the mass unit after the potential discovery of a mistake. Alan Steele said that there is no way to avoid such a possibility, and saw a similarity with existing unexplained instability in prototypes such as No. 9. Michael Borys expressed his concern that potential negative consequences might also affect NMIs. Carl Williams questioned the economic or safety impact of such an occurrence, concluding that in any case it would be negligible. Roman Schwartz went back to the questioned sentence in subclause 2.3 of the current draft mise en pratique. The defense, by Richard Davis, Philippe Richard and Michael Kühne, was that this examination of a claim is common practice with CODATA and, since the realization will be endorsed by the CCM, it is logical that the CCM examines the claim prior to accepting it. About the example showing the insufficiency of the existing conditions established by the CCM recommendation, Walter Bich commented that the present formulation, involving a 95 % level of 1 Martin Milton became BIPM Director on 1 January

9 confidence, is ambiguous and should be better specified. Carl Williams defended the need for changing the SI, in view of the needs in high technology and science. Richard Davis recalled the 2010 Metrologia paper by Michael Gläser et al, in which it is shown that the CCM recommendation is consistent with the needs of mass metrology. Alan Steele, while agreeing on the ambiguity about the level of confidence, confirmed that, overall, the existing recommendation is appropriate. Michael Borys, supported by Carl Williams, highlighted the difference between the uncertainty of h and that of a realization of the kilogram. Walter Bich reminded the workshop that the realization uncertainty is irrelevant to the mass community, if this uncertainty component were common to everybody. Michael Borys clarified his thought by commenting that the quantum Hall and Josephson effects and are much more reproducible than available Avogadro and watt balance results and that traceability to the SI kilogram is important for many applications. Horst Bettin then continued his presentation by noting that the uncertainty achieved with the silicon route is more than sufficient for a local dissemination towards any conceivable application. Michael Gläser suggested removal of the last sentence in 3.1 of the mise en pratique, dealing with the uncertainty of a local realization. The discussion moved on the role of (natural) silicon spheres in monitoring the stability of an ensemble. Concerning Annex A.3 of the mise en pratique, dealing with this issue, Horst Bettin clarified that the use of the AVO28 crystal is decided by its owners; however, more crystals will be available soon. He repeated the analysis of the time needed for a realization by XRCD, repeating his remarks of the first day that the only measurement for a realization is the surface characterization, and that this takes about one week. He subsequently made an overview of the future of XRCD, defending its validity as a primary route to the realization of the kilogram. Finally he laid down the recommendation of the WGAC concerning the timeline for the redefinition. First, the results should be confirmed, next, the discrepancies solved and the spheres re-calibrated with respect to IPK; then the mise en pratique should be revised accordingly and eventually the mass unit could be redefined. Alan Steele suggested that, beside the mise en pratique, a supplementary document, perhaps a handbook, specifically devoted to details of the various methods should be developed. Walter Bich asked for a timeline for such potential initiative. Ian Robinson asked whether it could be believed that many Avogadro projects do contribute separately or rather there is a unique contribution from many concurring sources. Nieves Medina (CEM) presented the viewpoint of the NMIs neither involved in watt balance nor in XRCD experiments. She recalled that these represent about 90 % of the total. She had collected opinions through a questionnaire sent to EURAMET Contact Person for mass and related quantities. There were very few replies, and not to all questions, so that little value could be credited to the questionnaire. This meager feedback was ascribed to lack of information or, perhaps, of interest. Carl Williams acknowledged that leading NMIs have responsibilities and should give due consideration to the global requirements of the other NMIs. Walter Bich recalled a recent experience with public enquiries on the VIM and GUM, which gave useful feedback. Carl Williams agreed, but was persuaded that people should be educated before being asked an opinion through a questionnaire. Also for Alan Steele the quality of responses showed different levels of understanding of the problem. Reaction from other Regional Metrology Organizations (RMOs). Abdeljelil Mekki (Tunisia), in the name of AFRIMETS, feared a separation among CMCs of laboratories, depending on whether or not they have a primary realization of the kilogram. Also for this reason, traceability to the BIPM is preferred. There was no immediate reaction from other RMOs. There was a general feeling that the mise en pratique should be revised in an effort to clarify it to a non-initiated readership. 9

10 Walter Bich gave a presentation on the risk that sometime in the future the mass community might be unable to realize the kilogram because of lack of primary realizations. He said that it seems reasonable to imagine that five independent realizations at the time of the redefinition would represent a redundancy sufficient to keep reasonably low the risk that <3 in the first ten years or so after the redefinition (where n is the number of realizations). He concluded by giving his views about the best way of disseminating. There are two possibilities, each represented by an existing example, the length unit and the time unit. The mass community can choose either way, but his preference goes to the latter way. Among the various comments from Alan Steele, Carl Williams, Dave Inglis, Enrico Massa on the number of independent realizations, and on the probability that an experiment will close, Jon Pratt pointed out that three methods are already available, the third being a voltage balance operating at the milligram level and below. Abdeljelil Mekki, in the name of AFRIMETS, expressed a word of caution, as the uncertainties required in the CCM recommendation are not yet reached. He felt that IPK should not be put in a museum, but be kept to validate watt balances in order to monitor their stability. Estefanía de Mirandés then talked about advantages and difficulties of the combined pool of artefacts maintained by the BIPM and NMIs with primary realizations of the kilogram. She gave an overview of the dissemination before and after redefinition. She then highlighted the advantages of a pool, which is readily available, can be routinely calibrated by means of primary realizations and can help comparing primary realizations available at different times. In addition, mass differences can be accurately measured frequently. Finally, a pool can maintain the realization between two primary campaigns. She then went on listing the characteristics of an ideal ensemble, i.e., a sufficiently large number of standards, a clean environment etc. She then described the present status of the BIPM pool, developed according to resolution 1 of the 24 th CGPM. Currently the pool is made of four platinum-iridium, four stainless steel, four silicon standards, and four stacks (one Pt-Ir, one Si, and two stainless steel). These standards, all having a nominal value of 1 kg, are kept under four different storage conditions, i.e., low-flow (0.3 l/min) argon and nitrogen, vacuum and air. Temperature, humidity and pressure are monitored in the cabin housing the containers. Many further details were given on storage conditions and analysis. Michael Borys asked how the stacks are stored and handled and highlighted a possible problem in the calculation of the sorption coefficients if the conditions were not the same as for the companion mass artefacts. In addition, Philippe Richard remarked that there are no two standards of the same material under the same conditions. Richard Green was reassured that any standard will always be exposed to the same atmosphere, including during weighings. Gérard Genevès suggested that smaller samples used for surface analysis also be included in the ensemble. Patrick Abbott asked a clarification about the kind of seal of the containers (which is an elastomer joint for all of them) and whether there is a correlation between seal material and results (which has not been found so far). In answer to a question by Jon Pratt, it was explained that surface analysis is not planned presently. The roughness of standards is as yet unknown. Carl Williams said that standards should be characterized in many aspects so that if something would go wrong the reason could be more easily found. Richard Davis agreed, but pointed out that this would take a very long time. Alan Steele asked about the connection with the world ensemble and how could NMIs collaborate to augment the pool. Michael Kühne suggested that having only one standard of a given material in each different environment might be a limitation, which NMIs could contribute to mitigate. Alan Steele stressed that the emphasis should be on dissemination from the BIPM, not on stability studies, and expressed scepticism about the initiative. Carl Williams asked whether the pool would be the same intended to be used for general dissemination. This is indeed the case, although not immediately after redefinition. Richard Green wished for more redundancy. Ian Robinson asked about the role of transfer 10

11 standards in the ensemble. The reply was that they will be assigned an uncertainty, but will not participate in the weighted mean. Walter Bich commented on the limitations in the possible comparison schemes among pool elements. Gérard Genevès said that what really matters is whether the structure of the ensemble does meet the expectations of NMIs, at which Michael Kühne declared that of course the BIPM is open to cooperation. Philippe Richard asked about the system running cost, which is mostly to be ascribed to the bottles of high-purity gases, and could run to about a year if the system shown in the slides were to be adopted without modification. BIPM are working on methods to reduce this cost by a factor of 10. Roman Schwartz asked whether there is an idea about the time when one environment will be selected, four years seeming a reasonable lapse of time. Richard Green suggested sampling the atmosphere within the containers at discrete time intervals, but it was replied that flow (and analysis) would be continuous. Walter Bich commented that once a specific environment is selected, the routine situation will be much simpler. He also asked whether the treatments given by Estefanía de Mirandés and Lars Nielsen are concurring or competing. Michael Stock (BIPM) suggested that the CCM meeting in February would be the right place to discuss the issue. Philippe Richard gave the views of the NMI directors on their long-term support to primary realizations. He had contacted the directors of all the NMIs with projects in the kilogram redefinition, and got five replies. All gave strong support to resolution 1 of the 24 th CGPM, and showed strong intention to have available primary realizations and tools for dissemination. All supported the ideas of comparing watt balances before redefinition and of developing simpler balances. As to the timeline, there was no urge to change the definition, and prudence seemed to dominate, in order to avoid possible negative impact. The majority of responses seemed to indicate five to ten years as a reasonable time lapse. A one-year further support will be given to the solution of the NIST-NRC discrepancy. Finally, the position of NMI directors does not impact on the mise en pratique. Then the time came for conclusions and decision of actions. All were invited to comment on the mise en pratique or to suggest alternative wordings by using a template that would be circulated. Also reconsideration should be given to the CCM recommendation and to removal of any ambiguity from it. The need for separate documents, among which a Frequently Asked Questions, was reviewed. Also the protocol for the comparison before redefinition needs to be prepared. It was asked that the stakeholders positions be published on the BIPM website. It was understood that the discussion would continue until the next CCM, considering the many technical issues that are still outstanding. Carl Williams raised the question whether the reply to CECIP should be responsibility of the CCM, and the universal feeling was that it rather falls to the CIPM 2. Finally, the Chairman Richard Davis warmly thanked all the participants for their contribution to the success of the event and closed the workshop. The rapporteur Walter Bich 2 Note : The President of the CIPM replied to CECIP on 6 December