News from the BIPM laboratories—2018

Significant progress has been made on the development of the BIPM Kibble balance. The suspension was modified in order to align all components more easily and independently. This improvement allowed more accurate alignment of the coil with respect to the magnet, which had previously been the main contribution to the type B uncertainty. The use of a programmable Josephson voltage standard (PJVS) for the current and the induced voltage determination further improved the measurement accuracy. The apparatus has been operating reliably in vacuum since mid-2018. Measurements of the Planck constant were carried out using a 1 kg stainless steel and a 1 kg Pt–Ir mass standard. As a consequence of the improvements, the measurement uncertainty has been reduced to the level of 2 parts in 10⁷. This uncertainty is dominated by instability in the vertical alignment of the interferometer beams when going from air, where the alignment is made, to vacuum for measurement. A new interferometer based on a novel design, and which is fixed on a much more stable support, is to be integrated into the apparatus to further reduce the measurement uncertainty. A detailed study has been carried out to evaluate the effect of the coil-current on the magnetic field [1–3]

A new 1 kg Pt–Ir prototype was delivered to Pakistan in 2018. Since the signing of the Metre Convention in 1875, 1 1 1 Pt–Ir 1 kg prototypes have been fabricated for 44 NMIs and the BIPM and they will continue to be made available for Member States upon request. During 2018, four Pt–Ir mass prototypes and eleven stainless steel standards were calibrated for eight NMIs. The mass stability of the BIPM working standards used for these calibrations is verified by an annual comparison with working standards of higher order, which are used only for this purpose.

The BIPM will continue to provide calibrations of mass standards for the NMIs of Member States following the redefinition of the kilogram, which comes into force on 20 May 2019. This follows the revision of the International System of Units (SI) by a decision adopted by the 26th meeting of the General Conference on Weights and Measures (CGPM) in Versailles on 16 November 2018 [4]. Detailed guidance will be provided on the introduction of an additional uncertainty of 10 µg for the mass of the IPK. The maintenance and dissemination of the kilogram following its redefinition is described in [5]. The final report on the pilot comparison of future realizations of the kilogram, carried out in 2016, has been published [6].

The department's electricity laboratories have organized a key comparison of electrical capacitance calibrations, CCEM-K4, with seven NMI participants from four regional metrology organizations (RMOs). This was the first consultative committee for electricity and magnetism (CCEM) comparison to adopt the star scheme, which consists of a set of parallel bilateral comparisons carried out between the participating NMIs and the BIPM. The comparison results agreed within  ±5 parts in 10⁸ at 10 pF and within  ±10 parts in 10⁸ at 100 pF, which is consistent with the claimed uncertainties. The comparison also allowed the evaluation of the difference between the value of the von Klitzing constant R_K measured by electrical means and based on the calculable capacitor, and the value recommended in the CODATA fundamental constants adjustment of 2014. The latter value is dominated by measurements of the anomalous magnetic moment of the electron and atomic recoil. A difference of (43  ±  23) parts in 10⁹ (k  =  1) has been found [7]. The comparison was completed within a period of less than two years. The CCEM community will consider if this model can be applied to comparisons of other quantities.

The department has organized two further on-site comparisons of quantum Hall resistance (QHR) standards, with the NRC (Canada) and the NMIJ (Japan) and the report of a previous comparison with METAS was published [8]. These comparisons require considerable logistics in transporting more than 1 tonne of equipment to the participating NMI: the main elements are the cryostat with magnet and QHR sample, a resistance bridge and thermo-regulated resistors of 1 Ω, 100 Ω and 10 kΩ. The measurands are the value of the 100 Ω resistor, measured using the BIPM and the NMI QHR standards and bridges, and the 1:100 ratios between the resistors, measured with the BIPM and NMI bridges. The comparison with the NRC has led to very good agreement at the level of 1–2 parts in 10⁹ [9]. The observation made during previous comparisons that the value of the 1 Ω standard depends on the cycle time of the comparison bridge was confirmed. The results obtained at the NMIJ are being analysed. In addition to the on-site QHR comparisons, the department organizes bilateral resistance comparisons using resistance transfer standards [10, 11].

A major renovation of the four terminal-pair coaxial bridge for ac-resistance measurements has been completed in collaboration with Dr Norihiko Sakamoto from NMIJ, on secondment to the BIPM for one year. This bridge plays an important role in linking the BIPM's 10 pF reference capacitors to its dc-QHR standard. It is used in particular to determine the frequency dependence of the two transfer resistance standards of the quadrature bridge from a Haddad resistor with calculable frequency dependence. After its renovation, complete characterization of the bridge was carried out, followed by a new measurement of the frequency corrections of the transfer resistors.

Two calculable Haddad resistors of nominal values 1000 Ω and 1290.64 Ω have been fabricated for the NIM (China) in collaboration with Dr Lu Huang from NIM, on secondment at the BIPM for four months. These resistors are intended to be used to link the dc-QHR of the NIM to its new calculable capacitor. After fabrication, the two resistors were characterized with the newly renovated four terminal-pair ac-resistance bridge of the BIPM. Further Haddad resistors are currently being fabricated for the LNE (France) and the BIPM.

The BIPM is supporting one of the first comparisons by GULFMET in the field of electricity; GULFMET.EM.BIPM-K11, a comparison of Zener voltage standards. Following the secondment of the organizer, Dr Steven Yang from SCL (Hong Kong (China)), to the BIPM for two months and a first series of calibration measurements in 2017, the travelling standards were measured for a second time during 2018 and their temperature coefficients were determined. BIPM participation will allow this comparison to be linked to the BIPM.EM-K11 comparison series and to compare the performance of the participants from GULFMET with those of NMIs from other regions.

The BIPM is developing a new protocol to extend its onsite comparison of quantum voltage Josephson standards from DC to AC voltages for frequencies below 1 kHz. A pilot study was carried out at the NPL (UK) in February 2018, following those previously undertaken with the NMIJ [12], the CENAM (Mexico) and the PTB (Germany). The BIPM contributed to a comparison of two cryo-cooled programmable Josephson standards at the NIST [13]. The aim of these studies is to gain experience in the comparison of AC voltages and to investigate the metrological behaviour of different AC sources used as transfer standards. At the NPL, an agreement within a few parts in 10⁶ could be achieved in measuring rms values of sinewaves at 1 V rms and 60 Hz, using the differential sampling method. The noise of the phase-locking process appeared to be the limiting factor. In addition, a comparison of AC signals was performed between the NMIA (Australia), using thermal converters, and the BIPM, using its PJVS, with a voltmeter as a transfer standard. In this comparison the input filter of the voltmeter was identified as a potential source of error. In addition to the onsite comparison of Josephson voltage standards, the department organizes bilateral voltage comparisons using secondary Zener voltage standards as transfer standards [14, 15].

In parallel, the influence of different types of samplers on the differential sampling technique was investigated with the support of Dr Mun-Seog Kim from KRISS (Republic of Korea), on secondment at the BIPM for one year. The observed differences between samplers, which increased with frequency, have been attributed to the differences in the input bandwidths. Work on the determination of leakage resistance to ground on PJVS, performed in collaboration with NIST, continued. The sources of leakage to ground were investigated using a direct comparison setup between two NIST cryo-cooled systems [13].

In addition to its comparison programme for electrical quantities, the department provided nearly 60 calibrations to NMIs in the field of resistance, capacitance and voltage during 2018.

There have been major changes within the Time Department during 2018 with the retirement of three physicists and the recruitment of two new physicists. Considerable effort has been devoted to ensure that specialist knowledge is maintained and that responsibilities are effectively transferred to guarantee the ongoing accuracy and reliability of the Time Department's products.

Most of the department's research work in 2018 that was carried out towards the computation of the reference time scale coordinated universal time (UTC), its rapid approximation UTCr, and the contribution to the key comparison CCTF-K001.UTC, has been devoted to the improvement and development of time transfer techniques. The introduction of clocks and frequency standards in UTC is based on the measures obtained by comparing clocks located in different laboratories, and these measures carry the most important uncertainty contribution.

Two families of time transfer techniques are currently used in the formation of UTC: those based on the passive use of global navigation satellite systems (GNSS) or those based on an active two way satellite time and frequency transfer (TWSTFT). Both techniques are well established and further improvements are being studied in different NMI time laboratories.

GNSS activities focused mainly on testing the new Chinese Beidou and European Galileo systems; satellites have been launched and the constellations and service availability are almost complete. These activities have benefitted from collaboration with NIM (China), which has produced and installed Beidou receivers in many laboratories, including the BIPM, which will be used for tests [16, 17]. As the Beidou constellation includes satellites in geostationary and inclined geosynchronous orbit, a more refined data analysis is necessary, particularly for the benefit of the use, a posteriori, of precise orbits and clocks. The first results with the Galileo system show very good stability, including with real-time broadcast parameters. The results were discussed at a dedicated workshop organized by the CCTF working group on GNSS time transfer (CCTF-WGGNSS) and will be presented at future international congresses.

Research is also continuing, in collaboration with the CNRS (France), on the use of integer precise point positioning (IPPP) as an improved treatment of the GNSS carrier phase measures to avoid cycle slips that degrade the stability of the time transfer solution [18, 19].

The GNSS calibration campaigns, in coordination with the RMOs, is progressing as expected, with the aim of visiting the G1 laboratories in each RMO every 2 years. Two new GNSS calibration campaigns have been carried out using the BIPM travelling apparatus in Russia and another in Asia, which visited three different G1 laboratories. A calibration report will be published. Results of GNSS calibrations are available on the BIPM website.

The department's work on TWSTFT has been devoted to the use of the software defined radio (SDR) receiver which has been evaluated in collaboration with the NMIs and the SDR baseline is now used as an 'alternative' (back up) solution in the computation of UTC [20, 21]. The results appear on the link comparison, which is regularly published on ftp://ftp2.bipm.org/pub/tai/timelinks/ and their analysis is continuing in collaboration with OP SYRTE (France).

To support the further development of the SDR technique to allow, for example, calibration, transmission, and testing on other codes, the BIPM, in cooperation with the CCTF Working Group on TWSTFT, has created a platform running GITLab for collaborative open source code development. A task group has been created to validate the different versions of the software and to provide a convenient and safe way for its download to NMIs.

In addition, the Time Department is working, in collaboration with the OP SYRTE, on the preparation of a calibration travelling box, which is aimed at calibrating the TWSTFT links by means of GNSS receivers and a carefully designed measurement chain. The goal is to have a calibration uncertainty of the order of 1 ns, which could be used when the traditional TWSTFT mobile calibrating station is not available or is not suitable.

The BIPM has started working with the Working Group for the exploitation of the atomic clock ensemble in space (ACES) experiment [22] on the International Space Station (ISS) with the aim of studying a high-accuracy microwave link for possible future use in UTC.

To ensure the accuracy of UTC the role of primary and secondary frequency standards is fundamental. The Department is following the development in the NMIs and supporting the introduction of frequency measurements in UTC [23–25]. In addition it is exploring the use of frequency standard for applications in geodesy and gravity field determination [26, 27]. A workshop on relativistic geodesy has been organized at the BIPM together with the International Association of Geodesy (IAG) (https://ife.uni-hannover.de/569.html)

The Time Department organized a training course within the framework of the BIPM CBKT programme on 'Effective participation in UTC'. The course, which was held at the BIPM on 13–14 February 2018, was supported by METAS (Switzerland) and included a practical session on the correct use of GNSS receivers in collaboration with the main GNSS timing receiver manufacturers. A significant number of applications were received to participate in the course and the positive feedback from the attendees supports the proposal to organize a further UTC training session within the RMOs.

The department supported initiatives to celebrate of the 50th anniversary of the atomic second in 2018 [28].

The work in the Ionizing Radiation Department in 2018 continued to focus on the provision and development of comparison and calibration services. These centralized services reduce the need for NMIs to pilot large-scale comparison exercises and also minimize the transport of hazardous materials or fragile instrumentation. The services form the basis of an established and robust measurement infrastructure in a field that impacts cancer therapy, medical imaging, safety of radiation workers, medical device sterilization, and environmental protection.

Comparisons and calibrations of standards for radiation dosimetry have continued using the BIPM x-ray and photon beams. A total of seven comparisons for four NMIs and 15 calibrations for three NMIs were carried out during 2018.

All dosimetry primary standards require key physical data and correction factors that are based on published international recommendations. These data have recently been updated to incorporate recent studies; the changes will affect the results from comparisons and calibrations and will also impact published CMCs. The changes to the BIPM standards have been published [29] and will come into effect on 1 January 2019.

A highlight for the department in 2018 has been the further development of the comparison service for primary standards for high-energy photon radiotherapy, based at the DOSEO research centre at Gif-sur-Yvette, France. Comparisons were completed for the KRISS (Republic of Korea) and METAS (Switzerland) and a procedure has been developed to reduce the measurement uncertainties due to small instabilities in the beam. In addition, a new project has started in collaboration with the LNHB (France) with help from a secondee from the NRC (Canada) to investigate the possible impact of variations on instrument calibration due to differences in photon spectrum between different models of accelerators.

The department's work has been affected by the increasingly stringent regulations on the use of sealed radioactive sources. The ¹³⁷Cs and ⁶⁰Co irradiators used for comparisons and calibrations for radiation protection standards have had to be closed; a project is under way to re-establish the former service in 2019 using the ¹³⁷Cs beam at the IAEA Laboratory in Austria. An older therapy-level ⁶⁰Co irradiator was also decommissioned and services were successfully transferred to a newer irradiator. For the same reason, a project has started with the IRA (Switzerland), the NPL (UK), the LNHB (France) and Triskem International to replace radium sources needed for radionuclide metrology; a suitable radionuclide (^166mHo) has been identified and source preparation is under way. A joint NIST-BIPM workshop was organized to investigate options to minimize the need for sealed reference sources of this type using a new technology for low current measurement. Following this workshop, it was agreed that a project will start with the PTB (Germany) in 2019 to study the use of Ultra-stable Low Current Amplifiers.

The international system for radionuclide metrology relies on a set of stable and reproducible specialist instruments. The international reference system (SIR) is used to compare activity standards of long-lived gamma-ray emitting radionuclides, and was used to compare standards of ²²³Ra and ⁶⁵Zn during 2018. This instrument, which is not suitable for short-lived radionuclides, is complemented by a transportable version (the SIRTI) that is used on site at NMIs. It was agreed with the Key Comparisons Working Group (CCRI(II)-KCWG(II)) that the SIRTI instrument will now be calibrated at the BIPM for additional radionuclides (⁵⁶Mn, ¹⁵³Sm, ¹⁶⁶Ho, ¹²³I) to prepare for a more efficient approach to organizing comparisons. An international consortium of the LNHB (France), POLATOM (Poland), PTB (Germany) and NPL (UK) is working with the BIPM to develop a third instrument for comparing standards of pure beta-emitting radionuclides; a specialist instrument has been commissioned and studies are under way to establish the reproducibility of measurements.

Fifteen visiting scientists from NMIs participated in the Chemistry Department Programme in 2018. Six comparisons were run by the department in this period, involving eighty participations by NMIs in these studies. Seven comparison reports were published as well as five papers in peer reviewed journals and three BIPM rapports. The capacity building and knowledge transfer (CBKT) programmes for 'Metrology for Clean Air' and 'Safe Food and Feed' continued, attracting seven visiting scientists from NMIs, spending between 3 months and 1 year at the BIPM.

Three visiting scientists from the NPLI (India), NMISA (South Africa) and KazInMetr (Kazakhstan) undertook the Metrology for Clean Air Course on FTIR Measurements on Gas Standards (NO₂, HCHO, HNO₃, CO₂) in the BIPM laboratories. They received training in the use of B-FOS software for use with Fourier transform infrared (FTIR) in gas metrology applications; the software being made available later for use within the participating NMIs. Short training courses on the use of B-FOS FTIR software were also provided to visiting scientists from NMIJ (Japan) and PTB (Germany).

The third meeting for the CBKT programme on 'Metrology for Safe Food and Feed', focusing on mycotoxin metrology and standards, was held at the BIPM in April 2018. The laboratory programme on mycotoxin standards was supported by three visiting scientists from NIM (China) working on related structure impurity analysis, calibration solution characterization, and qNMR for aflatoxin B1 and deoxynivalenol materials, as well as a visiting scientist from UME (Turkey) characterizing pure patulin material. In addition, two visiting scientists from UME (Turkey) and NMISA (South Africa) undertook three-month training secondments on non-related structure impurity quantification in mycotoxin pure materials. Stock solutions of zearalanone were provided to all NMIs that had participated in the programme to date (KEBS, INTI, NMISA, NIMT, INMETRO, UME), and the first key comparison (CCQM-K154.a) on mycotoxin calibration solutions was launched in the last quarter of 2018. NMI-prepared calibration solutions were scheduled to undergo comparison measurements at the BIPM in December 2018, with ten participations from NMIs. A paper summarizing the mycotoxin standard preparation and validation was published [30].

Within the BIPM's small organic primary calibrator programme, the final report of the CCQM-K55.d comparison on folic acid purity was published [31]. The final report for the CCQM-K78.a comparison on multi-component amino acid calibration solutions was completed and circulated to the CCQM for approval in November 2018. Samples for the CCQM-K148.a comparison (bisphenol A calibrator purity) were distributed, with 20 NMI participations, and results are to be submitted to the BIPM by February 2019.

Three reference data documents on 'Internal Standards for qNMR' were completed for maleic acid, dimethyl sulfone and potassium hydrogen phthalate and were published on the BIPM website as BIPM Rapports 2018/01 [32], 2018/04 [33] and 2018/05 [34], respectively. This was an output of the universal calibrator programme for qNMR at the BIPM, an activity initiated together with the NMIJ, and supported in 2018 by secondees from the NIM (China), with the remaining four reference data documents in preparation. A visiting scientist from INTI (Argentina) was seconded to the BIPM for knowledge transfer on qNMR techniques as well as to provide training to BIPM staff on NMR techniques for chemical structure and identity confirmation.

Support for the CCQM programme of peptide and protein primary standard comparison continued. Preparative work for the comparison on oxytocin (CCQM-K155.b) was completed in collaboration with NIM (China) and with the secondment of a visiting scientist from NIM. Samples for the comparison have been distributed with results to be submitted to the BIPM by the end of December 2018. There are 12 participations from NMIs in this comparison. Characterization work on pure hexapeptides of HbA1c for comparisons on the HbA1c glycated hexapeptide (GE), for CCQM-K115.c, and the non-glycated hexapeptide (VE), for CCQM-K115.2019, have continued with the support of a visiting scientist from NIMT (Thailand), and in collaboration with HSA (Singapore), LNE (France) and NIM. Methods developed for pure peptide characterization were published in Analytical and Bioanalytical Chemistry [35] and an invited review paper [36].

In the area of air quality measurement standards, the BIPM continued to contribute to the CCQM Working Group on Gas Analysis (CCQM-GAWG) Ozone Cross-Section Task Group. A paper summarizing the recommended best value and uncertainty for the ozone cross-section to be used in the key comparison BIPM.QM-K1 was submitted to Metrologia. Three NMIs: FMI (Finland), NILU (Norway), and NPLI (India) sent their ozone standards to the BIPM and participated in BIPM.QM-K1, with four reports [37–40] of the comparison published in Metrologia and the BIPM KCDB. One calibration of an ozone standard was performed for NILU. Collaboration with the NIST (USA) on the upgrade of the electronic module for the Ozone standard reference photometer (SRP) continued. The 2018 version of the prototype electronics module was successfully constructed and tested at the BIPM; the operating software is being developed by NIST. The Draft A report of CCQM-K137 (NO in N₂ at 30 µmol mol⁻¹ and 70 µmol mol⁻¹) was completed and the key comparison reference value (KCRV) was agreed by the CCQM-GAWG in October 2018. Measurements at the BIPM for the comparison of NO₂ in N₂ standards at 10 µmol mol⁻¹ (CCQM-K74.2018) and the pilot study of HNO₃ measurements in such standards (CCQM-P172) have started with 33 NMI participations.

In the area of greenhouse gas standards, the final report of the CO₂ in air standards comparison CCQM-K120 has been published [41]. The comparison demonstrated a reduction in the reference value uncertainty by a factor of four compared to comparisons performed previously. In addition, it demonstrated the improvements in the accuracy of standards and comparison methods that can now be achieved. The BIPM's CO₂ PVT primary system, which is based on manometry, was compared in the pilot study CCQM-P188, which was performed in parallel to the key comparison. Good agreement was observed with this method at levels of uncertainty equivalent to those that can be achieved using gravimetric methods. The development and validation of a manometric system for CO₂ measurements, in support of a future planned ongoing comparison of CO₂ standards (BIPM.QM-K2), has progressed with a second secondment from the NIST. Development work will continue with characterization of adsorption and trace gas effects as well as automation of the system planned. Preparation for a comparison on CO₂ isotope ratio standards, coordinated jointly by the BIPM and the IAEA continued, with an isotope ratio infrared spectrometer (IRIS) system for isotope ratio measurements integrated into the SIRM-GEN facility. The first blending experiments for CO₂ gases have been carried out with support from visiting scientists from INRIM (Italy) and VNIIM (Russia). Samples prepared at the BIPM were sent and measured at the IAEA. The NMIJ has donated highly-characterized pure CO₂ gas to the BIPM to support the validation work that is being carried out. A review paper summarizing standards activities for greenhouse and air quality gases has been published [42]. Also published is a review of SI traceable versus scale approaches for metrological traceability for greenhouse gas monitoring [43].