Key and supplementary comparisons of measurement capabilities across laboratories at an international scale are the backbone for a worldwide reliable quality infrastructure where measurements are of high accuracy and metrologically traceable to the definitions of the International System of Units (SI). Both, these comparisons and the related calibration services provided by metrology organizations under the CIPM Mutual Recognition Arrangement have a direct impact on national gross domestic products. However, rapidly evolving technology presenting new techniques for measurement, lack of experience of early career metrologists, and the negative impact of demographic changes are challenging the ability of pilots and participants of comparison campaigns to meet timely demands in their economies. There are cases where pilots need almost a year to perform data analysis using self-developed tools, and to prepare reports. Extending the capacities for data analysis in comparisons by well-defined digital approaches promises many benefits to the metrology community and its stakeholders. In the nearer term, an improved integrity of outcomes and a significant reduction of time for data analysis is foreseen. The availability of validated software for almost autonomous data evaluation and standards for machine-actionable data exchange are trailblazers for these developments. Medium- and longer-term gains will be higher levels of automation of end-to-end workflows and achieving a more sensitive data analysis using advanced FAIR data standards and Artificial Intelligence (AI). The Digital Metrological Expert (DME) approach is developing a powerful tool, enabling users to perform a digital analysis and to report data in key and supplementary comparison. The DME concept defines autonomously operating software with the ability of performing data and result processing for standard work as requested by human metrology experts. It provides processing steps such as the calculation of reference values, equivalence values, and model based measurands as well as a verification of metrological data, etc. and it can take over a significant amount of tedious human work. The DME exchanges machine-actionable information with external digital tools and services of the quality infrastructure through SI-based data and services that follow the FAIR principles for findable, accessible, interoperable, and reusable data. It can assess data and can propose suitable ways of processing depending on the intended application. This can include, for example, verification of data, transformation of data, application of filters, identification of outliers, and methods for propagation of measurement uncertainty. In future, innovative AI-based algorithms will be used to reveal inconsistencies in data that are not obvious to the human user and to foster the automation of the data processing. Finally, outcomes of the tool are provided by machine-actionable reports that clearly describe the (metrological) traceability of all outputs to the input data. Resolvable, unique, and persistent identifiers are the core to interlink all data and metadata for this purpose. Our contribution will give insights into the current state of the DME and its interrelation to developments of the SI Digital Framework, FAIR and metrological data formats, and SMART standards. Current use-case comparisons implemented in the domains of mass, temperature, and protein analysis will be presented.

The National Research Council of Canada's (NRC) Kibble balance contributes to the determination of the Consensus Value of the kilogram through participation in periodic key comparisons. This paper provides an overview of the Consensus Value of the kilogram and the realisation and dissemination of mass within the present phase of traceability through it. Description of NRC's participation in international comparisons used to calculate the Consensus Value is detailed, including methods to operate the Kibble balance, major uncertainties, and the evolution of key supporting measurements. The potential benefits of using a digital twin to optimise Kibble balance experiments and of digital comparisons to evaluate the equivalence of realisation experiments and determine the Consensus Value of the kilogram are also discussed. We offer perspectives on the possible future landscape of mass metrology once the use of individual realisations of the kilogram has been sanctioned and the final phase of dissemination from the Planck constant that began with redefinition of the SI in 2019 is reached.

We present the proof of concept for the first fully automated evaluation of a virtual mass comparison. The focus of this comparison is to demonstrate a possibility of digital transformation in metrology using an automated evaluation chain of different tools as well as machine-interpretable files for data transfer and reporting. We discuss the different tools and exchange formats as well as the performance of the virtual mass comparison in detail.

Digital twinning is a rapidly growing area of research. Digital twins combine models and data to provide up-to-date information about the state of a system. They support reliable decision-making in fields such as structural monitoring and advanced manufacturing. The use of metrology data to update models in this way offers benefits in many areas, including metrology itself. The recent activities in digitalisation of metrology offer a great opportunity to make metrology data ‘twin-friendly’ and to incorporate digital twins into metrological processes. This paper discusses key features of digital twins that will inform their use in metrology and measurement, highlights the links between digital twins and virtual metrology, outlines what use metrology can make of digital twins and how metrology and measured data can support the use of digital twins, and suggests potential future developments that will maximise the benefits achieved.

Main text This report describes the second CCM key comparison of realizations of the kilogram definition based on the fixed numerical value of the Planck constant, which came into force on 20 May 2019. The objectives were to determine the level of agreement between realizations of the kilogram using Kibble and joule balances and the x-ray crystal density (XRCD) method and to provide input for the calculation of the second "consensus value" of the kilogram. The consensus value serves as the basis for an internationally coordinated dissemination of the kilogram and is updated after each new key comparison. Its use will continue until satisfactory agreement between realization experiments has been achieved. Another objective was the determination of the reproducibility of the realization experiments by comparing the new results with those of the first key comparison of kilogram realizations, CCM.M-K8.2019. The comparison was organized by the BIPM and had nine participants. The BIPM, LNE, METAS, NIST, NRC and UME operated Kibble balances, the NIM used a joule balance and the NMIJ and the PTB participated using 28Si spheres, the masses of which were determined with the XRCD method. These realization methods were used to calibrate 1 kg mass standards under vacuum or in air. The standards were sent to the BIPM where they were compared with each other and with BIPM Pt-Ir working standards. The latter were calibrated traceable to the International Prototype of the Kilogram (IPK), the mass of which served as the definition of the kilogram until 20 May 2019. The results of the weighings at the BIPM together with the measurement results communicated by the participants allowed comparison of the values attributed to 1 kg mass standards using the realization experiments of the participants. The level of agreement between mass determinations with the realization experiments and the BIPM as-maintained mass unit, traceable to the Planck constant through the mass of the International Prototype of the Kilogram, could also be deduced. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/. The final report has been peer-reviewed and approved for publication by the CCM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

This study compared the surface properties and rising velocities of pristine and weathered plastic production pellets, to evaluate impacts of environmental conditions. Rising velocities were measured for 140 weathered pellets collected from a Spanish beach and compared with pristine low-density polyethylene, high-density polyethylene and polypropylene pellets. A subset of 49 weathered pellets were analysed by Fourier-transform infrared spectroscopy (FTIR), with all found to be polyethylene. Experimental rising velocities for the weathered pellets varied widely, from (2.36 ± 0.01) cm s-1 to (10.56 ± 0.26) cm s-1, with a mean value of (5.79 ± 0.06) cm s-1. Theoretical rising velocities were consistently higher than experimental velocities for all pellet types: on average 136% of experimental values for weathered pellets. This discrepancy was more distinct for less spherical pellets, which were often more weathered. Flatter pellets often oscillated as they rose, which explains at least some of this finding. Atomic force microscopy (AFM) analysis revealed that the roughness of the pristine and weathered pellets was (59 ± 11) nm, and (74 ± 26) nm respectively. X-ray photoelectron spectroscopy (XPS) analysis showed that the proportion of surface oxidised carbon species were 2.3% and 4.0% of the total carbon signal for a pristine and a weathered pellet, respectively; consistent with photochemical reactions changing the surface chemistry of weathered pellets. As determined by density column, weathered pellets had slightly lower experimental densities than pristine pellets. Overall, this study illustrates why it is important that modelling studies on the environmental fate and/or movements of microplastics validate or correct predictions using experimental data.

Data associated with publication https://doi.org/10.1595/205651321X16183288904988

Following the completion of the first key comparison of realizations of the kilogram, CCM.M-K8.2019, the internationally coordinated dissemination of the kilogram has entered into a new phase on 1 February 2021. The traceability of the mass unit to the Planck constant will now be taken from the ‘consensus value’ of the kilogram. This letter provides the background on the phases of the dissemination of the kilogram and describes the determination of the consensus value and its consequences for mass traceability.

It is known that platinum-rhodium thermocouples exhibit mass loss when in the presence of oxygen at high temperatures due to the formation of volatile oxides of platinum and rhodium. The mass losses of platinum, Pt-6%Rh and Pt-30%Rh wires, commonly used for thermocouples, were considered in this paper to characterise the mass loss of wires of the three compositions due to formation and evaporation of the oxides PtO 2 and RhO 2 under the conditions that would be seen by thermocouples used at high temperature. For the tests, the wires were placed in thin alumina tubes to emulate the thermocouple format, and the measurements were performed in air at a temperature of 1324°C, i.e. with oxygen partial pressure of 21.3 kPa. It was found that the mass loss of the three wires increases linearly with elapsed time, consistent with other investigations, up to an elapsed time of about 150 h, but after that, a marked acceleration of the mass loss is observed. Remarkably, previous high precision studies have shown that a crossover after about 150 h at 1324°C is also observed in the thermoelectric drift of a wide range of platinum-rhodium thermocouples, and the current results are compared with those studies. The mass loss was greatest for Pt-30%Rh, followed by Pt6%Rh, then platinum.

“When I use a word, it means whatever I want it to mean”: Humpty Dumpty in Alice’s Adventures Through The Looking Glass, Lewis Carroll. “Digital twin” is currently a term applied in a wide variety of ways. Some differences are variations from sector to sector, but definitions within a sector can also vary significantly. Within engineering, claims are made regarding the benefits of using digital twinning for design, optimisation, process control, virtual testing, predictive maintenance, and lifetime estimation. In many of its usages, the distinction between a model and a digital twin is not made clear. The danger of this variety and vagueness is that a poor or inconsistent definition and explanation of a digital twin may lead people to reject it as just hype, so that once the hype and the inevitable backlash are over the final level of interest and use (the “plateau of productivity”) may fall well below the maximum potential of the technology. The basic components of a digital twin (essentially a model and some data) are generally comparatively mature and well-understood. Many of the aspects of using data in models are similarly well-understood, from long experience in model validation and verification and from development of boundary, initial and loading conditions from measured values. However, many interesting open questions exist, some connected with the volume and speed of data, some connected with reliability and uncertainty, and some to do with dynamic model updating. In this paper we highlight the essential differences between a model and a digital twin, outline some of the key benefits of using digital twins, and suggest directions for further research to fully exploit the potential of the approach.

This report describes the first CCM key comparison of realizations of the kilogram definition based on the fixed numerical value of the Planck constant, which came into force on 20 May 2019. The objectives were to determine the level of agreement between realizations of the kilogram using Kibble and joule balances and the X-ray crystal density (XRCD) method and to provide input for the calculation of the first "consensus value" of the kilogram. The consensus value will serve as the basis for an internationally coordinated dissemination of the kilogram which will continue until sufficient agreement between realization experiments has been achieved. The comparison was organized by the BIPM and had seven participants. The BIPM, KRISS, NIST and NRC operated Kibble balances, the NIM used a joule balance and the NMIJ and the PTB participated using 28Si spheres, the masses of which were determined with the XRCD method. These realization methods were used to calibrate 1 kg mass standards under vacuum. The standards were sent (in air) to the BIPM where they were compared under vacuum with each other and with BIPM Pt-Ir working standards. The latter were calibrated (in air) traceable to the International Prototype of the Kilogram (IPK), the mass of which served as the definition of the kilogram until 20 May 2019. The results of the weighings at the BIPM together with the measurement results communicated by the participants allowed comparison of the values attributed to mass standards of 1 kg using the participating realization experiments. The level of agreement between mass determinations with the realization experiments and the BIPM as-maintained mass unit, traceable to the Planck constant through the mass of the International Prototype of the Kilogram can also be deduced. Main text To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

In order to demonstrate the equivalence in calibration of mass standards among National Metrology Institutes (NMIs) of EURAMET this key comparison (KC) on 1 kg stainless steel mass standards has been carried out under the auspices of EURAMET. The comparison was undertaken with reference to the International Prototype Kilogram (IPK) as the definition of the unit of mass. The overall result shows good consistency among the participants. Main text To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

The recent redefinition of the kilogram provides encouragement for widespread adoption of primary methods to realise the kilogram. To support this, NPL is building a "next generation" Kibble balance which, by eliminating the need for careful alignment of the coil and magnet, will be simpler in construction and operation than its predecessors. Work is currently concentrated on validating the operating principles of a prototype balance and ensuring its sensitivity and stability.

The results are presented of the key comparison EURAMET 1031 (EURAMET.M.D-K1.1) that covered the measurements of density and volume of silicon spheres of three different masses at 20 °C and 101325 Pa. The volume and density determinations of 15 national metrology institutes (NMIs) were checked and linked to the CCM.D-K1 key comparison. The measurements were carried out near 20 °C and at atmospheric pressure by the hydrostatic method in the time interval from 16 May 2008 to 18 Jan 2011. The comparison was performed in two petals with three spheres in each petal. The travelling standards of petal 1 have a mass of 1001 g, 200 g and 35 g (Petal 2: 984 g, 239 g, 35 g). Whereas the reference values of the 1 kg travelling standards could be determined by the link to the CCM.D-K1 comparison, the density reference values for the smaller spheres were determined by density comparison to the 1 kg spheres using the pressure-of-flotation method. One result was wrong due to a mistake in the mass determination. Additionally, four of the 57 volume (or density) values were discrepant with En values larger than 1.1, 1.2, 1.3 and 1.6. Five NMIs achieved density uncertainties of about 1 ppm (1 × 10−6 in relative terms) or less for the 1 kg spheres. This satisfies the needs of all customers who wish to calibrate solid density standards for other laboratories. Volume determinations of mass standards, air density artefacts or sorption artefacts should reach an uncertainty of about 1 mm3 in order to reduce the effect on the mass uncertainty to about 1 μg. At least for silicon spheres this is reached by eight NMIs. Due to the higher density of stainless steel this may be different for weights and will be checked within the CCM.D-K3 comparison. The results of the comparison can be used to submit new or improved entries in the calibration measurement capabilities table in the BIPM key comparison database. Main text To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Kibble balance experiments have allowed the kilogram to be redefined in terms of the Planck constant. Now that the redefinition is in place, the Kibble balance will allow SI traceable mass (or force) to be realised at any value and at any location. A feasibility study for a novel, scalable electrostatic ‘µKibble balance’, based on the National Physical Laboratory (NPL) next-generation Kibble balance system is presented and its expected performance calculated. A µKibble will allowin-situ, dynamic, small-scale (< g) mass measurements without the current precision limitation caused by subdivision of the kilogram. The measurements will be traceable through electrical calibration rather than transferable mass standards. The instrument will have a wide range of applications in both industry and research.

A comparison of volume standards was undertaken between CEM (ES), NPL (UK), PTB (DE), and INM (RO). For the comparison three silicon spheres are used as transfer standards with masses 1 kg (provided by PTB), 30 g (provided by PTB) and 125 g (provided by NPL). The aim of the EURAMET.M.D-S3 solid density comparison is to compare the results of the density (and volume and mass) determinations of solid samples of the participating laboratories and to evaluate the degrees of equivalence according to the Mutual Recognition Arrangement. The reference conditions for density and volume determination were 20 °C and 101 325 Pa. KEY WORDS FOR SEARCH Density, silicon sphere, Monte Carlo, comparison Main text To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

NPL is building a next generation Kibble balance to encourage more widespread primary realisations of the kilogram following the expected revision of the SI in 2019. The new balance does not require careful alignment of the coil and magnet and is aimed at a relative standard uncertainty of 2×10 -8 . This paper describes progress on the development of the two technology demonstrator instruments which will guide the design of the next generation of Kibble balances at NPL.

In November 2018 the General Conference on Weights and Measures (CGPM) will meet and are likely to ratify the revision of the international system of units (SI). This represents a major change to the way the base SI units are defined and realised, a major consequence being that the last of the definitions based on a material artefact, the international prototype of the kilogram, will be "retired" and the "new" kilogram will be realised in terms of the Planck constant via the Kibble balance or X-ray crystal density (XRCD) experiment. While the revision of the SI and the redefinition of the kilogram will almost certainly be endorsed by the CGPM there are issues around the current agreement of the Kibble balance and XRCD experiments. This will mean that in order to implement the kilogram redefinition a consensus value for the "new" kilogram will need to be adopted in order that a consistent value for the SI unit of mass be maintained. In order to maintain this consistency the storage, monitoring and use of current primary mass standards will be critical and this paper outlines ways in which the stability of artefact based mass standards can be optimised by careful storage and monitoring using surface analysis techniques and quartz crystal microbalance technology.

The complex and multi-parameter nature of chemical composition measurement means that establishing traceability is a challenging task. As a result incorrect interpretations about the origin of the metrological traceability of chemical measurement results can occur. This discussion paper examines why this is the case by scrutinising the peculiarities of the gas metrology area. It considers in particular: primary methods, dissemination of metrological traceability and the role of documentary standards and accreditation bodies in promulgating best practice. There is also a discussion of documentary standards relevant to the NMI and reference material producer community which need clarification, and the impact which key stakeholders in the quality infrastructure can bring to these issues.

The definition of the kilogram in the International System of Units (SI) is expected to be revised in 2018. The present definition of the kilogram, the mass of the International Prototype of the Kilogram (IPK), adopted in 1889, would then be replaced by a definition based on a fixed numerical value of the Planck constant. The Consultative Committee for Mass and Related Quantities has requested that, as one of the essential steps before the redefinition, a comparison of kilogram realizations based on future realization methods, Kibble(9) balances and x-ray crystal density (XRCD) experiments, be organized. This comparison was carried out during 2016 in the form of a 'Pilot Study'. One aim of the study was to determine the uniformity of mass dissemination after the redefinition by comparing mass calibrations based on different future realization experiments. Another aim was to test the continuity of the mass unit across the redefinition by comparing mass calibrations based on Kibble balances and XRCD experiments with those based on the IPK. This paper describes the organization of the comparison and presents its results.