– the intergovernmental organization through which Member States act together
     on matters related to measurement science and measurement standards.
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BIPM ensemble of mass standards
    As the redefinition of the kilogram approaches, the BIPM is preparing for the forthcoming challenges in mass metrology. In 2011, the General Conference on Weights and Measures (CGPM) encouraged the BIPM to develop "a pool of reference standards to facilitate the dissemination of the unit of mass when redefined"(Resolution 1 (2011)). Since then, the BIPM Mass Department has assembled a new ensemble of 12 reference mass standards (ERMS) and four stacks of disks which are now fully operational.

    View of the BIPM ensemble of reference mass standards

    The standards are stored in uncontaminated and continuously analysed environments to ensure the best possible mass stability. The artefacts are frequently inter-compared and the mass of each individual element compared to the mean mass of the ensemble, which is calculated by giving each element a statistical weight that reflects its stability. The mean mass should therefore be more stable than any of the individual masses. In addition, by calibrating one (or more) of the mass standards using the available Kibble balances and other primary realizations of the kilogram (with the help of transfer mass standards), traceability of the whole "ensemble" should be assured with respect to the fundamental constants.

    When the ensemble of mass standards is traceable to the fundamental constants, it will be possible to use it to disseminate the SI unit of mass, taking advantage of the high stability of the mean mass value of the ensemble. Using widely-available modern mass comparators, relative uncertainties of a few parts in 109 can be obtained when comparing 1 kg mass artefacts.

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    The standards are stored in uncontaminated and continuously analysed environments to ensure the best possible mass stability. Four different storage environments are used for the standards:

    • eight standards are housed in a low-flow gas storage network (four under argon and four under nitrogen);

    • four standards are maintained under vacuum (1 mPa); and

    • four standards are stored in air at ambient atmospheric pressure, under the same storage conditions that have been traditionally used at the BIPM.

    Three mass standards made of different materials:

    • a 1 kg Pt/Ir cylinder;
    • a 1 kg natural silicon sphere;
    • a 1 kg stainless steel cylinder;

    plus a 1 kg stack of disks (made of Si, Pt/Ir or stainless steel) are stored under each storage condition (argon, nitrogen, vacuum and ambient air).

    View of the BIPM ensemble of reference mass standards

    The combinations were chosen to test which had the optimum mass stability. The stack of disks is designed to have the same mass and volume as the single-piece standard of the same material but with a larger surface area, so that mass changes due to surface effects may be studied.

    The masses of the artefacts will continue to be frequently inter-compared, and the storage conditions will be maintained during these weighings.

    The photo below shows the gas storage network inside the cabin. A continuous gas flow of argon (left) and nitrogen (right) is maintained through all the containers. The argon gas is supplied from gas bottles with contents certified to contain a maximum of 50 nmol/mol of hydrocarbons, oxygen and water impurities. For the nitrogen network two different gas sources are used for comparison: ultrapure nitrogen in bottles (with the same impurity specifications as the argon bottles) and gas from a nitrogen generator which has up to 2 parts per million of moisture. After passage through the containers, the output gas is continuously analysed and compared to a control line. Any changes in the concentration of hydrocarbons, oxygen and water due to the mass standards stored inside the containers can be detected at the level of hundreds of parts per billion.

    The picture below shows the vacuum storage network.

    The BIPM ensemble of reference mass standards is fully operational. All the standards are stored in their respective containers. The mass of each individual element will continue to be compared to the mean mass of the ensemble. Therefore, the mean mass should be more stable than the mass of any of the individual standards.

    After the forthcoming redefinition of the kilogram, by calibrating one (or more) of the mass standards using available Kibble balances and other primary realizations of the kilogram (with the help of transfer mass standards), traceability of the whole ensemble will be assured with respect to the fundamental constants. It will then be possible to use the ensemble to disseminate the new SI unit of mass.


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