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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 been assembling a new ensemble of 12 reference mass standards and four stacks of disks.

    View of the BIPM ensemble of reference mass standards

    The standards will be stored in an uncontaminated and continuously analysed environment to ensure the best possible mass stability (see the tab 'Reference masses and storage conditions' above). The artefacts will be frequently inter-compared and the mass of each individual element compared to the mean mass of the ensemble, which will be 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 watt 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.

    Once 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.

    The standards will be stored in an uncontaminated and continuously analysed environment to ensure the best possible mass stability. Four different storage environments are being prepared for the standards:

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

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

    • four standards will be 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) will be stored under each storage condition (argon, nitrogen, vacuum and ambient air).

    This choice of combinations was chosen to test which leads to 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.

    In the future, the masses of the artefacts will 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 pictures below show the first working version of the vacuum storage network. The final version of the vacuum network is currently being automated, and will be soldered together.


    View of the vacuum storage network in place in the cabin (top left), connected to the residual gas analyser (RGA, bottom right). The RGA continuously analyses the chemical composition of the background vacuum.
    The storage containers are electro-polished, have an inner volume of about 4 litres and have been designed to accommodate all the different standards. All containers are equipped with manual isolation valves which allow each container to be removed and reinserted back into the network without altering the storage conditions of the standard inside. In addition, the gas storage containers have a bypass tube, so that the network can be purged without the purge gas flowing through the container.


    Photo: A container in place in the gas storage network, showing the (blue) isolation valves and bypass system.

    Inside each container is a holder used to maintain the mass. All the containers were constructed in the BIPM's mechanical workshop.


    Schematic: A container with a holder and test mass inside.

    Photo: The holder used to maintain the mass inside each container.

    The final preparations of the BIPM ensemble of reference mass standards are now under way:

    • The fabrication of the reference mass standards and the stacks of disks in Pt-Ir and stainless steel is complete and their characterization (mass, volume and surface) is finished.

    • The fabrication and characterization of the four spheres and the stack of disks in silicon is ongoing.

    • The gas and vacuum storage networks have been working continuously and satisfactorily for over six months. The storage networks will be ready to receive the mass standards of the ensemble within the next few months.

    In the future, the mass of each individual element will be compared to the mean mass of the ensemble, which will be calculated by giving each element a statistical weight that reflects its stability; the mean mass will therefore be more stable than any of the individual masses.

    After the forthcoming redefinition of the kilogram, by calibrating one (or more) of the mass standards using available watt 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.