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Mass and mechanical measurement

Storage of primary mass standards

Investigating the best methods for storing standard weights

In 2019 a major revision to the International System of Units (SI) is expected to take place. As part of this, the definition of the kilogram will change from being the mass of the international prototype kilogram (IPK) to a definition related to the Planck constant (h).

Traceability for mass calibrations will no longer come from the IPK but must be taken from the Kibble balance and Avogadro experiments. However, as demonstrated by the measurements contributing to the 2017 CODATA analysis which fixed the value of h, these experiments currently do not agree at the level of uncertainty required and lack the stability necessary to be able to unilaterally realise the unit of mass reliably. Therefore the medium and long-term stability of mass standards becomes critical in allowing these experiments to be compared and to provide continuous access to the SI unit of mass for end users.     

NPL has undertaken research which has demonstrated the benefits of storing weights in inert gas. We have developed apparatus for medium to long-term storage of mass standards in inert gas at a controlled and monitored positive pressure above ambient. The apparatus also has a built in quartz crystal microbalance (QCM) which allows real-time monitoring of surface cleanliness and also uses X-ray photoelectron spectroscopy (XPS) samples to characterise the composition of surface accretion before and after storage.

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