Storage of mass standards
Storage of mass standards in inert gas/vacuum to achieve improved mass stability compared with storage in air.
The National Physical Laboratory has weighing in vacuum facilities for measuring the mass of artefacts in vacuum. Weighing in vacuum is essential to provide mass traceability for the two approaches to redefining the kilogram. Research carried out at NPL over the last decade has shown that mass standards can become less stable when they are transferred between air and vacuum. Over time mass standards also gain hydrocarbon contaminants when stored in air.
Storage of mass standards in an inert gas such as Argon may reduce the potential for hydrocarbon contamination and improve their stability when transferred to vacuum. NPL has designed storage vessels that are capable of housing Platinum-Iridium, stainless steel and Silicon kilogram masses in inert gas (Figure 1). A transfer chamber has also been designed to enable the transfer of masses from their storage containers to vacuum on NPL’s vacuum balances without exposure to air (Figure 2).
NPL scientists are evaluating the stability of masses when they are repeatedly transferred from either inert gas or air to vacuum. This work is crucial not only to improving the stability of current mass standards but also in providing traceability to a new kilogram standard which will be defined in terms of a fundamental constant.