Figure 1: Vacuum balance
vacuum chamber.

The National Physical Laboratory has two mass balances designed to compare masses under vacuum.

NPL_one is the latest vacuum balance comprising a kilogram mass comparator located within a vacuum chamber (Figure 1). The balance has the capability to compare six masses in the range from 50 g to 1 kg in air, inert gas or vacuum. Its load-lock permits the transfer of masses to/from the balance whilst it is under vacuum.

Figure 2: Silicon
Avogadro sphere.

NPL_one can accommodate masses up to 90 mm in diameter which allows Silicon Avogadro spheres to be weighed (Figure 2). The balance is used to calibrate Platinum-Iridium (Pt-Ir) and stainless steel weights thus providing traceability to the International Prototype Kilogram (K) via kilogram 18 the UK’s national mass standard. It also provides mass traceability to the Watt balance redefinition via weighing in vacuum of Silicon transfer standards.

Figure 3: The GRS balance.

The GRS balance is NPL’s original vacuum weighing facility comprising a four station mass comparator within a vacuum chamber (Figure 3). It has the facility to weigh masses in the range from 50 g to 1 kg in air, inert gas or vacuum. The GRS balance was used to calculate mass correction factors for Pt-Ir, stainless steel and silicon masses for transferring them to vacuum (Figure 4). The balance also has the capability to measure the density of artefacts by weighing in an inert gas over a small pressure range.

Figure 4: Masses.