National Physical Laboratory

Where and how is the UK's National Standard Kilogram stored? (FAQ - Mass & Density)

The UK National Standard Kilogram - a cylinder of platinum-iridium alloy and numbered '18' - is kept in a basement vault at NPL.

It is stored in an air-tight enclosure, only exposed to the air via two micropore filters. Conditions in the basement are very stable - although not temperature controlled - with the temperature remaining in the range 19 °C ±1 °C throughout the year. While the temperature in the vault is monitored, the storage temperature of the kilogram is not critical provided it remains within a few degrees of 19 °C.

If is gets too cold condensation may form on the surface - resulting in physisorption of water into the surface layers.

If it gets too hot the rate of surface contamination and adsorption of hydrocarbons from the surrounding air may increase.

See more information about how NPL is redefining the kilogram.

Last Updated: 17 Feb 2012
Created: 8 Oct 2007


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