National Physical Laboratory

Density of solid artefacts

Density of solid artefacts 

Density measurements at NPL are normally carried out by the method of hydrostatic weighing. This requires the artefact under test to be weighed both in air, and in a liquid (normally water) of known density. By comparison of the two weight values and a knowledge of the density of the liquid the density of the artefact can then be determined using the Principle of Archimedes.

Where a lower uncertainty is required than that obtained from the above method, it is possible – for artefacts in the region of 1 kg – to compare them directly against an NPL volume standard also immersed in the test liquid. Using this method has the effect of removing most of the uncertainty due to the knowledge of the liquid density.

NPL has four hydrostatic weighing rigs, two of them fully automatic, allowing the density determination of artefacts with masses in the range from 5 g to 20 kg. Generally, the artefacts to be calibrated should be of regular shape (e.g. spheres or OIML class weights) although we are able to offer a service for artefacts of up to 1 kg of non-regular shape.

As the artefact is immersed in a liquid, the method of hydrostatic weighing is suitable only for solids that present no special difficulties, such as porosity. Where the artefact cannot be immersed in a liquid, NPL is able to measure the density of artefacts of mass approximately 100 g to 1 kg by weighing in an inert gas over a small pressure range. This method of determining the density is based on the fact that the apparent mass difference between a standard artefact of known volume and a test artefact will change as the value of air density changes. If the air density is varied by changing the air pressure, the gradient of the plot of apparent mass difference versus air density gives the volume difference between the standard artefact and the test artefact. If the volume of the standard artefact is known then the volume of the test artefact can be calculated.

Normally, all results will be calculated at a standard temperature of 20 °C. However, if required, results may be given with reference to a temperature other than 20 °C. It is also possible to calculate the coefficient of thermal expansion of the artefact by measurements over a range of temperatures.

The supplied certificate of calibration will give either the density or volume, as required, of the test artefact, together with an associated uncertainty. It can also state the mass of the artefact, normally determined by weighing carried out subsequent to its immersion in liquid.

Contact

Customer Service tel: +44 20 8943 6245
E-mail: mass_enquiries@npl.co.uk

Last Updated: 25 Mar 2010
Created: 14 May 2009

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