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Facilities and knowledge to meet your calibration and measurement requirements

The density of a solid or liquid may be fundamentally determined by hydrostatic weighing using Archimedes' principle. These measurements are traceable to either the density of pure water or to a solid density artefact. There are numerous other density measurement techniques including hydrometry, pycnometry and the use of vibrating cell meters. NPL offers formal training courses or, in what is becoming increasingly popular, the opportunity to work alongside NPL staff for one or more weeks. Typically, training in the density area is run in conjunction with training in mass metrology.

Density of solids

Density measurements at NPL are normally carried out by the method of hydrostatic weighing. This requires the artefact 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, it is possible – for artefacts in the region of 1kg – 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, such as 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, usually determined by weighing carried out subsequent to its immersion in liquid.

Density of particulates and powders

NPL has developed a liquid pycnometry technique for the density measurement of particulates and powders. The technique is also suitable for the density determination of small artefacts that are not suitable for measurement by hydrostatic weighing (usually less than 10 grams in weight). The certified density values are traceable to the national standard of mass and to the density of water and the density of the reference liquid used for the individual calibration (the liquid used will vary depending on the nature of the sample under test). The method has been validated by determining the density of samples of single crystal silicon.   

Typically, sample densities are determined at 20 °C but measurements can be made in the range 10 °C to 70 °C. The ability to precisely temperature control the samples additionally allows the accurate measurement of the coefficient of thermal expansion, which is difficult to determine using other measurement methods. Depending on the nature of the sample under test, uncertainties of the order of 0.1% can be achieved. 

Where appropriate, NPL can also calibrate the density of particulates and powders by conventional helium pycnometry techniques.

Density of liquids

For liquid density measurements two facilities exist. A vibrating cell density meter can be used where only a small liquid sample is available. Alternatively, where the highest accuracy is required, the density of the liquid can be determined by weighing a solid volume standard in the liquid. The supplied certificate of calibration will give the measured density of the liquid, together with an associated uncertainty. If requested, the coefficient of thermal expansion will also be provided.

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