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Density standards

Density meters are used in various industries for the final quality control of liquid commodities. Verification of the correct information printed on the labels of packaged goods is in the interest of the manufacturing companies as well as their customers. The aim is to prevent fines due to wrong tax payments and to save money by selling nothing more than what was labeled. This article discusses the important impact of certified reference material (CRM) like density standards on the performance of digital density meters.

Traceable results

A modern digital density meter is a widely used measuring device and as such needs density calibration in order to assess the accuracy of the results. Liquids with exactly known density are called certified reference materials and can be used for such calibrations thanks to the well-known reference value. This reference value has to be obtained with a traceable measuring method and is stated for given conditions (temperature, pressure) on the certificate of the certified reference material, i.e. liquid density standard.Density standards are provided by national metrology institutes or ISO 17025 accredited labs and ensure traceability back to national standards such as the International System of Units (SI) by a consistent measurement chain. For the determination of density with a preferably small measurement uncertainty a suitable method is the hydrostatic balance since it can be directly linked to the kilogram prototype. However, for routine density measurements this method would be too expensive and slow. State-of-the-art digital density meters are not directly traceable to SI units of measurement; their traceability is achieved by the use of certified reference materials. This way, liquid density standards close the gap and link modern digital density meters to primary national standards. Have a look at our website and find out how our in-house calibration workflow works.

Choosing the right density standard

Various certified reference materials are available from different providers so the right characteristics have to be considered first. The following points should be kept in mind:

⦁ The reference density of the CRM should be as close to the regular sample densities as possible.

⦁ Ideally, CRMs slightly above and below the regular sample densities should be chosen.


⦁ The CRM’s composition should be comparable to the regular sample’s composition; this means that the nature of the sample should be similar to the CRM. This includes the viscosity or vapor pressures but also whether it’s organic, aqueous, or alcoholic, etc.

⦁ The uncertainty of the stated reference values might vary greatly depending on the provider. This basically depends on the used measuring method to determine the reference value and on how consistently it can be linked back to national standards. We recommend choosing a density standard whose reference value was determined by means of hydrostatic weighing.

Handling certified reference materials

Recalling the above-discussed attributes of density standards, care also needs to be taken when handling these liquids. This already starts with storage conditions and adherence to the date of expiration stated on the certificate. Immediately after opening the sealed CRM container the liquid has to be injected into the density meter to start the density calibration. Therefore, the right syringe has to be chosen according to the chemical compatibility with the CRM.

Overall measurement uncertainty

Measurement uncertainty is another important aspect related to CRMs and digital density meters. Although a value for the accuracy of an instrument can be specified, the overall uncertainty of a measurement still includes much more, because the uncertainties of the CRM, the instrument, and the operating procedure add up.

Conclusion

For the international trade of goods it is crucial that a common understanding of quality can be guaranteed. Digital density meters offer an efficient way to characterize liquid products and liquid density standards are used to link them back to national standards. Traceable density calibrations with certified density standards are therefore the basis for international trade of liquid commodities.