Laboratory Density Meters

Many thousands of liquid density determinations are made worldwide every day and substantial sums of money depend upon the outcome. Although at one time, a wide variety of methods were employed, most laboratories now use either hydrometers or density meters.

Hydrometry has been used for many years and the uncertainties associated with it are reasonably well understood, if often appreciably underestimated. Laboratory density meters have been developed from the work of Stabinger et al. [1967] and use a miniature U shaped tube of glass as a measurement cell. The oscillating period of the cell is a function of the density of the liquid injected into it. About 20,000 bench top and 10,000 portable instruments are installed worldwide and since the majority of these are manufactured by Anton Paar K.G. of Austria, it is their machines which have been assessed.

Several papers have already been published on the effect of viscosity on the Paar instruments but unfortunately, most used aqueous sugar or glycerol solutions, giving a very tight correlation of density to viscosity, and as far as is known, few if any had access to a hydrostatic weighing system, and so needed to determine the density of viscous liquids by pyknometery, no easy task. No previous publications have been identified covering cell linearity or calibration.

H&D Fitzgerald Ltd developed a hydrostatic weighing system with an uncertainty of ± 0.01 kg m-3 at the 95 percent confidence level for which they hold ISO 17025 accreditation from UKAS. Liquids calibrated by this system were used to assess three of the most popular instruments in the current Paar range, the DMA 48 which indicates density to 0.1 kg m-3, the DMA 58 (0.01 kg m-3), and the portable DMA 35 (1 kg m-3). In addition, a DMA 55 (0.01 kg m-3), a type no longer manufactured, was tested since its cell temperature is controlled by use of an external water bath, and it uses the same cell design as the instruments used by National Bureau of Standards (now NIST) Whetstone et al. [1978] to gather the experimental data upon which the current Petroleum Measurement Tables are based.

All viscosities were measured by SGS Redwood Ltd, using IP 71 with an uncertainty of ± 0.4 percent of quoted viscosity at the 95 percent level.

Laboratory Density Meters PDF