After a brief description of the different methods employed in periodic calibration of hydrometers used in most cases to measure the density of liquids in the range between 500 kg m−3 and 2000 kg m−3, particular emphasis is given to the multipoint procedure based on hydrostatic weighing, known as well as Cuckow's method. The features of the calibration apparatus and the procedure used at the INRiM (formerly IMGC-CNR) density laboratory have been considered to assess all relevant contributions involved in the calibration of different kinds of hydrometers. The uncertainty is strongly dependent on the kind of hydrometer; in particular, the results highlight the importance of the density of the reference buoyant liquid, the temperature of calibration and the skill of operator in the reading of the scale in the whole assessment of the uncertainty. It is also interesting to realize that for high-resolution hydrometers (division of 0.1 kg m−3), the uncertainty contribution of the density of the reference liquid is the main source of the total uncertainty, but its importance falls under about 50% for hydrometers with a division of 0.5 kg m−3 and becomes somewhat negligible for hydrometers with a division of 1 kg m−3, for which the reading uncertainty is the predominant part of the total uncertainty. At present the best INRiM result is obtained with commercially available hydrometers having a scale division of 0.1 kg m−3, for which the relative uncertainty is about 12 × 10−6.

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The Istituto di Metrologia "G. Colonnetti" (IMGC, Italy), the Laboratoire National d'Essais (LNE, France) and the Mittatekniikan Keskus (MIKES, Finland) support primary calibration facilities used both for liquid density measurements and for hydrometer calibrations. In 1998, two independent bilateral comparisons of the calibration of standard hydrometers were carried out between the IMGC and the LNE and between the IMGC and the MIKES, the main purpose being to assess the present state of mutual compatibility in the density range 600 kg/m 3 to 2000 kg/m 3 among the three laboratories. The results show agreement within the uncertainties declared by the laboratories, although the MIKES measurements are in general 0.2 kg/m 3 lower than those of the IMGC and the LNE measurements are systematically 0.04 kg/m 3 higher than those of the IMGC. Both differences are smaller than half of the least-scale division of the transfer hydrometer used.

The usual method adopted for multipoint calibration of glass hydrometers is based on the measurement of the buoyancy by hydrostatic weighing when the hydrometer is plunged in a reference liquid up to the scale mark to be calibrated.An image processing approach is proposed by the authors to align the relevant scale mark with the reference liquid surface level. The method uses image analysis with a data processing technique and takes into account the perspective error. For this purpose a CCD camera with a pixel matrix of 604H × 576V and a lens of 16 mm focal length were used.High accuracy in the hydrometer reading was obtained as the resulting reading uncertainty was lower than 0.02 mm, about a fifth of the usual figure with the visual reading made by an operator.