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Determination of liquid density using hydrometers - A national comparison


Hydrometers are simple and inexpensive liquid density meters that achieve uncertainties up to 0.1 kg/m³. For this reason, hydrometers are as popular as ever! Based on their high importance for the dissemination of the density unit, a national comparison DKD-V 8-1 and a European comparison EURAMET.M.D-K4.2020 were performed.

Hydrometers are based on the Archimedes principle, whereby the float is immersed in a liquid until the weight of the displaced liquid is equal to the weight of the immersed body. With the depth of immersion of the hydrometer, the corresponding liquid density is indicated on the scale line in the float stem, that are to be calibrated.

Calibration of the hydrometer scale is performed using the Cuckow method [1], i.e. the hydrometer is first weighed in air to determine the mass. Then the hydrometer is weighed in a liquid of known density and surface tension, immersed up to the scale line.

In the national comparison DKD-V 8-1, on the one hand, the uncertainties of measurement given in the calibration of hydrometers in the density range from 0.600 g/cm³ to 2.000 g/cm³ with different reference liquids and types of line scale readings were checked. On the other hand, the mathematical models which are used for the consideration of temperature, surface and meniscus correction were reviewed [2]. In addition to PTB (as pilot laboratory), three DAkkS laboratories participated in the national comparison, whereby PTB provided the reference values which are linked to the worldwide key-comparison CCM.D-K4 [3].

Furthermore, a European key-comparison EURAMET.M.D-K4.2020 is currently being carried out with 8 European participants [4], which is also linked to the CCM.D-K4 comparison. This comparison is expected to be completed in 2022; Draft A is being prepared.

In the national comparison, PTB has provided four hydrometers in a density range from 0.600 g/cm³ to 2.000 g/cm³ (including one alcoholometer in a measuring range from 20 to 30 % vol) for different reference liquids with different surface tension grades, see Fig. 1. In this comparison, the corrections of the hydrometer scale indications were determined on three test points of the respective hydrometer scale. The corrections, i.e. the difference between the real value and the scale indication of the hydrometer, were given for the reference temperature t = 20 °C or t = 15 °C with the corresponding reference surface tension depending on the reference liquid and line scale reading.

The comparison confirmed the traceability of the hydrometer calibration of all individual laboratories to the national density standards of PTB. All measurement results are within the evaluation criterion │EN│ ≤ 1. The results of the comparison confirm the competence of all participated laboratories in their accredited range.


Hydrometers from left to right: Alcoholometers with a range of 20 to 30 % vol, hydrometers in the medium density range (1.100 - 1.150 g/cm³), in the low density range (0.600 - 0.620 g/cm³) and in the high density range (1.980 - 2.000 g/cm³).



[1] F. W. Cuckow, A new method of high accuracy for the calibration of reference standard hydrometers, J. Soc. Chem. Industry 68 (1949) 44-49. Opens external link in new windowLink

[2] PTB, Nationaler Vergleich von Aräometerkalibrierungen, Vergleichsbericht DKD-V 8-1, DOI: 10.7795/550.20210614B. Opens external link in new windowLink

[3] S. Lorefice et al., CCM key comparison CCM.D-K4 'Hydrometer’, Metrologia 53 (2016) 07003. Opens external link in new windowLink

[4] A. Malengo et al., Calibration of high resolution hydrometer, EURAMET.M.D-K4.2020, Report in progress, draft A. Opens external link in new windowLink



Daniela Eppers, FB 1.1, E-Mail: Opens local program for sending emaildaniela.eppers(at)ptb.de

Christiane Niedergesäß, FB 1.1, E-Mail: Opens local program for sending emailchristiane.niedergesaess(at)ptb.de