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Properties of Liquids

Working Group 3.32

Oscillation-type densimeters

Fig. 1: Oscillation-type densimeter


Working Group 3.32 uses these measurements to ensure the quality of density measurements made by laboratories in industry, by calibration services, in calibration authorities and in scientific institutes.  



Measurements of the density or density differences are made on liquid samples in the density range from 600 kg/m³ to 2000 kg/m³. Typically, these measurements are carried out in the temperature range from 0 °C to 90 °C at atmospheric pressure. In addition, the working group maintains oscillation-type densimeters for research purposes, which can measure density at temperatures down to -50 °C as well as for density measurements up to 700 bar.


Measuring method

Density measurements are carried out with densimeters based on the oscillator principle (cf. Fig. 1). The working group owns several such measuring instruments (Anton Paar), which are optimized/designed for the respective application. The measuring principle is based on a U-tube-shaped transducer which is clamped on one side and filled with a gas or a liquid which is excited to vibrate. The density of the medium is calculated from the period duration. The densimeter has built-in devices for controlling and measuring the temperature of the density measuring cell.

If the densimeters are calibrated with certified density calibration liquids, which are provided, for example, by the PTB or by DAkkS laboratories, the traceability of the measurements to the national standard for density is  guaranteed.

Important recommendations for measurement with such instruments can also be found in the standard DIN EN ISO 15212-1.



The traceability of the measurement results is ensured by connecting them to appropriate standards. The densimeters used to perform density or density difference measurements on liquid samples are traceably calibrated with density calibration liquids by determining the density measurement deviations, i.e., the deviation of the density displayed on the density meter to the calibration density value of the calibration liquid. The density values of the calibration liquids are determined with the hydrostatic weighing measuring device.



The determination of the density or the density difference on liquid samples with oscillation-type densimeters is carried out with an expanded measurement uncertainties down to 0.01 kg/m³ (for an expansion factor k = 2). By a substitution procedure, the expanded measurement uncertainty can be reduced to twice of the measurement uncertainty of the respective density calibration liquid used as a substituent. In the case of water, this value can reach 0.002 kg/m³.

Hydrostatic balance

Fig. 2: Hydrostatic balance


Liquids calibrated by hydrostatic weighing are provided as density calibration liquids, primarily to traceably calibrate oscillation-type densimeters to the national standard  and with that to ensure the quality of custody transfer, quality assurance and scientific measurements.



PTB's hydrostatic density weighing apparatus can be applied to liquids with a density in the range from 600 kg/m³ to 2000 kg/m³. The instrument allows measurements in the temperature range from -40 °C to +90 °C at atmospheric pressure if this is permitted by the physical and chemical properties of the liquids and the health and safety regulations applicable to the liquids.


Measuring method

The density of the liquid to be measured is determined automatically by the  hydrostatic weighing principle in an apparatus specially developed and designed by PTB (cf. Fig. 2). A calibrated silicon sphere is used as a density standard in the liquid to be investigated. A substitution procedure is used.

A measuring cycle for determining the density of the sample typically comprises the following two steps:
1st step: weighing of the empty suspension holder and substitution weights on the balance.
2nd step: weighing of the suspension holder with the solid density standard, no substitution weights.
For each step, the temperature in the sample as well as the air temperature, air pressure and air humidity are also measured and recorded to determine the air buoyancy correction.

From the difference of the 2nd weighing and the 1st weighing, the density of the sample can be calculated, since the mass, the volume at 20 °C and the thermal expansion coefficient of the solid density standard are known.

The traceability of the hydrostatically measured density of the  liquids is ensured by the connection to the corresponding standards.



Taking into account the uncertainty contributions caused by transport and storage of the calibration liquids, the density values can - depending on the calibration liquid - be guaranteed within an expanded uncertainty of 0.004 kg/m³ to 0.010 kg/m³ (coverage factor k = 2).