ABOUT Rholiq 17RPT02



Density bridges two classical quantities (mass and volume) which are the major driving elements in economic transactions with important goods, ranging from food to fuels. At present, only  few European NMIs possess the appropriate expertise to perform liquid density measurements from the primary‑level, i.e. hydrostatic weighing apparatus, to the secondary‑level, i.e. oscillation‑type density meters, with a level of accuracy and uncertainty that meets national (e.g. to fulfil national laws) and international (e.g. to fulfil European Directives and standards) needs (i.e. 0.002 kg/m3 to 0.005 kg/m3 (first-level), 0.01 kg/m3 to 0.05 kg/m3 (second-level), 0.1 kg/m3 to 0.5 kg/m3 (second-level up to 600 bar). As proof of this, in the latest EURAMET intercomparison (EURAMET.M.D‑K2 (1019)) [12], performed with the hydrostatic weighing method, around 36 % of the results obtained by the participants were unsatisfactory results (En > 1) and the mean measurement uncertainty obtained was from 4 to 13–fold of the ones obtained by the pilot laboratories (i.e. from 0.002 kg/m3 to 0.006 kg/m3). Therefore, as this method is at the top of the density traceability chain, inadequate performance is very likely to jeopardise the accuracy and the precision of the liquid density traceability chain down to the second‑level measurements at NMIs and accredited calibration laboratories, and down to the third‑level used in industry and research laboratories.


The overall objective of this project is to develop the national metrological capacity in liquid density metrology in the emerging NMIs, with the target of achieving the lowest measurement uncertainty possible when using state-of-the-art density measuring systems. This will include coverage of the quantities that influence the measurement of density, i.e. temperature and pressure, and viscosity. This objective will include a review and upgrade of existing capabilities and needs, validation of existing systems and, if required, the development of new systems, with the support of the more experienced NMIs, for the density, temperature and pressure intervals relevant for scientific and industrial needs.

The specific objectives of the project are:


  1. To develop first-level liquid density measurement capabilities by the hydrostatic weighing method in the emerging NMIs, at atmospheric pressure, in the temperature interval from 5 °C to 60 °C, with an uncertainty from 0.002 kg/m3 to 0.005 kg/m3.
  2. To develop second-level liquid density measurement capabilities performed with oscillation-type density meters in the emerging NMIs, for liquids with dynamic viscosity at least up to 2 000  mPa·s (ideally up to 10 000  mPa·s), in the temperature interval from 5 °C to 60 °C, at atmospheric pressure, with an uncertainty from 0.01 kg/m3 to 0.05 kg/m3, and for pressures up to 600 bar, with an uncertainty from 0.1 kg/m3 to 0.5 kg/m3.
  3. To establish the degree of equivalence of the density measurements performed by the emerging NMIs with the developed first and second-level measuring systems via comparisons.
  4. To study the robustness of first-level, i.e. hydrostatic weighing method, and second-level, i.e. oscillation-type density meter, liquid density measurement methods regarding the influence of the liquid’s physical properties: viscosity (from 1  mPa·s to 10 000  mPa·s); surface tension from 20 mN/m to 72 mN/m and viscoelasticity (behaviours other than Newtonian, such as yield point); and of independent properties (temperature (from 5 °C to 60 °C) and pressure (from atmospheric pressure up to 600 bar).
  5. To develop new guidance documents for first and second-level liquid density measurement methods and for the production of certified reference liquids for use in density measurements, such as EURAMET guides and a good practice guide, for the metrological and industrial communities. To provide input for the revision of relevant reference documents (ISO 15212, OIML Guide 14 and WELMEC Guide 6.4).
  6. To develop the emerging NMI’s individual strategies for the long-term operation of the capacity developed, including: new/upgraded measurement and calibration services; the production of certified reference liquids; the provision of training to disseminate best practices in liquid density metrology; the provision of national metrological infrastructures and proficiency testing schemes to support accreditation bodies and field laboratories.

Progres beyond the state of the art

Developed / improved liquid density measurement capabilities at first and second-level in 8 emerging NMI and developed capabilities for producing CRM for liquid density

First-level traceable measurement capabilities for liquid density will be established by developing the hydrostatic weighing method, in emerging 8 NMI. This will allow the dissemination of liquid density from the SI base quantities, mass and length, by establishing the first step of the liquid density traceability chain and it will enable the production of Certified Reference Materials (CRM) for density, targeting a lowest uncertainty in the interval from [0.002, 0.005] kg/m3, to be used for the dissemination of the liquid density quantity to a second-level of the traceability chain.

Second-level traceable measurement capabilities for liquid density will be established in the emerging NMI, using standard oscillation-type density meters, at ambient pressure and high pressure (up to 600 bar), with a uncertainty in the interval from [0.01, 0.50] kg/m3 and [0.1, 0.5] kg/m3, respectively. The emerging NMI will be able: to offer new/better calibration services with a high level of accuracy; to determine a liquid’s thermal expansion coefficient and compressibility coefficient; and to produce and certify reference materials (RM) with an uncertainty level sufficient to be used by accredited calibration laboratories and industrial companies. Additionally, these NMI will be able to produce accurate and traceable density measurements of liquids at high-pressure (up to 600 bar), and for high viscosity liquids (with viscosity (ideally) up to 10 000  mPa·s), with a target uncertainty within the interval from [0.10, 0.50] kg/m3, which is currently requested by several industries, such as the food and petrochemical industries.

All measurement capabilities listed above are intended in the most relevant intervals: of density from [600, 1700] kg/m3; of viscosity from [1, 10 000]  mPa·s and temperature from [5, 60] °C, i.e. in terms of the most common liquids measured by the end-users.

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ABOUT Rholiq 17RPT02