Profile
Research/Development
Cooperation in the project for the re-determination of the Avogadro constant
Density comparison by the pressure-of-flotation method
The pressure-of-flotation method allows investigations of very small density differences to take place by comparing measurements of two silicon samples (e. g. 1 kg silicon-28 spheres). To this end, both samples are immersed in a liquid mixture that has nearly the same density as silicon. The pressure in the liquid is adjusted in such a way that one of the two samples floats freely without rising or sinking. Currently, an apparatus for density comparisons that uses the pressure-of-flotation method is being set up; the desired relative standard uncertainty of the density comparison measurement using this apparatus is 0.01 ppm for silicon-28 spheres. To achieve this uncertainty, both a high resolution of the pressure changes and a temperature stability in the µK range are crucial.
Services
Information
Automatic hydrostatic weighing apparatus
The automatic hydrostatic weighing apparatus is used to determine the volume of solid samples with a mass of up to about 1030 g and a volume of up to 440 cm3. The measurements are performed at 20 °C. The apparatus has been included in the international CMC tables.
The following expanded measurement uncertainties (coverage factor k = 2) are accepted worldwide:
(0.15 + 0.0015 cm-3 · V) mm3 for the volume V ranging from 50 cm3 to 440 cm3.
Mass comparison apparatus
In this apparatus, the masses of solid samples with a mass of up to about 1000 g can be compared in order to determine the density of samples.
Mass comparisons can be performed with a measurement uncertainty of 0.10 mg (for a coverage factor k = 2).
Absolute mass determinations with a lower uncertainty (i.e. traceable to the SI mass unit, the kilogram) are performed by Working Group 1.11, “Realization of Mass.”
Calibration of hydrometers
Hydrometers for measuring densities in the range between 500 kg/m3 and 2000 kg/m3 can be calibrated with expanded uncertainties as low as 0.02 kg/m3 (coverage factor k = 2) using the Cuckow method. This is relevant for the following types of hydrometers: Alcoholometers, hydrometers for aqueous sucrose solutions, for wort, for sulpho spirit, for petroleum and for liquefied petroleum gas. The scale unit may be kg/m3, g/cm3, % mas, or % vol. The reference temperature should be in the range from 15 °C to 25 °C. The scale reading may be "in the liquid's level" or "at the upper rim of the meniscus."
