The Working Group 1.13 "Density of Solids" is capable of measuring the density of sufficiently big Hg samples with a relative uncertainty of about 3•10^-6 and thus provides the metrological basis of pressure measurements with the mercury manometer, on which the realisation of the pressure scale is based.

The manometer is accomodated in a walk-in enclosure which ensures thermal isolation from the laboratory environment. A stationary container and a container movable in the vertical direction, both partly filled with mercury, are connected via a sheathed flexible tube.The vertical displacement of the movable, evacuable container is measured by means of a counting laser interferometer.The mercury surfaces are localised by a capacitance sensing method using a measuring bridge. Level differences can be measured with uncertainties in the range of tenths of a micrometer. Platinum-resistance thermometers are used for precise temperature measurements. The uncertainty of the average temperature of mercury is 10 mK. The residual gas pressure in the evacuable container can also be measured. Under these conditions it is possible to measure, for example, absolute pressures in the order of 100 kPa with a relative standard uncertainty of about 3·10^-6.

The manometer above all serves to determine the effective cross-sectional areas of piston-cylinder pressure measuring assemblies of absolute pressure balances at the customer's request (the figure shows a gauge presssure comparison measurement).