The measurement liquid is placed in a chamber (1) contg. a rotor (10) which is immersed in the liquid during the measurements and which is magnetically coupled to a drive motor (23) outside the chamber. The rotor is mounted with play with its central aperture (11) about a stationary guide axle (5). Liquid can pass between the rotor and axle and the rotor is axially supported by a rotating, radially directed sealing surface (22) which rests on a radially directed, stationary counter sealing surface (9). The gap between the sealing surfaces communicates radially inwardly with the intermediate vol. between the rotor and the guide axle and radially outwardly with an outer vol. of the rotor. The counter sealing surface is formed on a plate (6) with a central opening enclosing the guide axle.
