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A rotational in-line-viscometer without mechanical bearings and seals is described; all wetted surfaces are made of PTFE or PFA. This arrangement is suitable for the ultra-cleanliness requirements of the semiconductor industry. A cylindrical slice rotor is magnetically levitated and driven without mechanical contact with the stator of the viscometer. The rotor is surrounded by the test fluid. If the rotor is driven at a constant rotation speed, the fluid is sheared in a measurement gap between the rotor and the casing, and a torque is applied to the rotor. This torque correlates with the viscosity of the fluid. Since there is no friction from bearings and seals, this torque can be calculated directly from the motor current. The levitation of the rotor in the sample fluid is realized by a combination of active and passive magnetic bearings, based on the technology of the Bearingless Slice Motor, making the whole system very compact.