Abstract: Apparatus for measurement concerning fluids in a rotor or container (2) during rotation, comprising an electric or magnetic sensor (4) mounted internally on a wall in the container (2), and means (5A, 5B) for contact-free and intermittent transmission of measurement signals from the sensor (4) to a stationary measuring unit (7) outside the container. The sensor (4) comprises an active electronic circuit (8) adapted to store measurement values that are recorded during at least a portion of a revolution of the container (2), before said transmission of corresponding measurement signals to the measuring unit (7). Electric power supply to the electronic circuit (8) is provided for by generator means comprising a stationary magnet (11) near the container (2) and a coil (12) mounted in the container so that a voltage is induced in the coil (12) during movement past the magnet (11) during the rotation of the container (2).