An apparatus for the separation of solids and liquids includes a perforated rotary basket arranged for rotation within a fixed outer casing, a washing liquid supply system for providing washing liquid to the interior of the basket and its contents, and a transducer device for establishing a control signal representative of the state of liquids centrifugally expelled from the basket when such liquids impinge on an inner surface of the fixed outer casing. The transducer is set in or on the inner wall of the outer casing for monitoring the conductance of liquids within.