Abstract: A level-sensing head is suspended from a capstan above a vessel by means of a flexible link which is wound around the capstan to raise or lower that head upon rotation thereof in one direction or the other. Depending upon the position of the sensing head relative to the level of a liquid or other flowable mass in the vessel, the head causes emission of either of two control signals triggering a pulse generator to step a motor, coupled with the capstan, in a sense tending to maintain that relative level between two narrow limits. The output of the pulse generator is also fed to recording equipment and/or to a digital comparator designed to establish a preselected level which may be modified from time to time by a programmer. The flexible link may serve as an electrical or fluidic transmission channel for control signals originating at the sensing head; with a floating sensing head such control signals can also be generated by a torsion monitor inserted between the motor shaft and the capstan.