Abstract: Apparatus for the capacitative measurement of the surface level of a fluid medium in a vessel comprises a probe mounted to provide a capacitance between itself and the surface, the probe being connected into one of two RC-circuits, respectively connected to opposite ends of a secondary winding of a transmitter, the primary winding of which is in a resonant high-frequency generator circuit. The two RC-circuits are earthed and respectively connected through rectifiers to the inputs of a differential amplifier, the output of which provides an output signal corresponding to the capacitance between the probe and surface level. The RC-circuit associated with the probe comprises a parallel manually adjustable capacitor and a capacitance diode arranged for the remote adjustment of the RC-circuit.