Abstract: Apparatus (1) for measuring the amount of a substance (2) in a vessel which comprises a detector means comprising at least two electrodes (3) adopted to generate an electric signal in response to contact of the electrodes with the surface of the substance (2), a first reference point (6) on the detector means which point is a fixed distance from the ends of the electrodes (3) said reference point being provided on a material suitable for reflecting waves from a wave transmitting-receiving device (8) e.g. a sonar device, preferably located in a fixed position above the first reference point for measuring the distance from the first reference point (6) to a second reference point (70) at the wave transmitting-receiving device (8) whereby the amount of the substance (2) in the vessel can be determined by computation, e.g.