Self-cleaning ultrasonic sensor with adjustable sensitivity for the continuous determination of the sludge level or the sludge height in settling tanks, vessels and reactors, expressed as depth from the surface or height from the tank floor. The sludge level is calculated without contact between the probe and the sludge, on the basis of the propagation time of the ultrasonic echo. User-friendly menu guidance by membrane keypad and illuminated large graphics display with curve generation by using the sc1000 controller.