A level sensor system according to the present invention includes a controller and a sensor assembly defined by a transducer assembly and a mounting assembly configured for a quick connection or non permanent attachment of the transducer assembly to the tank or container. The ultrasound transducer assembly emits ultrasound pulses in the tank along a detecting path, receives ultrasound echoes indicative of at least one of the first and second fluids and produces echo signals representative of the at least one of the first and second fluids. The mounting assembly is configured for receiving the ultrasound transducer assembly and for operatively mounting the ultrasound transducer assembly to the tank for ultrasound measurement. The controller is coupled to the ultrasound transducer assembly, receives the echo signals and determines the position of the fluid interface therewith; the level of the first fluid being defined by the position of the fluid interface. The fact that the sensor assembly is configured so as to be removably mounted to the tank allows replacing the tank, for example when it is empty, without having to replace the sensor assembly.