Detecting the presence or absence of a liquid

An ultrasonic signal is passed through a pipe D in a liquid (e.g. beer) dispensing system to determine the presence or absence of liquid within the pipe according to the attenuation of the soundwaves. The attenuation level of the received ultrasonic signal is compared (at F) with a predetermined reference level, and if the attenuation is low a "no liquid" indication (at K) is provided.This indication signal can be used to shut down the pump in the dispensing system or to shut a valve to prevent drainage of the system. The indication signal may be temporarily delayed (at H) in order to obviate spurious operation of the "No liquid" detector to prevent it tripping when a gas or foam pocket passes through the ultrasonic beam.