Abstract: A detector for detecting the presence or absence of an ultrasonically conductive medium such as a liquid within a container includes a transducer that generates and transmits an energy burst through a first wall of the container toward the opposing wall of the container. The frequency at which the energy burst is transmitted is selected so that the energy burst is greatly attenuated when it travels through air and is not greatly attenuated when it travels through liquid. The detector includes a window generator that generates an electronic window corresponding to a predetermined period of time during which the detector expects to receive an echo of the energy burst. If an echo is received during the window, then liquid is present in the container, whereas no liquid is present if no echo is received. The position of the window may be changed via a dial having discrete settings to accommodate pipes of different diameters.