a transmitting transducer is mounted on the outside wall of a vessel and supplies bursts of ultrasonic energy . if liquid is present , the energy is coupled to the other side of the vessel . otherwise , the energy is attenuated . a receiving transducer is positioned to receive the ultrasonic energy if liquid is present . it is gated to be active only when such energy should arrive in the presence of liquid . the receiving transducer therefore generates an electrical signal indicating presence or absence of liquid in the vessel at the level at which the transducers are mounted .