proposed is a fill - level indicator designed to indicate the level of the contents of a container or the storage level of a material at a storage place , the indicator having an ultrasonic transducer which is operated to act alternately as a pulse emitter and as a receiver . the ultrasonic transceiver is surrounded by the fill - level indicator housing to form an exactly cylindrical coaxial air gap and has boundary surfaces which produce acoustic impedance jumps to reflect the acoustic energy emitted by the surface of the piezoelectric crystal .