each barge in a group of river - going tanker barges , has a double - hull construction to protect the inner hull and avoid collision - induced cargo spills . the space between hulls is partitioned to individual empty , dry compartments with a vertical , open bottomed , measuring tube fixed in each . a closed - bottom master tube is fixed in one compartment . all tubes have a quick - connect , above - deck , flange to receive an ultrasonic transducer thereon . a hand - held measuring instrument containing a computer and controls and information display contains a transducer which is selectively pulsed to periodically send ultrasonic signals down the tube , and receive echo signals , with elapsed time representing the depth of a signal reflector surface which is the master tube bottom or , for the measuring tubes , the bottom of a dry compartment or the surface of liquid in a leaking compartment . a unique bar code with each tube , distinguishes it and the barge from all others . the measuring instrument includes a code reader . in one embodiment , an individual transducer is on each tube , and connectable by an electrical cable to the measuring instrument . the apparatus is also used in cargo compartment tanks for cargo surface level measurement .