Noninvasive noncontact fluid detection in submerged containers using swept frequency ultrasonic technique

A noncontact technique has been developed for the remote interrogation of submerged and limited access metal containers for determining the presence of fluid inside. The technique is based on the radiation damping effect of water on the thickness mode resonance of the container wall. A transmitter-receiver pair of piezoelectric transducers is placed at a standoff distance of 5 mm from the container wall with the water outside the container providing ultrasonic coupling medium. The excitation frequency applied to the transmitter is swept between 0.8-4.0 MHz and the receiver transducer detects the signal, in the form of a frequency spectrum, returned from the wall. By analyzing the variation in the observed spectrum, it is straightforward to determine whether the container is fluid or air backed thereby detecting if the container has leaked.