Page 505 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

500 CHAPTER 18 Ancillary Sensors
range) is generated by a transducer (either in close proximity to (or in direct contact with) a metal surface) and then transmitted through a metal. The sound will bounce off the opposite wall. The time difference is then measured to derive a very high-accuracy metal thickness measurement. These readings, however, are subject to a host of errors and interpretations.
Modern UT gauges make use of the multiple echo technique for higher accuracy reading. When sound is passed through metal at a particular frequency, the sound will resonate within the metal. This allows the taking of later echo bounces to more accurately measure the echo with regard to the metal (versus other couplants like coatings, dirt, etc.). In Figure 18.24, a diagram of the tech- nique of measuring a subsequent echo isolates the sound timing within metal from the outside sources. This allows for accurate measurement of flat plate metal thickness along with some types of circular metal structures.