Disclosed is a method for determining the thickness of an object, including the steps of generating a higher order mode ultrasonic wave at a first location on the object, varying the frequency of the generated wave, and measuring, at a second location on the object, the minimum frequency at which the...http://www.google.de/patents/US4305294?utm_source=gb-gplus-sharePatent US4305294 - Ultrasonic apparatus and method for measuring wall thickness
