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Ultrasonic Gauging

Thickness measurements are performed using a conventional flaw detector and a compression wave probe, which sends longitudinal waves into the component at normal incidence to the surface. Signals are displayed on the flaw detector screen in the form of an A-scan, in which the horizontal axis represents distance and the vertical axis represents signal amplitude. Since a 0° compression probe is being used, the horizontal axis is equivalent to depth from the scanning surface. When the probe is placed on the surface of the component, a reflection appears at a range corresponding to the thickness of the component at that point. The use of an A-scan display allows the operator to distinguish more easily between signals originating from embedded plate flaws and the nominal back wall response. Also, the dynamics of the back wall echo can be observed on the A-scan display to detect the presence of pitting.

Conventional twin-crystal 0° compression probes are generally used to detect hidden corrosion. However, where pitted surfaces are being assessed for remaining thickness, pencil probes are used. These have a pointed tip which is designed to fit into the pits, so that the remaining thickness can be measured where the external pitting is at its most severe.

Internal corrosion pitting and general erosion in most metals. A-Scan thickness surveys are also used for the inspection of parent material for inclusions and laminations.

Generally used for thickness surveys on pressure vessels, pipelines, storage tanks and ship hulls.