Multiple coil eddy current probe system and method for determining the

Abstract

Both a system and a method for determining the length of a discontinuity such as a crack or other fault in the interior wall of a steam generator heat exchanger tube is disclosed herein. The apparatus generally comprises an elongated probe assembly which includes a plurality of eddy current coils mutually separated at known distances with respect to each other along the longitudinal axis of the probe, and an eddy current coil actuating device for separately and independently actuating each of the coils to provide an adjustable electromagnetic sensing field. In the method of the invention, the probe assembly is positioned adjacent a discontinuity and then each eddy current coil, separated by a known distance, is independently actuated and its reading recorded to determine which of the coils actually detects the discontinuity. The length of the discontinuity may then be generally inferred from the known distances that separate the eddy current coils that detect the extremities of the crack or other discontinuity. A more specific sizing of the length of the crack is then obtained by actuating the eddy current coils that are located just outside the extremity-detecting coils at progressively lower frequencies until field of these outside coils finally couples with the extremities of the crack.