Remote field flaw sensor including an energizing coil, first and second

Abstract

A flaw sensor for a metal pipe has an exciting coil for generating remote field eddy currents in the pipe. First and second groups of receiving coils are spaced from the exciting coil by respective distances along the axis of the pipe in a region of the generated remote field eddy currents. The first and second groups have respective pluralities of receiving coils with axes perpendicular to the pipe axis and disposed circumferentially about the pipe axis at predetermined angular increments wherein the receiving coils of the second group are staggered relative to the receiving coils of the first group. A third receiving coil is disposed coaxially with the pipe at still another distance from the exciting coil along the pipe axis but also in the region of the generated remote field eddy currents. When a defect in the thickness of the metal pipe is encountered, a magnetic field caused by the defect to be perpendicular to the axis of the pipe is detected by one or more of the receiving coils of the first and/or second group. The third receiving coil detects phase changes to the magnetic field generated parallel to the pipe by the eddy currents. A ratio of phase changes produced by a standard thickness of a reference pipe and a sample subject pipe is used as a correction factor for detected phase changes in a defective pipe to accurately calculate the defective wall thickness.