Patent ID: 7804295

Claim:
An apparatus for detecting discontinuities on or below a surface of a magnetizable material comprising: a support arranged to be located adjacent the surface; a drive arrangement for causing relative movement of the support along the surface; a magnetizing device carried on the support for inducing a magnetic field in and above the material; an array of sensors carried on the support where each sensor is arranged to detect magnetic flux leakage from the material and produce an output signal corresponding thereto; and an analyzing device to detect the presence of discontinuities in the material based on the signals from the sensors and generate data on said discontinuities therefrom; and a lift-off distance measuring device for continuously measuring a lift-off distance between the sensor array and the material surface so as to generate lift-off distance data and wherein the analyzing device is arranged to use the lift-off distance data to calibrate the signal from the sensors to the appropriate magnitude according to the lift-off distance.