Fig 1Magnetic flux leakage inspectiondrag to move · space to playpeak · sensor at the pit · odo 10
Magnetic flux: along the wall between the poles, leaking out at the pitTool: drive cups, magnets and steel brushes, Hall sensor ring, joint, recorder, odometer wheelsExternal metal loss, at the edge of the cutTrace: sensor signal against odometer distance, no unitsIllustrative, not to scale. Product pressure behind the cups pushes the tool along the pipe. Magnets and steel brushes carry flux through the wall between two poles, close to saturation. Where the wall is thinner the flux cannot all stay in the steel and some leaks out, more on the side the metal was lost, a little into the bore, where the Hall sensors riding the wall read it. The recorder logs that signal against the odometer, so the peak marks where along the pipe the loss is. The figure shows the principle only: no sizes, depths or severities.