Differential Coil Technique

Two coils connected in opposition cancel out uniform interference fields. Only when a defect such as a crack or pore passes under the arrangement does the field become asymmetrical and deliver a clear signal. This allows defects in the weld to be located much more reliably than with a single coil. Make sure to keep the alignment clean and the distance to the test piece constant, otherwise you'll get usable indications. Before testing, perform a zero adjustment on a defect-free area so that the system is properly set.