Study of Inhomogeneities in Non-Magnetic Tubes by Means of a Contactless Inductive Technique

Abstract

An accurate technique for contact-less measurement of resistivity in non-magnetic conductive tubes has been developed. It is intended for application in non-destructive testing (NDT) in tube manufacturing by simple study of the induced currents. Inhomogeneities or minimal imperfections in the tube thickness are immediately detected. This measuring technique is also suitable for determining the thickness of very thin metal coatings on non-conductive tubes. The experimental setup corresponds to an electrical transformer, the tube being the core. A first coil is placed around the tube under test, whereas the detecting coil is placed inside it. The study of the magnetic screening caused by the currents induced in the tube, allows its conductivity to be determined. The use of low frequency guarantees the almost complete absence of the skin effect in such a way that the results are obtained in a very simple manner. The method is based on the determination of the phase angle of the complex mutual inductance between the two coils working under low frequency conditions.