Instrument for detecting discontinuities in pipe walls - with spaced transformers inducing magnetic field in pipe

摘要

An instrument for detecting discontinuities, e.g. flaws, couplings on perforations in a metallic pipe, e.g. oil well pipe or gas pipe, includes two spaced transformers with magnetic circuits completed by the pipe wall during movement of the instrument along the pipe, the secondary windings of the transformers having different numbers of turns and being oppositely connected in series and coupled to the input of an amplifier circuit, the primary windings of the transformers having identical numbers of turns and being matchingly connected in series and coupled to the output of the amplifier circuit. Due to the difference in the number of turns of the secondary windings and to the matching connection of the windings and the amplifier circuit, there is positive feedback in the amplifier circuit, the amount of positive feedback being defined by the product Kbeta, K being the gain of the amplifier circuit and beta being the resultant transformation ratio of the interconnected transformers, the value of Kbeta being made greater than unity by selecting a suitable value for the ratio of the number of turns of the primary windings and the difference in the number of turns of the secondary windings. Any discontinuity in the pipe wall causes a disturbance of the magnetic symmetry of the transformers' magnetic circuits and a consequent change in the value of beta, resulting in a characteristic change in frequency of the output signal of the amplifier circuit. There is no change in frequency of the output signal due to accidental variation in the distance of the instrument from the pipe wall.