Coil apparatus mounted on an axially split spool for indicating wall

Abstract

A method and apparatus are provided for effecting the inspection of a ferromagnetic element, or piping, and particularly piping that is already installed. The invention incorporates a device for indicating wall thickness of a length of the ferromagnetic element, including means for producing a value of magnetic flux extending longitudinally through a portion of the element. A voltage generating device is disposed in surrounding relationship to the element and is movable axially relative to the length of the element containing the value of the magnetic flux. Means for indicating the voltage generated in the voltage generating means by the value of the magnetic flux also are provided. Preferably, an axially split spool is provided which may be closed in surrounding relationship to an installed pipe. A saturation level of magnetic flux is induced in the pipe by a flexible cable which is wrapped around the spool in a multi-turn configuration and supplied with electrical current sufficient to produce saturation. The spool incorporates in its periphery three axially split pickup coils, the ends of which are brought into interengagement by the closing of the spool around the length of pipe to provide three separate multi-turn current paths around the enclosed pipe. As the spool is moved along the pipe, the voltage induced in the central pickup coil is amplified, integrated and applied to one Y-axis of a voltage recording device. The differential of the voltages generated by the two end pickup coils is amplified, integrated and applied to a second Y-axis of the recorder. The position of the coils along the pipe is indicated by a transducer which generates a voltage signal applied to the X-axis of the recorder. The recorder thus provides concurrent indication of both the average thickness of the pipe and any discontinuities from the average thickness as indicated by the amplified, integrated differential voltage signal.