GB952106A - Improvements in or relating to methods of and apparatus for measuring the wall thickness of tubular bodies - Google Patents

Abstract

952,106. Determining physical dimensions. BRITISH INSULATED CALLENDERS' CABLES Ltd. April 27,1962 [April 28, 1961], No.15492/61. Heading G1N. The thickness of a tubular body of metal, or of non-metallic material overlying a metallic layer, is measured by means of an A. C. energized detector coil system which helically scans the surface of the body, and the signals produced by the reaction in the coil of eddy currents in the metal are applied to an indicator or recorder through a phase discrimination circuit adjusted to reject that part of the signal due to curvature variation and leave a signal indicative of thickness variation. It is stated that the output signal due to change in curvature is about 90 degrees away in phase from the output signal due to change in thickness, while the output due to temperature variation lies between these phase directions. Detector coil system 1, including primary and secondary windings W1, W2, is resiliently mounted on a carriage resting on tube 14 under test. A support arm 17 rotatably supports the carriage through sleeve 16 and fits into terminal box 22 at its other end. Box 22 is mounted with two degrees of freedom on a support member 25 attached to a ring 36 (Fig. 4), arranged for rotation about tube 14. The tube moves longitudinally relative to ring 36 and coil system 1, the carriage mounting also permitting lateral and angular movements of the tube. Slip rings 54 are provided or ring 36. A balancing coil system 2 with primary and secondary windings W3, W4 is also mounted on the carriage either spaced from the tube surface or adjacent the surface, with its axis parallel to the tube axis. The primaries and secondaries may each be on separate non-magnetic formers,or bifilarly wound on two formers. Primaries W1, W3 are bridge connected to oscillator O, and secondaries W2, W4 are bridge connected to a band pass amplifier BPA whose output is passed through two phase detector channels, one, PD1, rejecting curvature variation signals, and the other PD2 rejecting temperature variation signals. The output of PD1 is applied to a CRO and through switches S1, S5 and S6 to chart recorder R. Capacitor C5 blocks slow changes due to temperature drift. The output of PD2 is integrated by capacitor C4 to provide a mean thickness indication. Potentiometers P2, P3 are initially adjusted using a tube of correct thickness and subsequent readings on recorder R with switches S1 to S6 in their centre positions indicate mean thickness plus temperature drift, the latter then being eliminated by adjusting P3 and turning the switches to their lower positions. Rotary potentiometer P1 provides a time-base signal for the CRO.