Patent GB-872855-A (GB)
Title: Improvements in methods of and apparatus for measuring a tubular member of electric insulating material, such as the covering of a coated conductor
Inventor: KNIGHT ERIC ALFRED
IPC: G01R31/02
Filed: 1959-12-31
Abstract: 872,855. Measuring-apparatus. ADDISON ELECTRIC CO. Ltd. Dec. 31, 1959, No. 44477/59. Class 37. A method of measuring the wall thickness and/or bore eccentricity of a tubular member of electric insulating material manufactured as in a continuous extrusion process comprises measuring the capacitance between a conductor disposed within the tube and one or more external electrodes having a liquid disposed between them and the tube. The electrode(s) are employed in conjunction with a guard electrode to confine the measurement to the required zone. The external electrode 4, Fig. 1, embedded in an insulating medium 5, is in contact with the wall 1 of the insulating tube having an inner conductor 2, the whole being immersed in a liquid 3 and partially enclosed in a metallic container 6 which is connected to a guard potential via a coupling flange 7 and the screen of a connecting cable 8, the inner conductor of which serves to connect the electrode 4 to a measuring circuit. A second electrode 10 symmetrically positioned on the opposite side of the tube serves to confine the thickness measurements to the required zone of the wall. The wall capacitance is measured by a bridge circuit, Fig. 3, a generator 11 being connected to the junction of the capacitive ratio arms 12 and 13. The lower bridge arms are formed by the wall capacitance and a variable standard condenser 14, which is set to the required nominal value, any variations of the wall capacitance being indicated by the detector 15. A second trough 17 is connected to the guard potential of generator 10 to shield the electrode 4 from objects at ground potential. The electrode 10 can also be connected to guard potential or can take the place of condenser 14 in which case the reading of detector 15 indicates the horizontal displacement of the conductor 2 from its central position. The assembly may be rotated around the wall to measure the thickness in other planes or alternatively further electrode assemblies may be arranged around the wall. In a further embodiment, Fig. 4, a pair of electrodes 18 and 19 are mounted perpendicular to each other on an insulator 20 and backed by a guard plate 21, the assembly being carried on an arm 22 projecting from the pivot 23, through the interiors of which are passed the shielded connecting cables (not shown). The arm 22 is lightly spring-urged to press the electrodes 18, 19 against the wall 1 to be self-adjusting to different sizes of tubular wall. An electrode assembly not in contact with the wall, Figs. 6 and 7, comprises electrodes 24, 25, 26 and 27 equally spaced around the inner wall of an insulating tube 28 whose outer surface is enveloped by a metallic guard tube 29, protected by a further insulating tube 30. The tubular assembly is hinged at 31 to allow for introduction of the cable 1, 2 and two V-guides 32, 33 serve to maintain the tube wall 1 in the centre of the four electrodes. A measuring circuit similar to that in Fig. 3 may be used, a commutating switch successively connecting each electrode into the bridge circuit, the others being connected to guard potential. An alternative measuring circuit, Fig. 8, comprises a generator 11 connected to electrodes 26, 27 via the centrepoint of the primary winding of a balanced transformer 34. If the bore or central conductor of tube 1 is displaced a voltage appears across the secondary of transformer 34 and this is applied through an amplifier 35, and phasesensitive rectifier 36 to a galvanometer 37, giving a positive or negative voltage depending on whether the conductor 2 is displaced to the right or left. The vertical eccentricity is similarly displayed on the galvanometer 41. The D.C. outputs from the two sets of electrodes can also be fed to the deflection plates 44, 45 of a cathode-ray tube 46. A third bridge circuit comprising a balanced transformer 47, variable capacitor 48, amplifier 49, phase-sensitive rectifier 50 and galvanometer 51 enables any variations in the average wall thickness to be determined, the capacitor 48 being adjusted so that the current through it is balanced by the sum of the currents supplied to the four electrodes, which depends on the average wall thickness. The output signals may be employed for control purposes.