GB846029A - Method of and apparatus for minimizing spurious signals in an electromagnetic flowmeter - Google Patents

Abstract

846,029. Electric tests. FOXBORO CO. Aug. 27, 1957 [Sept. 6, 1956], No. 27033/57. Class 37. In an electromagnetic flowmeter head comprising a stainless steel pipe section 10, Fig. 1, through which the fluid passes and in which a transverse alternating magnetic field is set up across the pipe by two saddle coils 16, 18 and in which the flow of the fluid through the magnetic field generates an electrical signal voltage of amplitude proportional to the rate of flow in an indicating and measuring, and if desired a controlling, circuit such as an oscilloscope 64, by means of a pair of electrodes 46, 48 (see also Fig. 2) inside the pipe connected by respective leads 48, 50 to the oscilloscope 64, spurious signals produced in the output circuit particularly those in phase with the desired output signal, are eliminated or minimised by a conductive mass such as aluminium plate 62 which is moved manually in axial and radial directions to the best position, as indicated by a straight horizontal trace on the oscilloscope 64. Plates 62 of different sizes may be tried until the optimum size is found. Plate 62, when finally adjusted, is secured by layers of adhesive tape to the inner surface of helically wound silicon steel tape 20 which serves as a low reluctance return path for the magnetic flux. Out of phase spurious signals may be minimised or eliminated by previously adjusting saddle 26 and shield tubes 52, 54 for the output leads 48, 50 longitudinally to a position giving minimum coupling between the magnet coils and output leads 48, 50, i.e. so that there is minimum flux linkage between the magnet coils and the circuit loop comprising the electrode leads and oscilloscope 64. The output leads are twisted together over the horizontal portions thereof (Fig. 1) so as to be as non-inductive' as possible. The tube 10 has an insulating inner coating formed by baking vitreous enamel on to the surface or by plastic material. Identical magnet coils 16, 1.8 are conected in parallel to an A.C. source and energised in phase so the magnetic fields are additive. Adjacent turns of silicon steel helical band 20 are insulated from one another to prevent eddy currents by two strips of glass fibre tape, Fig. 4 (not shown) and the space between the strips is filled by a coating of insulating cement, preferably including an epoxy potting resin, the resistance between adjacent turns of band 20 being thus made equal to within 1 % of the end to end resistance of band 20 if unwound. Two bridges 26, 28 secured to the pipe by arch members 30 welded to the pipe and to the bridges support the coils 16 and the band 20. The magnet coils are held by curved clamps 32a, 32b having bolts 34a, 34b and nuts 36a, 36b and band 20 is held by a flat plate 38 having bolts 30 and nuts 42. The electrodes 44, 46 are inside but insulated from the pipe and the output leads extend through copper shield tubes 52, 54 joined together to form a saddle supported by apertured member 58. An insulating sheath 56 of fibre glass-silicon composition surrounds the twisted portions of the output leads. The oscilloscope 64 has one pair of deflecting plates thereof energised by the flowmeter output and the other pair energised by the power voltage supply which energises coils 16, 18. The output signals may be amplified before being applied to the oscilloscope. The magnetic coils can be wrapped in aluminium sheaths 72a, b, c, d to assist in the suppression of eddy currents. Diagonally opposite magnet coils are connected by leads 74, 76; 80, 82 to terminals 78, 84 on flat support plate 38. A prior art arrangements in which a shorted turn of wire is wound around band 20 is referred to.