Patent GB-816107-A (GB)
Title: Measuring apparatus
IPC: H03F3/02
Filed: 1955-08-30
Abstract: 816,107. Electric tests. FOXBORO CO. Aug. 30, 1955 [Sept. 14, 1954], No. 24905/55. Class 37. In a flowmeter of the type in which the flow of liquid through an alternating magnetic field produces an E.M.F. between electrodes situated in the flow, the leads connecting the electrodes to an amplifier are mounted in shield tubes which have a common junction located centrally with respect to the electrodes and earthed, and the amplifier has a chassis which is insulated from a casing connected to a point at pipe potential, the amplifier having a reference point connected to the shield tube junction about which the amplifier circuit is balanced. As shown, the liquid under test flows in pipe 10 lined with insulating material such as vitreous enamel or a layer of chemical and heat-resistant plastic. Upper and lower coils 16, 18 are A.C. energized to produce a uniform flux in the cross-section of the pipe and are surrounded by a laminated core 40 of silicon steel. Both the core and the coil are supported by bridges 42, 44 extending along the pipes to which they are welded. Clamps 48a, 48b fasten the coils to the bridges. Electrodes 20 to measure the voltage induced in the liquid by its velocity, are positioned 180 degrees apart in the wall of the pipe from which they are insulated. The electrodes are connected to output leads 24, 26 which are insulated with a heatresistant plastic and extend through copper shield tubes 28, 30 which are joined together at 29 to form a saddle on the pipe. The shield tubes are then twisted together to reduce inductive pick-up and increase rigidity and are encased in an insulating sheet of fibre glasssiliconed composition. The junction of shield tubes 28, 30 is connected to earth by lead 32a. The wires 24, 26 are connected through leads 34, 36 to the input terminals of an amplifier (see below). Shielding tubes 28, 30 are connected to a shield 35 surrounding leads 34, 36 which in turn is connected to the chassis of the amplifier through terminal 112. A still further shield 37 is connected to flow meter pipe 10. Circuits.-The alternating output from the flow meter is applied to terminals 70, 72 which are connected through feedback winding 74, 82 to the grids of push-pull cathode followers 80, 88 between the cathodes of which are connected the condenser 138 and the primary of transformer 140. The transformer secondary is tuned to the mains frequency and feeds resistancecapacity coupled amplifying stages 150, 174 and 190. Stage 190 is provided with feedback path 202, 204 and also feeds stage 214 in the grid of which is tank circuit 216 which is tuned to near the supply frequency but is slightly mis-tuned to introduce a phase-shift, which compensates for phase-shift in the amplifier, before application of the signal through condenser 228 to phase-sensitive rectifier 230. Phase-sensitive rectifier 230 is also fed with reference antiphase alternating voltages from transformer secondaries 244, 246. According to the direction of flow and hence the phase sense of the voltage input to the phase-sensitive rectifier, a D.C. output is produced between the ends of resistors 264, 266 to control the relative anode currents of triodes 276, 286 which each energize half of winding 297 to position a member 295. Member 295 in turn positions a movable element 298 to adjust the relative coupling between primary winding 304 and differentially connected secondaries 308, 110. The difference signal developed by the secondary is fed to the primary winding 322 of transformer 94 the secondaries 74, 82 of which are in series with the feeds from terminal 70, 72 to the amplifier input stages. The feedback backs off the input signal to substantially zero. The " earth " lead of the device is constituted by lead 110 which is taken to terminal 111 on the D.C. supply and is insulated from the flow-meter and the case but is connected to the amplifier chassis. Capacitors are connected between terminals 70 and 72 to shortcircuit unwanted signals. Specification 648,981 is referred to.