GB1009721A - Improvements in or relating to electric measuring arrangements - Google Patents

Abstract

1,009,721. Automatic mixture control. ELECTRONIC SWITCHGEAR (LONDON) Ltd. Jan. 5, 1961 [Jan. 5, 1961], No. 544/61. Heading G3R. [Also in Division G1] In an arrangement for measuring the electrical conductivity of a liquid, comprising a selfbalancing A. C. bridge including a conductivity cell in one arm, the frequency applied to the bridge is much higher than that applied to the servomotor and means are provided for controlling the energization of the servomotor by the bridge output to provide a reversal of phase in the servomotor feed in coincidence with reversal of phase in the bridge output. The bridge which comprises cell CMC, temperature sensitive resistor R 2 , resistors R 1 , R 3 , and potentiometer PR, is energized by an oscillator V1, (1000 c/s) and the output is applied to the grid of valve V2a which is the input stage of a multi-stage amplifier, the lead from point C of the bridge being screened and the screen being connected to the potentiometer wiper (the diagonally opposite bridge point). A phase sensitive network in the amplifier output is supplied with a reference signal from oscillator V1 and controls the respective impedance of valves V4a, V4b, to energize winding W1 of a servomotor M from the mains supply in one phase sense or the other in dependence on the bridge output to adjust the wiper of potentiometer PR and thus rebalance the bridge. Overshoot error is eliminated by a network R7, D3, C4, in series with a winding W2 of the servomotor, which network produces a braking current component. Alternatively or additionally the servomotor may be coupled to a rotatable cam shaft for controlling fluid control valves to correct any divergence of the measured liquid condition from a chosen value. The application referred to is salinity measurement and control in ship borne fresh water supply evaporator equipment. A conductivity measuring cell, Fig. 2, for insertion in a pipe line shown at 10, 11, comprises metal end electrodes 14 which are counterbored to house hard carbon electrodes 15 which have a bore contiguous with the pipe line. The carbon electrodes are held by similarly bored insulators 16 which each screw into a central carbon electrodes block 17 and the two outer electrodes are interconnected. A block 21 in good heat transfer relation and within a protective casing 22 is provided for mounting the resistors R 1-3 . The use of a single bridge with several measuring cells is referred'to. Specification 984,419 is referred to.