GB1148091A - Improvements in or relating to electrolytic conductivity measuring and control systems - Google Patents

Abstract

1,148,091. Automatic mixture control. ELECTRONIC SWITCHGEAR (LONDON) Ltd. June 30, 1966 [March 30, 1965], No.13482/65. Heading G3R. In a system for automatically maintaining the composition of a liquid within predetermined limits, a conductivity cell in the liquid forms part of an A.C. bridge circuit associated with the input of a transistor amplifier the output of which controls the composition of the liquid through a phasesensitive detector and a contact-making relay. In one arrangement for maintaining rinsing water below a predetermined level of contamination, a conductivity cell in the water forms one arm of a mains energized adjustable bridge circuit the output of which is applied to a multi-stage transistor amplifier the first stage of which utilizes a field effect transistor. The amplifier output, together with alternate half cycles at mains frequency, are applied to a transistor discriminator so that, when the bridge output has a phase sense corresponding to excessive contamination, the discriminator causes a Schmitt-trigger operated relay to be in such a condition that a motor or solenoid operated valve is opened to supply fresh water to the rinsing tank. Pilot lights indicate the condition of the circuit which may be arranged to fail safe. A known resistor may be switched in place of the conductivity cell for test purposes. A switch may also be operated to maintain the trigger circuit in the condition it has at any time, and to connect an indicator, with suitable compensation and gain adjustments, so that the conductivity may be measured by manually balancing the bridge circuit. In a modification for maintaining a desired concentration of a chemical process solution, the bridge and discriminator are energized at a higher frequency from a transistor oscillator and a temperature variable resistor is included in the bridge circuit to compensate for temperature variations. Two conductivity cells, one in the liquid and the other in a reference liquid are arranged in the bridge circuit in an alternative arrangement.