Abstract: A system for controlling the silver ion concentration of a silver halide emulsion made by mixing alkali halide and silver nitrate solutions which are delivered continuously to a vessel at primary addition rates. The silver ion concentration in the vessel is measured to establish the deviation from a desired concentration. Where the error is within a limited or normal range, a low signal selector circuit causes a pump in a secondary flow loop to deliver a secondary amount of whichever solution is needed to offset the deviation. Where the error is abnormal, a high signal selector circuit alters the primary addition rate of the alkali halide solution to offset the deviation.