Abstract: A replenishing solution is mixed from liquid concentrate and tap water. Rather than measure the flow of liquids, apparatus measures static volumes of reagents, then transfers the reagents to a replenishing tank where they are mixed and brought to a predetermined temperature. Precise volumes are measured by weirs in each of the containers for water and concentrate. The replenishing solution is added to a developer in amounts determined by the pH of the developer. The electrode of a meter used for measuring pH are protected from scale or deposits by a sequestering agent, preferably EDTA, in the replenishing solution.