Ryznar Stability Index Calculator

About this calculator

Water treatment engineers need to know whether a given water chemistry will deposit protective mineral scale on pipe walls or actively corrode them, since both are costly problems in cooling towers, boilers, and distribution systems. John Ryznar refined the older Langelier Saturation Index into the Ryznar Stability Index: RSI = 2·pHs − pH, where pHs is the pH at which water is exactly saturated with calcium carbonate, and pH is the water's actual measured pH.

Unlike the Langelier index, the RSI scale is designed so that lower values (below about 6) indicate scaling tendency and higher values (above about 7) indicate corrosive tendency, with a numeric “sweet spot” around 6-7 for balanced water. This calculator computes RSI from the two pH values, a standard check for water treatment operators and HVAC/industrial engineers managing cooling water and boiler feedwater chemistry.