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Ryznar Stability Index Calculator

Enter the saturation pH and measured pH to evaluate water stability.

How it is calculated

The Ryznar Stability Index compares the water's actual pH with its saturation pH: RSI = 2 × pHs − pH. pHs is the pH at which the water would be exactly saturated with calcium carbonate. It is worked out separately from temperature, total dissolved solids, calcium hardness and alkalinity, and this tool does not calculate it for you. pH is the pH you measured. The tool reads the result in three bands: below 6, the water tends to form scale; from 6 to 7 inclusive, it is balanced; above 7, it tends to be corrosive. The higher the pH gets above pHs, the lower the index and the stronger the scaling tendency. The result is a tendency indicator, not a guarantee, and it does not check that your inputs are in a realistic range.

Worked example

Water has a saturation pH of 6.8 and a measured pH of 8.4. Step 1: 2 × 6.8 = 13.6. Step 2: 13.6 − 8.4 = 5.20. Because 5.20 is below 6, the tool reports that the water tends to scale with calcium carbonate deposits. For comparison, a water with pHs 7.2 and pH 7.8 gives 6.60, which is in the balanced band, and pHs 8.1 with pH 7.2 gives 9.00, which is corrosive.

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Common mistakes

  • Entering the measured pH in both fields. The first field is the saturation pH (pHs) calculated from the water analysis, not a reading.
  • Swapping the two values. Because pHs is doubled, swapping them gives a very different index and can reverse the conclusion.
  • Reading a low RSI as good. Low values mean scaling tendency, high values mean corrosive tendency, and the balanced range is only 6 to 7.
  • Using a pHs worked out at a different temperature from the one at which the sample is used. pHs changes with temperature.

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.

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Comments (1)

  • Editorial team

    Two samples show how the bands behave. A water with pHs 7.2 and measured pH 7.8 gives 6.60, inside the balanced band. A water with pHs 6.5 and measured pH 8.5 gives 4.50, which points to scaling. The index describes a tendency only; it says nothing about how fast scale or corrosion develops, and it does not compute pHs for you. What kind of water are you checking, a pool, a boiler or a cooling loop?

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