Ryznar Stability Index Calculator

Use the Ryznar Stability Index (RSI, also shown as RI) to screen water for calcium carbonate scaling tendency. The historical corrosion descriptions are qualitative guidance, not a prediction of metal loss.

Reading the tables: both tables interpret the same calculated RSI; the Carrier table is an alternative set of empirical categories. Below RSI 4, the Carrier result is reported as outside the tabulated range. Classification uses the unrounded RSI, so a displayed value at a boundary may fall in the neighbouring category.

What does the Ryznar Stability Index mean?

The Ryznar Stability Index is an empirical indicator based on calcium carbonate chemistry. It is calculated as RSI = 2pH_s − pH, where pH is the measured water pH and pH_s is the calculated pH at calcium carbonate saturation. Lower RSI values generally indicate a stronger tendency to form calcium carbonate scale. Higher values indicate less scale formation and fall within the historical aggressive-water categories shown above. The boundaries are empirical guidelines and differ between interpretation tables.

Use the correct analysis and units

Enter dissolved calcium as Ca²⁺ and bicarbonate as HCO₃⁻ in the selected units. Do not enter total hardness or alkalinity reported as CaCO₃ directly into these fields. If alkalinity is almost entirely bicarbonate, bicarbonate in mg/L is approximately 1.22 times alkalinity in mg/L as CaCO₃; this approximation becomes unsuitable when carbonate, hydroxide or other alkalinity contributors are significant.

Use measured TDS in mg/L where available. The ppm option is treated as numerically equal to mg/L, an approximation for dilute water. Conductivity alone does not uniquely determine TDS: any estimate needs a conversion factor appropriate to the water composition. This calculator therefore accepts TDS directly. Enter pH and temperature representing the same water conditions. Use a decimal point; the input range for this screening tool is pH 0–14 and temperature 0–100 °C.

How this calculator estimates saturation pH

The calculation retains the existing temperature-dependent equilibrium expressions and estimates ionic strength from TDS using I = 2.5 × 10⁻⁵ × TDS, with TDS in mg/L. Calcium and bicarbonate are converted to mol/L and adjusted using simplified activity coefficients. This is an approximate model; the input limits are software limits, not a validated accuracy range. It does not replace a full water analysis and a suitable chemical equilibrium calculation, particularly for seawater, concentrated brines or unusual water compositions.

Limits of the interpretation

RSI does not calculate scale mass, deposition rate, corrosion rate or drinking-water safety. Corrosion also depends on pipe material, dissolved oxygen, chloride, sulfate, flow conditions and treatment chemicals. A historical label such as “little scale or corrosion” does not guarantee that a system is protected. Assess operating conditions and material-specific corrosion evidence before selecting treatment.

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Warning: Lenntech BV cannot be held responsible for errors in the calculation, the program itself or the explanation. For questions or remarks please contact us.