Scaling indices: Langelier (LSI) and Ryznar (RSI)
Calculate the Langelier Saturation Index and Ryznar Stability Index from pH, temperature, TDS, calcium hardness and alkalinity, with interpretation of the scale-forming or undersaturated (aggressive) tendency.
Results
| Saturation pH (pHs) | 7.93 |
|---|---|
| Langelier Saturation Index (LSI) | -0.43 |
| Ryznar Stability Index (RSI) | 8.36 |
LSI -0.43: approximately balanced water, close to calcium carbonate saturation at these conditions. RSI 8.36 reads undersaturated while the LSI does not. The two are different empirical fits and can disagree near the boundaries. Treat the water as borderline.
Formula used
LSI = pH − pHs
RSI = 2 × pHs − pH
pHs = (9.3 + A + B) − (C + D)
A = (log10(TDS) − 1) ÷ 10
B = −13.12 × log10(T + 273) + 34.55
C = log10(Ca) − 0.4
D = log10(Alk)
- pH
- measured pH of the water
- pHs
- pH of calcium carbonate saturation
- TDS
- total dissolved solids, mg/L
- T
- water temperature, °C
- Ca
- calcium hardness, mg/L as CaCO3
- Alk
- total alkalinity, mg/L as CaCO3
Carrier method approximation of the Langelier Saturation Index; Ryznar Stability Index per Ryznar (1944). Indicative for waters roughly 0–80 °C and TDS below 10 000 mg/L. LSI above +0.5 suggests scaling, below −0.5 an undersaturated, aggressive tendency; RSI below 6 scaling, above 7 increasingly aggressive. These are saturation tendencies, not corrosion rates or scaling speeds. Lab report lists bicarbonate instead of alkalinity? Alkalinity as CaCO3 ≈ HCO3 (mg/L) × 0.82. Calcium given as the ion? Ca hardness as CaCO3 = Ca (mg/L) × 2.5.
The background
- Scaling indices (LSI and RSI) in plain languageThe Langelier and Ryznar indices condense pH, hardness, alkalinity, temperature and TDS into one number that predicts whether water will deposit calcium carbonate or dissolve it. It answers a scaling question and does not give a corrosion rate.
- pH and alkalinity — and why they are not the same thingpH says how acidic the water is right now; alkalinity says how hard it will fight you when you try to change that. Confusing the two is the most common water chemistry mistake.
- Water hardness: what it is and how it is measuredHardness is dissolved calcium and magnesium, the minerals behind scale in geysers, kettles and membranes. The units are the confusing part; the chemistry is simple.
Equipment for this duty
Send the water analysis and duty point, and the branch confirms exact selections, pricing and lead times, usually within one working day.
Request a quoteResults are indicative engineering estimates based on standard formulas and typical assumptions. They are not a design service; confirm any design against a full water analysis and the relevant manufacturer’s data.