LSI & RSI: Scaling Indices Explained & Calculated
The Langelier Saturation Index (LSI = pH − pHₛ) and the Ryznar Stability Index (RSI = 2pHₛ − pH) tell you whether water will scale or corrode from calcium carbonate. A positive LSI (or RSI below ~6) means scaling; a negative LSI (or RSI above ~7) means corrosive; LSI near zero is balanced. Both are calculated from five inputs — pH, temperature, TDS, calcium hardness and total alkalinity — via the saturation pH (pHₛ).
What the indices tell you
LSI and RSI both predict the calcium-carbonate scaling or corrosive tendency of water — the single most common scale in cooling systems. They are the starting point for setting the operating window (cycles of concentration, pH) and the antiscalant dose. They only describe CaCO₃, not sulfate, silica or general corrosion, so they are a guide, not the whole story.
Langelier Saturation Index (LSI)
LSI = pH − pHₛ, where pHₛ is the pH at which the water is saturated with calcium carbonate. Interpretation: LSI > 0 = scaling (CaCO₃ tends to precipitate), LSI < 0 = corrosive/undersaturated (water dissolves CaCO₃ and its protective film), and LSI ≈ 0 = balanced. Many cooling programs aim for a slightly positive LSI (a thin protective film) with an antiscalant preventing actual deposition.
Ryznar Stability Index (RSI)
RSI = 2pHₛ − pH. It refines LSI for higher-TDS cooling water and always returns a positive number: RSI < 6 = scaling, RSI ≈ 6–7 = balanced, RSI > 7 = corrosive (and >8 heavily corrosive). RSI is popular for cooling towers because it separates scaling from corrosive water more sharply than LSI alone.
The five inputs and pHₛ
Both indices need pH, temperature, total dissolved solids (TDS), calcium hardness (as CaCO₃) and total alkalinity (as CaCO₃). pHₛ is computed as pHₛ = (9.3 + A + B) − (C + D), where A depends on TDS, B on temperature, C on calcium hardness and D on alkalinity. Higher temperature, calcium and alkalinity lower pHₛ and push the water toward scaling.
Worked example
For water at pH 8.0, 30 °C, TDS 500 mg/L, calcium 240 mg/L (as CaCO₃) and alkalinity 200 mg/L (as CaCO₃), pHₛ works out to about 7.2. Then LSI = 8.0 − 7.2 = +0.8 (scaling) and RSI = 2×7.2 − 8.0 = 6.4 (mildly scaling/balanced) — so this water will tend to scale and should run with an antiscalant and controlled cycles.
How to use them
- Target a slightly positive LSI (thin protective film) and dose an antiscalant to prevent actual scale — do not run corrosive (strongly negative LSI) water.
- Recalculate at the concentrated (tower) conditions, not just make-up, and at the hottest skin temperature.
- Remember the indices cover only CaCO₃ — check sulfate, silica and general corrosion separately.
LSI and RSI interpretation
| Tendency | LSI | RSI |
|---|---|---|
| Heavy scaling | > +0.5 | < 5.5 |
| Slightly scaling | 0 to +0.5 | 5.5 – 6.2 |
| Balanced | ≈ 0 | 6.2 – 6.8 |
| Slightly corrosive | −0.5 to 0 | 6.8 – 8.0 |
| Corrosive | < −0.5 | > 8.0 |
Watch
Frequently asked questions
What is the Langelier Saturation Index (LSI)?
LSI = pH − pHₛ, where pHₛ is the pH at which water is saturated with calcium carbonate. A positive LSI means the water tends to scale (deposit CaCO₃); a negative LSI means it is corrosive and dissolves protective carbonate film; LSI near zero is balanced. It is calculated from pH, temperature, TDS, calcium hardness and alkalinity.
What is the Ryznar Stability Index (RSI)?
RSI = 2pHₛ − pH. It refines the LSI for cooling water and always gives a positive number: below about 6 the water is scale-forming, around 6–7 is balanced, and above 7 it is corrosive (above 8 heavily corrosive). It distinguishes scaling from corrosive water more sharply than LSI alone.
What inputs do you need to calculate LSI and RSI?
Five: pH, water temperature, total dissolved solids (TDS, or conductivity converted to TDS), calcium hardness as CaCO₃, and total alkalinity as CaCO₃. From these you compute the saturation pH (pHₛ = 9.3 + A + B − C − D), then LSI = pH − pHₛ and RSI = 2pHₛ − pH.
What LSI should cooling water run at?
Many cooling programs aim for a slightly positive LSI — around 0 to +0.5 — so a thin protective carbonate film forms while an antiscalant prevents actual scaling. Strongly positive LSI risks scale; negative LSI is corrosive and should be avoided. Always evaluate at concentrated tower conditions and the hottest surface, not just the make-up water.
Do LSI and RSI cover all cooling water scaling?
No. Both indices predict only calcium-carbonate scaling and corrosivity. They do not cover calcium sulfate, silica, phosphate scale or general/pitting corrosion, which must be assessed separately. Use LSI/RSI as a starting point, then confirm the full program and dose — VCYCLETECH can help evaluate your water analysis.
About the manufacturer
VCYCLETECH is a China-based manufacturer of water treatment chemicals — cooling-water scale & corrosion inhibitors, phosphonates, dispersants, biocides, coagulants and defoamers — ISO 9001 / 14001 / 45001 certified, with a COA on every batch and OEM/ODM service. See our quality & certifications.
References
- Langelier saturation index — Wikipedia
- Hard water & calcium carbonate — Wikipedia
- Calcium carbonate — Wikipedia
Related: HEDP · ATMP · Phosphonates · Cooling water treatment · Cooling tower guide