Water Treatment Indices
The Blueprint of Water uses a small set of empirical indices to estimate how a given water will behave toward mild steel and toward calcium carbonate. One index (Larson-Skold) flags corrosivity, and three (LSI, RSI, PSI) estimate calcium carbonate scaling pressure. None of them predict corrosion rate or deposition rate; they describe thermodynamic tendency and are most useful as comparative tools.
| Index | What it predicts | Inputs | Interpretation bands |
|---|---|---|---|
| Larson-Skold Index (LS) | Corrosivity of water toward mild steel (ratio of aggressive anions to protective inhibitors, potential to interfere with protective film formation) | Chloride, sulfate, bicarbonate, carbonate, all in meq/L. LS = (Cl⁻ + SO₄²⁻) / (HCO₃⁻ + CO₃²⁻) | Less than 0.8, lower corrosion risk, film likely stable. 0.8 to 1.2, potential risk, film interference likely. Greater than 1.2, high risk, localized corrosion and pitting likely. |
| Langelier Saturation Index (LSI) | Whether calcium carbonate tends to dissolve or precipitate (thermodynamic bias) | pH and pHs. pHs from TDS (mg/L), temperature (°C), calcium hardness and total alkalinity (mg/L as CaCO₃). LSI = pH minus pHs | Less than 0, dissolution favored. Equals 0, equilibrium, no net scaling or dissolution. Greater than 0, precipitation favored. |
| Ryznar Stability Index (RSI) | Actual scaling or dissolving behavior of water, based on field observations | pH and pHs (pHs calculated same as LSI). RSI = (2 × pHs) minus pH | Less than 6, scale forming. 6 to 7, borderline, balanced zone. Greater than 7, dissolving tendency, often more corrosive. |
| Puckorius Scaling Index (PSI) | Calcium carbonate scaling pressure adjusted for buffering capacity, uses equilibrium pH after precipitation | pHs (same as LSI) and pHeq. pHeq = 1.465 × log₁₀(Total Alkalinity as CaCO₃) + 4.54. PSI = (2 × pHs) minus pHeq | Less than about 6, scale forming. 6 to 7, borderline, balanced zone. Greater than 7, dissolving tendency. |
Related concepts
Corrosion Cell · Passivation · Film Saboteurs · Pitting Corrosion · Mils Per Year (MPY) · Galvanic Series · Supersaturation · Cycles of Concentration · Scale Inhibitors · Calcium Carbonate Scale · Cathodic Control
Part of The Blueprint of Water