Hardness, pH, dissolved oxygen, cycles of concentration and conductivity — these five core indices are the keys to interpreting system behavior and predicting scaling or corrosion risk. Based on real operating data, this article walks through what each index means, how to measure it and how to apply it in industrial water systems.
1. Langelier Saturation Index (LSI)
LSI is the most widely used scaling tendency indicator, reflecting the saturation state of calcium carbonate in water.
pHs is the saturation pH of calcium carbonate at the given water quality, obtained from charts or formulas based on temperature, TDS, calcium hardness and alkalinity.
| LSI value | Interpretation | Operating strategy |
|---|---|---|
| < −0.5 | Strong corrosion tendency | Increase corrosion inhibitor, adjust pH or raise hardness |
| −0.5 ~ +0.5 | Essentially stable | Maintain current operating conditions |
| > +0.5 | Strong scaling tendency | Add scale inhibitor, lower cycles of concentration |
2. Ryznar Stability Index (RSI)
RSI fills the gap that LSI indicates direction but not intensity:
- RSI < 5.5 : heavy scaling
- RSI = 5.5–6.2 : mild scaling
- RSI = 6.2–6.8 : water stable
- RSI = 6.8–8.5 : mild corrosion
- RSI > 8.5 : severe corrosion
3. Puckorius Scale Index (PSI)
PSI introduces the equilibrium pH (pHeq), which is more accurate for high-alkalinity, high-cycle cooling water:
When total alkalinity is high or the water deviates from the assumptions behind LSI, use PSI to evaluate scaling tendency.
4. Reading hardness, alkalinity and pH together
In industrial cooling systems these three need to be interpreted jointly:
- Total hardness (Ca²⁺+Mg²⁺): scaling risk rises notably > 300 mg/L (as CaCO₃);
- Total alkalinity: watch for carbonate scaling > 200 mg/L;
- pH: recommended range 8.2–9.0, which can be raised with the right scale inhibitor.
5. Conductivity & cycles of concentration
Conductivity is fast to measure and is the common on-site way to monitor cycles of concentration:
Abnormally high conductivity often indicates blowdown valve failure or an unbalanced water flow — inspect the blowdown system and make-up flow immediately.
6. Dissolved oxygen and corrosion
In open cooling towers, dissolved oxygen (DO) approaches saturation (8–10 mg/L) and is the main driver of carbon-steel corrosion. Common control strategies:
- Use film-forming corrosion inhibitors (molybdate, zinc-based);
- Maintain proper pH to form a dense γ-Fe₂O₃ passivation layer;
- Keep flow velocity in the 0.6–2.5 m/s range to avoid local stagnation.
Operating tip: Plants should build monthly water quality trend curves and diagnose LSI / RSI / PSI together, combined with corrosion coupon and scale probe readings—only then can chemical programs be adjusted scientifically.
7. Conclusion
Water quality indices are not isolated numbers; they are the combined result of water chemistry, metallurgy and process conditions. Pairing TRISPE® scale inhibitor + POLYMER® corrosion inhibitor with our smart O&M platform allows dynamic chemical adjustment based on real-time indices — a true closed-loop control of both scaling and corrosion.