-0.16
7.66
34
66
0.16
pH
-0.16
7.66
34
66
0.16
pH
The Langelier Saturation Index (LSI) Calculator determines whether water has a tendency to deposit calcium carbonate scale or to dissolve existing scale (corrosion). Developed by W.F. Langelier in 1936, the LSI is the most widely used index for assessing the scaling and corrosion potential of water in distribution systems, cooling towers, swimming pools, and industrial water circuits. The index compares the measured pH of water with the theoretical saturation pH (pHs) — the pH at which water would be in equilibrium with calcium carbonate. A positive LSI indicates oversaturation and a tendency to deposit scale, while a negative LSI indicates undersaturation and a tendency to dissolve scale (corrosion). The index is essential for corrosion control, pipe protection, and maintaining water infrastructure integrity.
The LSI is calculated as the difference between actual pH and saturation pH:
$$LSI = pH - pH_s$$
Where the saturation pH is estimated using the empirical formula:
$$pH_s = (9.3 + A + B) - (C + D)$$
With the following components:
Interpretation: LSI = 0 means water is in equilibrium with CaCO₃; LSI > 0 means oversaturated (scale-forming); LSI < 0 means undersaturated (corrosive). The magnitude indicates the intensity — values beyond ±0.5 indicate significant scaling or corrosion potential.
An LSI of +0.5 to +1.0 suggests moderate scaling — a thin protective layer may form on pipe surfaces, which is actually desirable in distribution systems. An LSI of +1.0 to +2.0 indicates heavy scaling that can reduce pipe diameter, block heat exchangers, and increase energy costs. An LSI of -0.5 to -1.0 indicates moderate corrosion potential, where metal pipes may leach iron, copper, and lead into drinking water. An LSI of -1.0 to -2.0 indicates aggressively corrosive water that can rapidly degrade infrastructure. The ideal target for most water systems is LSI between 0 and +0.5, providing mild scale protection without excessive buildup.
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LSI of +0.18 indicates slightly scale-forming water — ideal for pipe protection. A thin CaCO₃ layer will form, providing corrosion protection without problematic scaling.
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Strongly negative LSI of -2.46 indicates aggressively corrosive water that will dissolve pipe materials and leach metals. pH adjustment and alkalinity supplementation are needed.
An LSI of exactly zero means the water is in perfect equilibrium with calcium carbonate — it will neither deposit scale nor dissolve existing scale. In practice, this is a theoretical balance point. Most water treatment professionals target a slightly positive LSI (+0.1 to +0.5) to provide mild protective scale without excessive buildup.
Higher temperature increases calcium carbonate's tendency to precipitate (decreasing its solubility), shifting the LSI in the positive (scale-forming) direction. This is why hot water heaters, boilers, and heat exchangers are particularly susceptible to scaling. A water that is slightly corrosive at 20°C may become scale-forming at 60°C.
The LSI (Langelier Saturation Index) gives a qualitative indication: positive = scaling, negative = corrosive. The RSI (Ryznar Stability Index) provides more differentiation in the scaling and corrosion ranges. RSI = 2pHs - pH. RSI of 6-7 indicates stable water, below 6 indicates scaling, and above 7 indicates increasing corrosion.
Yes, LSI is commonly used for hot water systems, but temperature must be adjusted to the operating temperature. Hot water at 60°C will have a significantly higher LSI than the same water at 20°C. For boiler water at very high temperatures, other indices or direct solubility calculations may be more appropriate.
To increase LSI (reduce corrosion): add lime or caustic soda (raises pH and alkalinity), add calcium chloride (raises calcium hardness), or add soda ash (raises alkalinity). To decrease LSI (reduce scaling): add acid (lowers pH), reduce alkalinity, or use scale inhibitors. The specific adjustment depends on which parameter is most economically modified.
No, LSI only addresses calcium carbonate saturation equilibrium. It does not account for other corrosion mechanisms such as microbiologically influenced corrosion (MIC), galvanic corrosion, oxygen pitting, or corrosion by chloride, sulfate, or dissolved gases. A comprehensive corrosion assessment requires additional indices and water quality parameters.
The LSI specifically models calcium carbonate (CaCO₃) equilibrium. Magnesium carbonate has different solubility characteristics and is much more soluble. Since calcium is the primary scale-forming cation, only calcium hardness is used in the calculation. Magnesium hardness is irrelevant for CaCO₃ saturation assessment.
LSI is widely used for cooling tower water management but has limitations at high TDS and high cycles of concentration. For concentrated cooling water (TDS > 5000 mg/L), ionic strength corrections improve accuracy. Some practitioners use modified indices or computer-based speciation models for more precise scaling prediction in cooling systems.
Swimming pool water should have an LSI between -0.3 and +0.3, with a target near 0. Negative values corrode metal fixtures and etch plaster surfaces, while positive values deposit scale on surfaces, tile grout, and equipment. Pool chemistry (pH, alkalinity, calcium hardness, temperature) should be balanced to maintain this range.
LSI is an indirect indicator — water with positive LSI tends to form protective CaCO₃ coatings that reduce lead and copper dissolution. However, the US EPA Lead and Copper Rule uses more specific methods including corrosion coupon testing, metal release measurements, and specialized indices. Orthophosphate addition is often more effective than LSI adjustment for lead control.
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