The Langelier Saturation Index (LSI)

Introduction

The Langelier Saturation Index (LSI) is one of the most important tools for anyone maintaining pools, spas, or any system with water in contact with calcium-based surfaces. It predicts whether water will dissolve calcium carbonate (making it corrosive and aggressive) or deposit it (making it scale-forming). Balanced water protects surfaces like plaster, tile, fiberglass, and vinyl fittings, keeps heaters and pumps efficient, and ensures swimmers enjoy comfortable, clear water. For professionals, understanding LSI is the difference between preventative maintenance and costly surface damage.

History

Dr. Wilfred Langelier introduced the LSI in the 1930s while studying municipal water distribution systems. His goal was to prevent pipes from corroding or clogging with scale. In the 1970s, John A. Wojtowicz expanded the concept for swimming pools, introducing the Calcite Saturation Index (CSI). The CSI adjusted LSI to reflect modern chemical constants, new understanding of alkalinity, and the effects of cyanuric acid on carbonate chemistry. While both terms are used in the pool industry, the principle remains the same: water is always seeking equilibrium, and it uses calcium carbonate as its balancing agent.

Six Key Factors of Balance

pH: The most powerful driver. A small swing in pH changes the LSI significantly.
Calcium Hardness (CH): Needed to prevent etching. Too little calcium causes surfaces to dissolve; too much promotes scale.
Total Alkalinity (TA): Buffers pH. Must be corrected for cyanuric acid (CYA) to determine true carbonate alkalinity.
Temperature: Warm water encourages scale. Cold water promotes aggressive, etching conditions.
Total Dissolved Solids (TDS): Influences solubility. Higher TDS reduces the water’s ability to hold more calcium.
Cyanuric Acid (CYA): Stabilizes chlorine but contributes to alkalinity chemistry. Must be accounted for.

Reference Tables

Temperature Factor (°F)

60	0.4	70	0.5	80	0.6	90	0.7

Calcium Hardness Factor (ppm as CaCO₃)

150	1.9	200	2.0	250	2.1	300	2.2

Alkalinity Factor (ppm carbonate alkalinity)

60	1.7	80	1.9	100	2.0	120	2.1

CYA Correction: At pH 7.6, subtract one-third of CYA from total alkalinity. At 7.4, subtract 0.25×. At 7.8, subtract 0.42×.

Scale and Etching on Pool Surfaces

The most obvious signs of LSI imbalance are on pool surfaces:

Plaster: Aggressive water dissolves calcium, creating pits, gray discoloration, and a rough feel. Scale leaves plaster looking blotchy and uneven. Acid washing may help but slowly erodes plaster lifespan.
Pebble and exposed aggregate finishes: Pebble surfaces hide some wear, but scale deposits dull their shine. Aggressive water weakens cement binders that hold pebbles in place.
Vinyl liners: Liners don’t contain calcium but are still affected. Negative LSI destabilizes seams and fittings. Scale clings to ladders, returns, and skimmers.
Fiberglass: Gelcoat surfaces resist water longer but are not immune. Scale forms a gritty texture. Aggressive water fades colors and reduces gloss.
Tile and grout: Evaporation at the waterline concentrates calcium, making tile a prime target for scale. Low LSI dissolves grout, loosening tiles. Acid cleaning restores appearance but damages surfaces over time.

Severe scale can clog heaters and filters, while aggressive water silently eats away at surfaces until expensive repairs are necessary. Consistent monitoring is the only way to prevent both outcomes.

Seasonal and Regional Impacts

Climate and seasons shape how LSI must be managed. Cold winters mean water is more aggressive, requiring higher calcium hardness and slightly higher pH targets. Hot summers concentrate TDS through evaporation, encouraging scale. Pools in soft-water regions (low calcium in source water) are prone to etching unless calcium chloride is routinely added. Pools in hard-water regions constantly battle scale and must rely on lower alkalinity and careful pH management. Saltwater chlorine generators create especially high-risk zones for scale where electrolysis occurs on the cell plates.

Advanced Chemistry and Interactions

LSI focuses on calcium carbonate, but real-world water includes many more interactions. Sulfates weaken cement-based plasters. Chlorides increase corrosion risk in heaters. Magnesium can partially replace calcium but increases scaling risk when excessive. Organic debris introduces phosphates that feed algae, which in turn alter water chemistry. Professionals must account for these additional factors when interpreting LSI in practice.

Troubleshooting by Symptom

Cloudy water when heating: Calcium carbonate precipitation due to sudden rise in temperature and pH.
Rough or sharp plaster: Long-term etching. Often permanent without resurfacing.
White deposits on tile line: Evaporation-driven scale. Best prevented, difficult to remove.
Gray mottling on plaster: Chronic low LSI dissolving binders unevenly.
Chalky feel on fiberglass: Localized scale deposits from high CSI zones.
Vinyl wrinkles or puckering: Caused indirectly by unstable water balance stressing seams.

Myths and Misconceptions

Myth: LSI measures metal corrosion.
Fact: LSI measures calcium carbonate stability, not metal corrosion.
Myth: Pebble finishes never etch.
Fact: Cement matrix still dissolves under aggressive water.
Myth: Vinyl liners don’t require calcium.
Fact: Heaters, ladders, and fittings still need balanced water.
Myth: Once scale forms, it can be scrubbed away harmlessly.
Fact: Acid cleaning erodes finishes and weakens surfaces over time.

Professional Workflows

Test all six factors weekly: pH, CH, TA, CYA, TDS, temperature.
Correct TA for CYA to find carbonate alkalinity.
Calculate LSI with a table or modern calculator.
Adjust gradually: calcium chloride for CH, sodium bicarbonate for TA, muriatic acid for pH.
Retest and document. Long-term logging helps predict seasonal patterns.
Target -0.3 to +0.3 LSI consistently year-round, shifting slightly for winter vs. summer.

Calculator Access

Use the TrackLiquid LSI Calculator for precise CSI/LSI results. It applies modern constants, cyanuric corrections, and removes guesswork, making it easier for pros to document water balance and explain results to clients.