Pool LSI Guide: Water Balance Every Tech Should Know

Pool LSI Guide: Water Balance Every Tech Should KnowKey Takeaway: The Langelier Saturation Index (LSI) reveals whether pool water is corrosive or scale-forming—something individual test readings don't show. Target LSI of -0.3 to +0.3. Below -0.3 corrodes plaster and equipment; above +0.5 causes scaling.Master LSI calculations to prevent scale, corrosion, and costly callbacks. Quick guide with charts and calculator tips for pool techs.Frequently Asked Questions

What is LSI in pool water?

LSI (Langelier Saturation Index) is a calculated value that indicates whether pool water is corrosive (negative LSI), scale-forming (positive LSI), or balanced (near zero). It combines pH, temperature, calcium hardness, and alkalinity into a single number. Target range is -0.3 to +0.3.

How do you calculate pool LSI?

LSI = pH + Temperature Factor + Calcium Factor + Alkalinity Factor - 12.1. Each factor is determined by lookup tables based on your test readings. Pool service software can calculate this automatically when you enter water test results.

What is a good LSI for a pool?

A good LSI is between -0.3 and +0.3. Zero is ideal. Slightly positive (0 to +0.3) is preferred for plaster pools and saltwater systems. Below -0.3 is corrosive and will damage surfaces; above +0.5 causes scale buildup.

How do you fix low LSI in a pool?

To raise a low (corrosive) LSI: first raise pH with soda ash, then increase calcium hardness with calcium chloride if needed, or raise alkalinity with sodium bicarbonate. Raising pH is usually the easiest and most effective fix.

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