Water Chemistry
A composite calculation (pH + temperature + calcium hardness + alkalinity + CYA adjustment) that measures whether pool water is corrosive, balanced, or scale-forming.
The LSI combines five chemistry variables to produce a single balance score. LSI at 0 means the water is neutral — not corrosive, not scaling. Negative values (below -0.3) mean corrosive water that pulls calcium out of plaster and etches grout. Positive values (above +0.3) mean scale-forming water that deposits calcium on tile, heater elements, and filter media. Commercial operators use LSI to catch long-term chemistry drift that daily chlorine/pH testing can miss. A pool that reads `normal` on every daily log but trends -0.5 LSI for six months is quietly damaging its plaster.
A `balanced` pool in daily terms can still be LSI-unbalanced over time.
LSI is not a regulatory requirement — it's an operational quality signal.
High calcium hardness + high alkalinity can push LSI into scale-forming territory even when pH is normal.
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Related terms · Water Chemistry
Cyanuric Acid (CYA / Stabilizer)
A pool additive that protects chlorine from UV degradation in outdoor pools. Essential in moderation, counterproductive above 70ppm.
Chlorine Lock
A state in which chlorine test readings appear normal but the chlorine is ineffective at actually sanitizing the pool. Usually caused by excessive cyanuric acid.
Combined Chlorine (Chloramines)
Chlorine that has already reacted with organic contaminants and lost most of its sanitizing value. The primary cause of the strong `chlorine smell` in pools.
pH (Pool Water)
A measure of how acidic or basic pool water is on a 0–14 scale. Illinois IDPH requires semi-public pools to hold pH between 7.2 and 7.8, with 7.4–7.6 as the practical operating target.
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