Pool LSI Guide: Water Balance Every Tech Should Know

Pool LSI Guide: Water Balance Every Tech Should KnowKey Takeaway: A pool can have textbook-perfect chlorine, pH, and alkalinity readings and still be aggressively corroding plaster or scaling salt cells. LSI catches that. A 85°F Florida pool with pH 7.4, alkalinity 90, and calcium 250 calculates to LSI ≈ -0.4, corrosive enough to etch plaster over months. Target: -0.3 to +0.3. The math takes 30 seconds; the damage from skipping it costs thousands.Master LSI calculations to prevent scale, corrosion, and costly callbacks. Quick guide with charts and calculator tips for pool techs.Frequently Asked Questions

Can a pool have perfect test readings but still damage the plaster?

Yes, and this is the single most expensive lesson in pool service. pH 7.4, alkalinity 90, calcium 250, chlorine 3 all read 'normal' on a strip or kit. But if the water is 85°F (typical Sun Belt) and TDS is elevated, the combined LSI is approximately -0.4, which is corrosive. The pool slowly etches plaster, dissolves calcium from grout, and pits metal components. The damage isn't dramatic; it's accumulated over 6–18 months, and most operators only notice when a customer's surface starts looking 'chalky' or stained.

Why does my LSI sometimes disagree with the pool store's reading?

Three usual culprits: (1) the pool store's software uses a different TDS constant (12.1 vs 12.0 vs a temperature-adjusted value): small math difference, sometimes shifts LSI by 0.1–0.2; (2) their water temperature reading is the store's ambient air, not your actual pool water temp: a 10°F gap shifts LSI by ~0.1; (3) calcium hardness varies by test method (titration vs strip vs digital meter) and is the single biggest input to LSI. When numbers disagree, trust the field reading taken in actual pool water with a calibrated kit.

What's the fastest way to fix corrosive water without overshooting?

Raise pH first with soda ash (washing soda). It's the highest-leverage knob and the easiest to titrate. Most low-LSI fixes only need pH moving from 7.2 to 7.6, which is a soda ash dose of roughly 1.5 lbs per 10,000 gallons. If pH is already at 7.6+ and LSI is still low, the issue is calcium hardness: raise it with calcium chloride flakes, but stop at 250 ppm for traditional pools (350–400 for saltwater) to avoid scaling later. Avoid touching alkalinity unless it's actually low; it overcorrects pH unpredictably.

Is LSI more important than chlorine for water safety?

They protect different things. Chlorine protects swimmers (sanitization). LSI protects the pool itself (plaster, grout, equipment, salt cells). A pool with bad LSI and good chlorine is safe to swim in but slowly self-destructing. A pool with good LSI and bad chlorine is intact but a health risk. Both need to be right, but LSI is the one that costs the homeowner $8,000–15,000 in resurfacing if you ignore it, which is why professionals who check LSI consistently retain customers far longer than ones who don't.

Back to Blog | Home | Features | Pricing