Range Chemistry & the LSI Reality Check

Send us Fan MailThis Friday episode digs into one of the most argued topics in pool care: range chemistry and the Langelier Saturation Index (LSI).Rudy takes us back to 1936 and the work of Wilfred F. Langelier, who developed a model to prevent municipal water pipes from dissolving or scaling shut. LSI was never designed for swimmers. It was built to answer one simple question:Will this water dissolve calcium carbonate… or deposit it?That’s it.Pools adopted LSI later because plaster behaves like municipal concrete. Your pool is essentially a miniature water system — just with sunscreen and cannonballs.What LSI Does (and Doesn’t Do)LSI predicts calcium carbonate equilibrium. It protects:PlasterGroutHeatersSalt cellsTile linesWhat it does not tell you:If chlorine is killing pathogens fast enoughIf chloramines are risingIf nitrification is occurringIf biofilm is formingIf oxidation demand is being metLSI protects the vessel. It does not guarantee sanitation.Where 7.2–7.8 Came FromNo single person invented the modern pH range. It evolved from the overlap of:Human physiology (comfort and irritation)Chlorine chemistry (HOCl vs OCl⁻ balance)Cement durability researchRegulatory standardsEven phenol red test kits influenced it — operators standardized what they could clearly see and control.The Cyanuric Acid Blind SpotIf you don’t subtract roughly one-third of CYA from total alkalinity before calculating LSI, your saturation balance is wrong.And LSI does not account for chlorine kinetics at all.You can have:A perfect 0.00 LSIHigh CYASlower disinfectionRising combined chlorineBiofilm quietly developingThe plaster may be safe. The water may not be optimal.Salt Cells, Heaters & MicroenvironmentsLSI models bulk water.Inside salt cells and heaters, localized pH spikes can create scaling even when your overall LSI reads balanced. Context matters. Temperature matters. Ionic strength matters.Water chemistry is not binary — it’s gradient-based.The Real TakeawayRange chemistry isn’t stupid. It’s probabilistic. It works under average conditions in average pools.The mistake is believing ranges are universal laws.LSI is necessary — but not sufficient. Balance is not a number. It’s interaction between thermodynamics, kinetics, microbiology, and material science.Stop worshiping the calculator. Start managing the system. Support the showThank you so much for listening! You can find us on social media:FacebookInstagramTik TokEmail us: [email protected]