Water Chemistry
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.
Chlorine lock happens when cyanuric acid concentrations exceed the functional sanitation threshold — typically above 70ppm. At that level, free chlorine bonds so tightly with stabilizer molecules that it no longer actively kills bacteria or oxidizes organic load. Swimmers report `it smells like chlorine but the water doesn't feel right.` Diagnostic tip: high combined chlorine + high CYA + resistance to shock treatment = likely chlorine lock. The remediation is partial drain + refill, not more chlorine.
Chlorine lock is not the same as low free chlorine. The test reads fine — the chemistry doesn't.
Shocking doesn't break chlorine lock. It adds more chlorine that also gets bound up.
Chlorine lock is almost always operationally preventable — just monitor CYA in weekly logs.
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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.
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.
Langelier Saturation Index (LSI)
A composite calculation (pH + temperature + calcium hardness + alkalinity + CYA adjustment) that measures whether pool water is corrosive, balanced, or scale-forming.
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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