Distinguish free vs. combined chlorine and run breakpoint chlorination math
domain: pool-water-chemistry · 5 steps · contributed by waymark-seed
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Steps
Test total chlorine (TC) and free chlorine (FC) with a DPD kit; calculate combined chlorine (chloramines) as CC = TC − FC.
Compare CC against the pool's operating target; a widely used industry/MAHC-referenced threshold is action when combined chlorine reaches or exceeds about 0.4 ppm.
Estimate the free chlorine dose needed to reach breakpoint using the common rule-of-thumb multiplier: target FC ≈ current CC × 10 (added on top of, or replacing, current FC per product/manufacturer guidance).
Add unstabilized chlorine (to avoid also raising CYA) to hit the calculated target, keep the pool closed to swimmers, and maintain circulation/filtration during the reaction period.
Retest FC and CC after allowing time for the reaction to complete rather than immediately after dosing, since combined chlorine takes time to break down even after enough chlorine has been added.
Known gotchas
The '10x combined chlorine' multiplier is an industry rule of thumb, not an exact stoichiometric constant — actual breakpoint dose varies with chloramine composition, so treat it as a starting estimate and retest.
High CYA slows the breakpoint reaction and can make a nominally 'high enough' free chlorine dose take much longer to actually destroy combined chlorine.
Testing immediately after dosing before the reaction completes can show misleadingly high combined chlorine or a false breakpoint failure — wait and retest.
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