How to use it
- Run a test that reports both free chlorine (FC) and total chlorine (TC) — an FAS-DPD kit reads both cleanly.
- Enter FC, TC and your volume. We work out your combined chlorine (CC = TC − FC) — the chloramines — and the breakpoint dose.
- If CC is above 0.2 ppm, add the liquid chlorine all at once at dusk with the pump running, then re-test before anyone swims.
What combined chlorine (chloramines) actually is
Free chlorine is the chlorine still available to sanitize. Combined chlorine is chlorine that has already reacted with sweat, urine, oils and nitrogen to form chloramines. Chloramines are weak sanitizers, and they are what cause the harsh "chlorine smell," red eyes and irritated skin at a busy pool. Total chlorine is simply free + combined, so CC = TC − FC (we clamp it at zero if a test quirk makes FC read higher than TC).
The smell means you need MORE chlorine, not less
This is the paradox that trips everyone up: a strong "chlorine" odor almost never means too much chlorine. It means chloramines have built up and you are short on free chlorine to destroy them. The fix is to add chlorine to breakpoint — not to back off and let it get worse.
The 10× breakpoint rule
To break chloramines apart — oxidizing them to nitrogen gas that leaves the water — free chlorine has to clear a threshold. The pool-operator rule of thumb is to raise free chlorine to about 10× the combined chlorine, added in one dose. Get part-way there and you can actually create more chloramine, so breakpoint is all-or-nothing.
Where does 10× come from? The theoretical breakpoint ratio is about 7.6 parts chlorine to 1 part ammonia-nitrogen by weight; operators round up to 10× to cover the mix of mono-, di- and trichloramine plus organic demand. It is deliberately generous — the true requirement is often lower — so treat the number as a ceiling: add it, keep the pump running, and re-test rather than chasing it higher. (Some pros instead add 10× the CC on top of current free chlorine; our target is 10× CC as the total free chlorine, the slightly more conservative reading of the rule.)
When to breakpoint-shock
Shock when combined chlorine climbs above about 0.2 ppm — the maximum many health codes and the PHTA allow — or after heavy bather loads, a rainstorm, or when a chlorine smell shows up. At 0.2 ppm or below there is essentially nothing to burn off.
Non-chlorine shock (MPS) — an honest note
Potassium monopersulfate (MPS, or "non-chlorine shock") oxidizes organics and lets swimmers back in sooner, but on its own it does not reliably break down inorganic chloramines the way breakpoint chlorination does. It is a fine weekly oxidizer; for a real combined-chlorine problem, chlorine breakpoint is the dependable fix. Note that MPS can also make some DPD tests read a false "combined chlorine" until it clears.