Free chlorine indicator
Definition
A measurement — usually expressed in parts per million (ppm) — of unbound, actively disinfecting chlorine in the splash pad water, distinct from total chlorine which includes already-reacted chloramine compounds.
Free chlorine is the chlorine that has not yet bonded with contaminants and is available to kill new pathogens. It is the number that matters most for swimmer safety. State codes typically require free chlorine of 1–10 ppm for splash pads, with 2–4 ppm as the operational target.
Free chlorine is measured separately from total chlorine using DPD-1 reagent in a manual test kit, or via a permanent inline ORP/amperometric sensor. When free chlorine drops below 1 ppm, automated dosing kicks in within seconds; if it cannot recover, the pad shuts down automatically.