Water chemistry
Chlorine demand
Chlorine demand is the amount of chlorine the water uses up before any is left to sanitize.
When water has a lot of algae, leaves, sweat, or other waste, chlorine reacts with all of it first. That amount is the chlorine demand. Until it is met, your test will show little or no free chlorine.
This is why a green pool can swallow a big dose and still test at zero. The fix is to keep adding chlorine in steps and brushing until the reading holds overnight. Check cyanuric acid and pH while you do, because both change how well chlorine works. High demand that keeps coming back points to a deeper issue.
Related terms
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