Free and combined chlorine

  • Free chlorine: what actually disinfects. Target 1–3 ppm (3–5 in a spa).
  • Combined chlorine (chloramines): "spent" chlorine bound to dirt. It's what smells like "chlorine" and irritates. It should be near zero.

If it smells strongly of chlorine and stings your eyes, it's not that chlorine is lacking: there is combined chlorine and you need to shock.

The stabilizer (cyanuric acid)

The sun destroys chlorine within hours. Cyanuric acid (30–50 ppm) acts as sunscreen for the chlorine. But in excess (>80 ppm) it "locks" it and it stops working; then the only fix is to replace part of the water.

Rule of thumb: free chlorine should be at least 7.5% of the cyanuric acid. With 60 ppm of cyanuric acid you need ~4–5 ppm of chlorine, not 1.

Shock chlorination

It burns off chloramines and algae. Raise free chlorine to 10 times the combined chlorine (or ~40% of the cyanuric acid). Do it at dusk, with the pump running, and don't swim until it drops below 3 ppm.