The chart
| CYA | Minimum | Target | Top of range | Shock (SLAM) | Salt water minimum / target |
|---|---|---|---|---|---|
| 0 | 1 | 4 | 6 | 10 | 1 / 2 |
| 20 | 1.5 | 4.5 | 6.5 | 10 | 1 / 2 |
| 30 | 2.3 | 5.3 | 7.3 | 12 | 1.5 / 2.5 |
| 40 | 3 | 6 | 8 | 16 | 2 / 3 |
| 50 | 3.8 | 6.8 | 8.8 | 20 | 2.5 / 3.8 |
| 60 | 4.5 | 7.5 | 9.5 | 24 | 3 / 4.5 |
| 70 | 5.3 | 8.3 | 10 | 28 | 3.5 / 5.3 |
| 80 | 6 | 9 | 10 | 32 | 4 / 6 |
| 90 | 6.8 | 9.8 | 10 | 36 | 4.5 / 6.8 |
Why CYA changes your chlorine target
Cyanuric acid shields chlorine from sunlight, which is why outdoor pools need it. But it also binds most of the free chlorine, leaving only a small active fraction. The active fraction depends on the ratio of free chlorine to CYA, so as CYA rises, free chlorine has to rise with it to keep the same sanitizing power. That is the idea behind the FC/CYA approach popularised by Trouble Free Pool.
Where to keep CYA
- Manual chlorine (liquid chlorine or bleach): 30 to 50 ppm.
- Salt water chlorine generator: 60 to 80 ppm.
- Indoor pools: little or none, since there is no sun.
- Above 80 to 90 ppm, chlorine needs become impractical. Only replacing water lowers CYA.