EDIT: You will find a simplified and standardized version of this chart at Pool School, Chlorine / CYA Chart.
Chlorine/CYA Chart by Chemgeek
FC = Free Chlorine in parts-per-million (ppm)
CYA = Cyanuric Acid in parts-per-million (ppm)
CYA ........... Min FC3 ..... Target FC ...... Yel/Mstrd Min ...... Shock FC ..... Yel/MstrdShock
0 ................. 0.071 ........... 0.111 ................ 0.151 .................. 0.641 .............. 1.501
10 ............... 0.81 ............. 1.21 .................. 1.61 .................... 4.5 .................. 7.1
20 ............... 1.51 ............. 2.4 .................... 3.1 ..................... 8.3 ................ 12.7
30 ............... 2.2 ............... 3.5 .................... 4.6 ................... 12.2 ................ 18.2
40 ............... 2.9 ............... 4.6 .................... 6.1 ................... 16.0 ................ 23.8
50 ............... 3.7 ............... 5.7 .................... 7.5 ................... 19.8 ................ 29.42
60 ............... 4.4 ............... 6.8 .................... 9.0 ................... 23.7 ................ 34.92
70 ............... 5.1 ............... 8.0 .................. 10.5 ................... 27.52 ............... 40.52
80 ............... 5.8 ............... 9.1 .................. 12.0 ................... 31.42 ............... 46.12
90 ............... 6.6 ............. 10.2 .................. 13.5 ................... 35.22 ............... 51.72
100 ............. 7.3 ............. 11.4 .................. 14.9 ................... 39.12 ............... 57.32
120 ............. 8.7 ............. 13.6 .................. 17.9 ................... 46.72 ............... 68.42
1A minimum FC level is needed as a "reserve" for usage so in practice at least 2 ppm FC is required even at low CYA levels. The table above shows the amount needed for disinfecting chlorine for equivalent killing power (rates), but does not take into account the amount needed in reserve to prevent getting used up as this varies by pool.
2The shock levels shown have equivalent disinfecting chlorine amounts (in a column) but at high CYA levels it may be impractical to use such high FC levels. A partial drain/refill to lower the CYA level is usually what is needed or one can shock at a lower level but will take longer to kill the algae.
3Most saltwater chlorine generator (SWG) pools appear to prevent algae at a minimum FC level of 4.5% of the CYA level as compared with the roughly 7.5% of the CYA level shown in the "Min FC" column for manually dosed pools.
NOTE: A reasonable approximation to the above table is the following:
.... "Min FC" is 7.5% of the CYA level
.... "Target FC" is 11.5% of the CYA level
.... "Yel/Mstrd Min" is 15% of the CYA level
.... "Shock FC" is 40% of the CYA level
.... "Yel/MstrdShock" is 60% of the CYA level.
Since chlorine is more effective at lower pH, it is normally recommended to lower the pH before shocking at a high FC level, especially for yellow/mustard algae where lowering the pH to 7.2 before shocking at the above levels is best (and remember that the pH test will not be valid during shocking due to high FC levels).
ACKNOWLEDGMENT: The original chlorine/CYA chart was developed by Ben Powell and is shown here based mostly on experience plus some conversations with chemists. With the help of users at The Pool Forum and this forum, we expanded the chart to include yellow/mustard algae and I made the chart consistent with chemical theory.
Chlorine/CYA Chart by Chemgeek
FC = Free Chlorine in parts-per-million (ppm)
CYA = Cyanuric Acid in parts-per-million (ppm)
CYA ........... Min FC3 ..... Target FC ...... Yel/Mstrd Min ...... Shock FC ..... Yel/MstrdShock
0 ................. 0.071 ........... 0.111 ................ 0.151 .................. 0.641 .............. 1.501
10 ............... 0.81 ............. 1.21 .................. 1.61 .................... 4.5 .................. 7.1
20 ............... 1.51 ............. 2.4 .................... 3.1 ..................... 8.3 ................ 12.7
30 ............... 2.2 ............... 3.5 .................... 4.6 ................... 12.2 ................ 18.2
40 ............... 2.9 ............... 4.6 .................... 6.1 ................... 16.0 ................ 23.8
50 ............... 3.7 ............... 5.7 .................... 7.5 ................... 19.8 ................ 29.42
60 ............... 4.4 ............... 6.8 .................... 9.0 ................... 23.7 ................ 34.92
70 ............... 5.1 ............... 8.0 .................. 10.5 ................... 27.52 ............... 40.52
80 ............... 5.8 ............... 9.1 .................. 12.0 ................... 31.42 ............... 46.12
90 ............... 6.6 ............. 10.2 .................. 13.5 ................... 35.22 ............... 51.72
100 ............. 7.3 ............. 11.4 .................. 14.9 ................... 39.12 ............... 57.32
120 ............. 8.7 ............. 13.6 .................. 17.9 ................... 46.72 ............... 68.42
1A minimum FC level is needed as a "reserve" for usage so in practice at least 2 ppm FC is required even at low CYA levels. The table above shows the amount needed for disinfecting chlorine for equivalent killing power (rates), but does not take into account the amount needed in reserve to prevent getting used up as this varies by pool.
2The shock levels shown have equivalent disinfecting chlorine amounts (in a column) but at high CYA levels it may be impractical to use such high FC levels. A partial drain/refill to lower the CYA level is usually what is needed or one can shock at a lower level but will take longer to kill the algae.
3Most saltwater chlorine generator (SWG) pools appear to prevent algae at a minimum FC level of 4.5% of the CYA level as compared with the roughly 7.5% of the CYA level shown in the "Min FC" column for manually dosed pools.
NOTE: A reasonable approximation to the above table is the following:
.... "Min FC" is 7.5% of the CYA level
.... "Target FC" is 11.5% of the CYA level
.... "Yel/Mstrd Min" is 15% of the CYA level
.... "Shock FC" is 40% of the CYA level
.... "Yel/MstrdShock" is 60% of the CYA level.
Since chlorine is more effective at lower pH, it is normally recommended to lower the pH before shocking at a high FC level, especially for yellow/mustard algae where lowering the pH to 7.2 before shocking at the above levels is best (and remember that the pH test will not be valid during shocking due to high FC levels).
ACKNOWLEDGMENT: The original chlorine/CYA chart was developed by Ben Powell and is shown here based mostly on experience plus some conversations with chemists. With the help of users at The Pool Forum and this forum, we expanded the chart to include yellow/mustard algae and I made the chart consistent with chemical theory.
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