Target FC levels in Chlorine/CYA chart too low?

kevinskii

Gold Supporter
Aug 6, 2019
94
Los Angeles, CA
According to the simplified Chlorine/CYA chart, the target FC for a SWG pool with 70ppm CYA is 5ppm.

But according to the Pool Water Chemistry thread, the minimum recommended level of HOCL (disinfecting chlorine) required to kill both bacteria and algae is 0.05ppm. The rough formula to calculate HOCl (assuming the CYA:FC ratio is high enough) is:
(HOCl as ppm Cl2) = (FC as ppm Cl2) / ( 2.7*(ppm CYA) - 4.9*(FC as ppm Cl2) + 5 )

Plugging in 70ppm CYA and 5ppm FC, this works out to:
HOCl = 5 / (2.7*70 - 4.9*5 + 5)
= 5 / (189 - 24.5 + 5)
= 0.03

Thus, it seems that a target FC level of 5ppm at 70ppm CYA is too low? Richard Falk's (chem_geek's) detailed Chlorine/CYA chart recommends a target of 8ppm FC at 70ppm CYA, with 5ppm being the minimum.

Granted, I haven't read through the entire latter thread, and I had to struggle to make it through the "Pool Water Chemistry" thread. Please let me know if I'm missing something. Otherwise, it seems that perhaps the simplified Chlorine/CYA chart in the first link above should be updated to reflect the target values in Richard's more detailed chart.
 
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JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
17,213
Tucson, AZ
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.
SWG pools have lower min requirements and the formula you are using is only a rough estimate that doesn’t take into account pH. Richard’s PoolEquations Excel spreadsheet calculates much more accurate values.
 
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kevinskii

Gold Supporter
Aug 6, 2019
94
Los Angeles, CA
Ah, thank you. Richard's spreadsheet actually calculates the same value that I did when I plug everything in, but your quote explains it. 4.5% of 70ppm CYA is approx 3ppm, which is the recommended minimum in the simplified chart.
 
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JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
17,213
Tucson, AZ
Also, there are a lot of assumptions that go into the 50ppb figure for hypochlorous acid killing bacteria and algae. There are many different species of algae and bacteria all with different sensitivities to chlorine. 10ppb is roughly equivalent to 650mV ORP which is a minimum standard set for water purification (bacterial sanitation) and his assumption is that studies of marine plankton are shown to be inactivated at a level of 0.1ppm FC which is 50ppb HOCl at a pH of 7.5. So it’s plausible that common blue-green algae found in swimming pools would also be susceptible to 50ppb HOCl thus the bracketed sanitation range is somewhere between 0.01 and 0.05ppm [HOCl] .
 
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