CYA / Pool Pilot Question

[chem geek]

Mark,

Thanks for the link to the discussion. Ben's chart is conservative, partly because it is for manually-dosed pools where the chlorine level fluctuates so "could" get low at some point. The accurate relationship between disinfecting chlorine levels and the amount of FC and CYA in the pool is calculated in the following chart FC.htm and graph HOCl.gif

Ben's table roughly correlates with this data, at least for his minimum and maximum FC ranges (the shock area is another matter, but we are still figuring that out). I can say with reasonable confidence that the 0.011 ppm HOCl level is a minimum level needed for disinfection in a commercial pool environment, but it may very well be that in a home environment it simply isn't that big a deal if a bug is killed in minutes or even hours compared to seconds if it's just your own family in the pool (and one should note that even the 0.011 ppm HOCl level does not kill hard-to-kill bugs). Nevertheless, if you look at the 0.010 column in the chart, you can see that you can most certainly run at much lower FC levels with higher CYA levels. So for minimum disinfection this makes sense. Even 80 ppm of CYA at 2 ppm of chlorine gives an HOCl level of close to 0.010. ([EDIT]Oh, I see you said that in your post -- sorry for not getting that the first time [END-EDIT])

What is not so clear is what is required for algae prevention. Ben has seen pools develop algae even when the HOCl level (that I calculated based on the FC and CYA he mentioned) was just under 0.05 and the middle range of his table is roughly in this area. So it would be interesting to figure out if and why SWG pools running at 70-80 CYA and 2-3 ppm chlorine don't get algae. [EDIT]Your explanation of slow algae growth with some superchlorination through the salt cell sounds reasonable except for algae that grows on pool surfaces and doesn't pass through the cell. Maybe for some reason the algae in salt water needs less chlorine to prevent it from growing. Of course, I always thought algae liked salt (e.g. ocean). At any rate, the determination of algae prevention and algae kill (shock) levels both for non-SWG and SWG pools is something I want to try and figure out using the resources of this poolforum. I plan to start a thread for that purpose sometime soon. Thanks for your input.[END-EDIT]

As far as what level of CYA is appropriate, it is true that once you figure you want to run your pool at 2-3 ppm, then higher CYA levels will reduce the chlorine loss. Remember that my argument was the other way around -- setting a disinfection level and then determining the level of CYA needed for that level. The loss rate of chlorine at 2 ppm FC in sunlight with 80 ppm CYA is about 0.187/hour while with 40 ppm CYA it's about 0.210/hour so this isn't a very big difference, but the disinfecting chlorine (HOCl) level at 80 ppm CYA is only 0.010 ppm compared with 40 ppm CYA that produces 0.020 ppm or twice as much disinfecting capability. That was the main point I was trying to make. A little CYA goes a long way so I believe having your salt cell work 12% more to get twice the disinfection and algae prevention capability is worth it. [EDIT]Some of the posts in the link you gave indicate that the lower CYA required cells to work much more than the 12% I stated, but they also said they had algae problems which doesn't make sense unless they couldn't keep up. waterbear reported some of this so I'll try and get more info and see if we can figure this out. The most logical explanation I saw, so far, was the one that CYA helps assist in the chlorine generator cell itself to prevent some chlorine from breaking down under the high concentrations.[END-EDIT]

Richard