Quote Originally Posted by PoolDoc View Post
But for practical purposes, if CYA only provides significant shielding at 80ppm or higher, for forum response purposes, that's probably a complication we don't need to include in posts to general users
The higher CYA level seems to be best for SWCG pools since they tend to have a big problem with rising pH and by having the higher CYA levels they are able to lower chlorine demand and turn down the SWCG on-time and reduce the rate of pH rise. This, along with a lower TA, seems to control the SWCG pH rise problem reasonably. Adding 50 ppm Borates can help as well.

At 80 ppm CYA, SWCG pools seem to be able to do well even with 4 ppm FC rather than the 5-10 in the Best Guess CYA chart that seems better suited to manually dosed pools. However, I agree with you that for manually dosed pools the risk of missing a dose, getting low chlorine, and then having to shock to deal with algae becomes harder at the higher CYA levels. There are some people with manually dosed pools in very hot sunny areas that are running at higher CYA levels, but most don't. Then there's Pool Chlor who use 100 ppm CYA and add chlorine to 14 ppm FC each week where it drops to around 4 ppm by the next visit. So even in hot sunny areas like Arizona, the daily chlorine loss rate at that high CYA level is around 16% per day.

It's not that magically there is protection at 80 ppm, but the increased level of protection seems to be non-linear and somewhere between 60-80 ppm it climbs fairly rapidly. This is the sort of thing that can be determined from experiment. I got your E-mail and replied.