Hot climates = option of high CYA?

[chem geek]

Mark (mas985) did an experiment described in this post that showed that a higher CYA level protects chlorine loss from sunlight in a non-linear way that more than makes up for the greater loss from proportionately raising the FC level to keep the FC/CYA level constant. We still don't know exactly how this works though my best guess is the CYA shielding effect of lower depths since that would be a non-linear effect if there was not perfect mixing from surface to depth. This effect has been seen in many pools, but not every pool. The Taylor book is wrong on this (it's a good book, but it's wrong on other issues as well and has no discussion of the chlorine/CYA relationship).

So having a higher CYA isn't something special for SWCG pools and can be done with any pool that has a lot of loss of chlorine from sunlight. However, there are reasons why it makes more sense to have the higher CYA with SWG pools since the SWCG maintains a more constant chlorine level where the risk of having the chlorine too low is lower since you don't "forget" to add chlorine. If you let the FC get too low at a higher CYA level, it takes a lot of chlorine to shock the pool so it's more difficult to control. Also, most SWCG pools rise in pH and one of the ways to lower that is to lower the SWCG on-time so lowering the chlorine loss due to sunlight is a way of doing that. Finally, it seems that SWCG pools can operate without algae by maintaining an FC that is roughly 5% of the CYA level which is lower than the minimum FC in manually dosed pools (i.e. 3 ppm FC with 60 ppm CYA or 4 ppm FC with 80 ppm CYA compared to 5 ppm FC).

Richard