CYA for Salt Chlorinator Pools II

[chem geek]

Thanks Richard,

I guess I have a 'lucky pool' then.

Using just bleach and muriatic acid I run it at 1.5-2ppm chlorine as measured with a dumb little OTO test kit, with 30ppm CYA at about 7.2-7.4pH, adjusted via aeration to about 80ppm alk.

Clear blue water.

I mean -really- clear blue water.

TW

An FC of 1.5-2, CYA of 30, pH of 7.3 gives an HOCl of 0.022-0.030 which is probably equivalent to the Min. FC in Ben's chart (not if you look it up directly with his "ranges", but if you convert Ben's chart into HOCl levels, then his Min. FC column is approximately 0.02 or 0.025 though he has it rise up at the lowest CYA levels, probably just to ensure a minimum FC as a reserve). So your pool isn't just lucky, it's also within the relam of the theory we're trying to develop.

On the other hand, Ben has said he has seen pools with algae that were maintained with levels that, after I run through the calculations, are closer to 0.05 ppm HOCl which seems to be the worst-case scenario. So the real answer for algae prevention is likely to be somewhere in that 0.02-0.05 range. Of course, the "real world" creeps in with factors like uneven pool circulation that can cause localized conditions to be worse than the minimum. This is why Ben's chart tries to be conservative -- to account for such conditions.

By the way, what is your CH level and pool water temperature? The saturation index I calculate at 80ºF and 300 CH (with 7.3 pH) would be -0.28 (using my newer index, though the old index is -0.30 so about the same in this case). This isn't horrible and may be a very mild corrosive condition (if you have a plaster pool), but if you want to be in balance, then CH would need to be 700 (with my new index; 600 with the old). Of course once you commit to this high a CH, you have to keep your pH and alkalinity at your current low levels unless you drain your pool water to dilute the calcium.

I try to keep my pool's alkalinity closer to 100-115 with a CH of 260 and a pH of 7.4-7.5 and a water temperature of 88ºF and CYA of 15-20 (hard to tell for sure given the tube stops at 30). My primary motivation for this balance is that the higher alkalinity makes the need for pH adjustment much less frequent, though in practice I hardly ever have to adjust it anyway. I keep an FC level of 2-3 which is safe and conservative since I can drop to 1 FC and still have about 0.020 ppm HOCl similar to your pool. I only lose about 0.5 ppm FC or less per day due to an electric opaque pool cover. I, too, have never had algae and the water is crystal clear blue.

By the way, do you ever get CC > 0 and have to shock your pool?

Richard