CYA for Salt Chlorinator Pools II

[chem geek]

I understand Richard,

It just seems very odd to me. I'm an architect, and contrary to public perception that we're all about making things look nice, our first and foremost task is ensuring public safety. As is that of every other professional that works in constructing/managing the built environment. It just seems too odd that this particular part of the built environment is somehow exempt from that goal.

The manufacturers recommend keeping the FC at 3.0 or higher and that this is OK to do for CYA up to 70 and they say you shouldn't go above 100. So from a disinfection point of view at normal pH (7.5), even at 100 ppm CYA the HOCl is 0.012 which is a minimum for disinfection that roughly corresponds to an ORP near 650 mV which is the same level that commercial pools use as a minimum. So the issue is not one of health safety, at least for easy-to-moderate-to-kill bugs (i.e. up to E.coli), but of having sparkling clean pools without algae. Now some in the industry claim that this 3 ppm rate will also prevent algae, but Ben's table and real-world results dispute this (except, perhaps, for SWG pools where the jury is still out on the exact chlorine level needed to prevent algae in such pools).

The study I looked at done by one manufacturer, however, used statistical averages to prove their point saying that the average chlorine level in a group of pools (with varying amounts of CYA) without algae was 3 while in a group of pools with algae the average was below 3. That is, of course, not the right way to look at data. It should be the maximum chlorine level (actually HOCl level) in the pools with algae that is a starting point for analysis. The minimum chlorine level in the pools without algae is also useful and these will likely overlap due to "lucky" pools without enough chlorine but that haven't yet developed algae. Of course, algae growth depends on lots of other factors including nutrients in the pool, sun exposure, rate of introduction of algae into the pool, etc.

So I do not believe that the manufacturers are trying to skirt public safety. My analogy with the tobacco companies was again facetious and way too extreme. A better analogy might be drug companies who promote their solutions rather than non-patentable alternatives that sometimes work as well or better. Also, it could just be that the manufacturers just aren't being as careful with their studies, not intentionally, but just based on who did the study -- these aren't scientifically peer-reviewed articles, but industry conference presentations (the O'Brien paper, on the other hand, was done scientifically with university professors and may have had peer-review).

Richard

[aquarium]

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

[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

[aquarium]

The calcium hardness is about 430ppm.

Arm's length down into the deep end with the pumps running at 3:45PM (full sun in central Texas) the water temp is 84 degrees F.

Plaster pool, about 15,000 gallons, 6 foot deep end.

TW

EDIT: I don't bother with much beyond the OTO test kit, so I don't know CC and I've never shocked since doing BBB starting in June. Just lucky again I guess.

[waterbear]

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.


Richard

Langelier Saturation Index is, IMHO, a bogus measurement for pools. It was designed for closed systems and a pool is an open system. See these posts (2 of mine and one from Ben):
http://www.poolforum.com/pf2/showpos...91&postcount=7
http://www.poolforum.com/pf2/showpos...3&postcount=13
http://www.poolforum.com/pf2/showpos...1&postcount=17

[chem geek]

I'd like to bring this thread back to its original topic so I'm putting my response in a new Saturation Index thread where I hope to hear from you and Ben (and others).

Richard