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

[Poolsean]

Mark,
What is your cya level currently? It would be interesting to see what would happen to your chlorine demand if you lowered your cya back to 30ppm. Or do you normally operate at 60 - 80 ppm and then tested the effects by dropping down to 30 ppm?
You stated in your post today "Many SWG users operate 60-80 ppm of CYA and 2-3 ppm of chlorine, including myself, and do not have a cloudy pool or algae that many others who post on this forum do." but in the post you sent the link to (your post dated 6-22-06) you stated "My current preference is #4 since I have a plaster pool and not quite at Ben's CL ppm. I am operating at a CYA of 30, 21000 gallon pool, setting 90% and run the pump 6 hours a day. I am able to maintain 2 ppm."

As a comparison, my 20,000 gallon outdoor pool @ 70 - 80 ppm cya was operating last year with my Pool Pilot at about 30% output, 6-hrs per day with my SC-48 cell (max output = 1.56 lbs/day) and was maintaining 2-3 ppm (as tested with the Taylor K-2006 test kit). I kept pH at about 7.5 - 7.8 by manually dosing acid. I converted to the Total Control system back in Aug of last year and get about a 22% utility reading (which equates to about a 22% output setting), all other parameters the same as before the Total Control but with pH @ 7.3 electronically controlled.

FYI, the 1.45 lbs/day you're using to calculate your chlorine and ppm are swayed (no offense). This is a number that iwas submitted to NSF for testing under "IDEAL" conditions, which are not usually real world conditions.
Such as indoor (no UV), cooler water temps, higher salt levels, no bather loads(or chlorine demands), and most importantly, lower flow rates.
How does NSF validate the output claim? They start with a known free chlorine base level (no chloramines), run the systems, take periodic water samples, and tests for free chlorine. So as the salt system generates at full output, NSF records the free chlorine, calculates the increase per hour for the 24 hr period, then determine the max output. They repeat this several times to confirm consistent output.
The question is this, what does a manufacturer actually submit as the operating parameters to get the max lbs/day output?

I cannot dispute your test results for your pool. However, I think Waterbear and I both see many more backyard pools (water samples in his case) to state with certainty that the higher Cya levels do allow you to decrease the output of "a" salt system and still maintain a safe and clean pool.

Man, I can see this heading to the China Shop...

But let me state this. The Salt Chlorine Generator industry have NO vested interest in cyanuric acid sales, so we're not claiming a high-er cya is needed, just for profits. It's quite the opposite. We know from experience, that Cya helps to protect the chlorine from the sun's UV rays, not just from the heat of the sun. While you can get away with maintaining lower cya in a salt pool, your cell will suffer as a result.

Interesting article on UV radiation.
http://yourskinandsun.com/article1072.html
Northern climates may justify maintaining a lower cya due to it being further away from the equator, thus less UV exposure. But climb into the mountains and you should require a higher cya level.

Ben, have you ever read this article? I'm not a chemist by any means so any of you chemist types that disagree with this paper, please feel free to educate me.
http://www.magma.ca/~dougdela/chemed03/pool1.pdf
I do see that he states anything above 50 ppm cya is of little effect...darn it. Well, it's not just debated here. It is debated within the industry itself.
Just a worthy note, most Department of Health limits commercial pools to 100 ppm. More recently lowering that to 80 ppm. In fact, NY State does not permit CYA, except for special circumstances, such as outdoor pools, where pool operators can request the special exception to use it. In speaking to several of these pool operators in northern NY, they see the tremendous cost savings when permitted to use cya. Guess what level they maintain? OK...50 to 60 ppm. Not quite the 60 - 80 ppm we recommend, but not quite 30 ppm either.

...and so it continues...

[chem geek]

Sean,

I moved my conversation over to the
CYA for Salt Chlorinator Pools thread, but at the end of this linked post I mention how I did some rough calculations that, somewhat to my surprise, justify the theory of higher CYA levels making the salt chlorine generator cells more efficient.

To more fully validate the theory, I need some basic parameters of information such as cell chamber size, plate size, typical water flow rates through the cell, etc. since my estimates were pretty much guesses. It sounds like you're a sales guy, but if you can get your technical folks to help out (unless you know this information), I'd really appreciate it. Rather than bore the heck out of the people on this forum, you can send any information directly to me via E-mail since I allow that (just click on my "chem geek" name to get to my profile where you can send an E-mail). I can then post my net results that I expect will confirm what my very rough calculations have verified.

It's funny how the whole thing pretty much hinges on the rate constants (which I have) for the taking up of chlorine by CYA and how this gives half-lives in the sub-second ranges that roughly correspond to the time water spends inside the cell. I truly didn't expect that -- funny how even someone like myself who believes in the power of science still gets amazed when something that *should* come out a certain way actualy *does* come out that way.

[EDIT]By the way, making your cells larger (longer) and breaking up into multiple plates with space between them (or using multiple cells running at lower "power" generation rates) would let people use lower levels of CYA. The key is to give the generated chlorine time to combine with CYA and not build up too much in concentration. So a multiple staggered plate design would also work (but alas, is more complicated). Just giving you food for thought.[END-EDIT]

Richard

[Poolsean]

hmmm, not sure if to take offense at your comment...
"It sounds like you're a sales guy, but if you can get your technical folks to help out (unless you know this information), I'd really appreciate it. "
Ouch...all my efforts of being unbiased and informative, and I get called a sales guy. Back to the ole' drawing board.

I also posted on the other link. I'll move to that one too.

[chem geek]

hmmm, not sure if to take offense at your comment...
"It sounds like you're a sales guy, but if you can get your technical folks to help out (unless you know this information), I'd really appreciate it. "
Ouch...all my efforts of being unbiased and informative, and I get called a sales guy. Back to the ole' drawing board.

I also posted on the other link. I'll move to that one too.

Sean,

As you have figured out from my posts on the other board, no offense was meant and my wording was poor. I'm sorry about that.

Richard