CYA for Salt Chlorinator Pools II

[mas985]

Last week I raised my CYA from 30 to about 55. After a week of running with the higher CYA, I have come to the conclusion that it had no affect on my residual chlorine level. During this week I did not change my pump run time or the chlorinator setting. Also, pool temp and PH where about the same.

CL = 2 ppm @ CYA 30 ppm
CL = 2 ppm @ CYA 55 ppm

So the graphs from the CYA study are correct in that chlorine retention does not increase significantly for CYA levels more than 25 ppm even for a SWG pool.

One other question. Assuming the same residual, is a SWG any different than an ORP controller with liquid chlorine? Both would need to add chlorine to the pool at the same rate for the same residual. To me at least, once you get past the chlorine injection method, there is no difference. Therefore, why should the CYA levels be any different? Also, the argument about shocking in the cell for SWGs also applies to the liquid chlorine. The chlorine concentrations would be just as high near the injection point.

Sorry for keeping the debate going but there seems to be a lot of conflicting information, theories and experimental results.

[waterbear]

Last week I raised my CYA from 30 to about 55. After a week of running with the higher CYA, I have come to the conclusion that it had no affect on my residual chlorine level. During this week I did not change my pump run time or the chlorinator setting. Also, pool temp and PH where about the same.

CL = 2 ppm @ CYA 30 ppm
CL = 2 ppm @ CYA 55 ppm

So the graphs from the CYA study are correct in that chlorine retention does not increase significantly for CYA levels more than 25 ppm even for a SWG pool.
but you are still not within the recommend range of 60-80 ppm. Try raising it up to 70 ppm and see what happens. I find that I lose chlorine quickly when my CYA is below 60 ppm and even 60 is iffy.
One other question. Assuming the same residual, is a SWG any different than an ORP controller with liquid chlorine? Both would need to add chlorine to the pool at the same rate for the same residual. To me at least, once you get past the chlorine injection method, there is no difference. Therefore, why should the CYA levels be any different?
Because the FC levels in the cell when the cell is generating are VERY high. It is not just shock level...more like 'supershock' level. The level is much higher than can be achieved by manual chlorination!
Also, the argument about shocking in the cell for SWGs also applies to the liquid chlorine. The chlorine concentrations would be just as high near the injection point.
No, it would be MUCH higher with a SWG which is generating chlorine gas which then dissovles in the water to form hypochorous acid. If you had an automated chorine gas injection system that was introducing the gas into a very small chamber of water (the size of a generator cell) it might be similar but a dosing pump with liquid chlorine will not reach the same concentrations nor will it come close.

Sorry for keeping the debate going but there seems to be a lot of conflicting information, theories and experimental results.

Hope this helps.

[waste]

Hi all, esp Karin who started this (I'm going to TRY not to rant) Karin, the tech did lie to you, chlorine is chlorine is chlorine - Pat and Evan might be on to something with the introduction method, but the chlorine is the same (and made the same way most chlorine sold is made, be it a gas, puck, liquid or granual).
My problem has always been with the reason for a higher cya for salt pools, the reason proffered by Pat and Evan is about as intellectually satisfying as a ricecake satisfies a dieter. They understand chemistry a LOT better than I, but it sounds like they're quoting chapter and verse from the SWG makers. I started a rant down in the "China Shop" about this (and something else), yet none of this was brought up. So what if you are super, super shocking the water as it passes through an energized cell? Killing combined chlorine is one of the things we all do when we add bleach (chlorine) to the pool, it just does a better job of it. If it takes the entire production of cl to counteract the cc, the unit isn't dialed up enough (though I doubt that cc takes all the cl that's being produced). If it's the uv that is stealing the cl, cya will help that, but at the same time, the kill time for ... yuckies is reduced, people who follow Ben's Best Guess won't be able to generate enough to satisfy it's requirements - if you're saying that BBG doesn't apply to SWG's then SAY SO, Ben is an intelligent person and if you have data that contradicts his 'best guess', he'll probably welcome it and ammend the BBG to reflect that. I, until I hear otherwise, will follow the Best Guess and keep my pool owners @ 30 -40 cya, because they won't (or can't) understand that violating all the literature they've recieved and instructions they've been given, will be a good thing (they are pool store zombies and cl > 3 ppm = BAD). Are you sure that the SWG manufacturers aren't just trying to get people to generate more cl - they'd sell a lot more replacement cells if every one they sold had a longer run time? I hope I haven't ranted here, just a few points that came to mind as I read and reread this topic.
mas985, thanks for the link, now I know that Arch is within 1/2 hr from me, I'll gladly accept donations for an assault rifle and ammo, and a defense fund to protect me when I go down there and 'set them straight'

[Lenny]

waste,

Doesn't it make sense to try the 60 to 80 CYA range and see how it works with a SWG? If the manufacturers, etc. have determined that the super chlorination of the water as it passes through the cell helps reduce the need for active chlorine (not tied up by CYA) in the pool than why not give it a whirl? The important thing is to keep algae and other nasties at bay so if people are running their SWGs sucessfully in the 60-80 range and not reporting any problems than there must be something to it.

If we run with a CYA under 60 than the cell would need to work harder to sustain a given residual chlorine level, which should shorten the cell life. So it's not a matter of the manufacurers trying to sell us more replacement cells.

I just installed a SWG so I'll see how it goes while follwing the manuf. suggestions. Otherwise, I might be shortening the cell life unnecessarily.

[waterbear]

Hi all, esp Karin who started this (I'm going to TRY not to rant) Karin, the tech did lie to you

I have to disagree with this!

, chlorine is chlorine is chlorine - Pat and Evan might be on to something with the introduction method, but the chlorine is the same (and made the same way most chlorine sold is made, be it a gas, puck, liquid or granual).
My problem has always been with the reason for a higher cya for salt pools, the reason proffered by Pat and Evan is about as intellectually satisfying as a ricecake satisfies a dieter. They understand chemistry a LOT better than I, but it sounds like they're quoting chapter and verse from the SWG makers. I started a rant down in the "China Shop" about this (and something else), yet none of this was brought up. So what if you are super, super shocking the water as it passes through an energized cell? Killing combined chlorine is one of the things we all do when we add bleach (chlorine) to the pool, it just does a better job of it. If it takes the entire production of cl to counteract the cc, the unit isn't dialed up enough (though I doubt that cc takes all the cl that's being produced). If it's the uv that is stealing the cl, cya will help that, but at the same time, the kill time for ... yuckies is reduced, people who follow Ben's Best Guess won't be able to generate enough to satisfy it's requirements - if you're saying that BBG doesn't apply to SWG's then SAY SO, Ben is an intelligent person and if you have data that contradicts his 'best guess', he'll probably welcome it and ammend the BBG to reflect that. I, until I hear otherwise, will follow the Best Guess and keep my pool owners @ 30 -40 cya, because they won't (or can't) understand that violating all the literature they've recieved and instructions they've been given, will be a good thing (they are pool store zombies and cl > 3 ppm = BAD). Are you sure that the SWG manufacturers aren't just trying to get people to generate more cl - they'd sell a lot more replacement cells if every one they sold had a longer run time?

Here is where your rant falls apart. I test a lot of water and I can tell you that raising the CYA into the 60-80 ppm range allows you to turn the output DOWN on the cell...not up! I have oberved with my own system that when the CYA drops to about 50 ppm I have to almost double my run percentage!

I hope I haven't ranted here, just a few points that came to mind as I read and reread this topic.
mas985, thanks for the link, now I know that Arch is within 1/2 hr from me, I'll gladly accept donations for an assault rifle and ammo, and a defense fund to protect me when I go down there and 'set them straight'

I guess we're headed for the china shop again!

[Tredge]

I hear you waste

I also trust the chem experts here but I really would love to see more evidence. The SWG manufacturers must have that evidence to support their recommendations right? Where is it?

I use a SWG and currently have my CYA at 40. The SWG is on its lowest setting and is maintaining Free chlorine very well, maybe it could do better but I've been watching this thread before I add more CYA.

I figure if my pool is sparkling and my SWG isnt dying I'm ok. I can always add later if I become convinced that a high CYA is really superior.

[mas985]

After thinking about this a bit more, I wondered what the actual chlorine ppm would be leaving the cell given the production rate of the cell. To find out, I started with the maximum production level of my Aqualogic cell which is 1.45 lbs/day of chlorine (assumes 100% setting running 24 hours/day). Thus the production rate in GPM would be;

1.45 lbs/day * .125 gallons/lbs * 1 day / 24 hrs * 1 hr / 60 min = 1.26e-4 GPM

For my pool, the water flows at about 90 GPM (estimated from pump head curve, psi and suction in. mg.) so the maximum ppm leaving the cell would be 1.26e-4/90 or 1.4 ppm continuously. This is of course is added to the existing ppm but still it is not exactly shock level.

So if it is true that the cell does indeed have a very high level of chlorine in the cell, then the cell must either produce chlorine that is very unstable and reverts back to salt shortly after production and/or, very little of the water going through the cell is actually between the plates of the cell (ie. reduce volume of water, increased CL ppm).

Looking at the interior of my cell, I noticed that the plates only take up about half of the area of the cell which means about half or a bit more of the water never touches the cells but goes around them. Still this would only double or triple the ppm level in the cell.

So the only plausible explanation for a very high chlorine levels in the cell is that the chlorine gas is very unstable, which I think someone has already pointed out. It would seem that most of the chlorine reverts back to salt before or shortly after exiting the cell.

Therefore, the only explanation I can gather for the need of high CYA, is that during this temporary phase of high chlorine, the high CYA may prevent some of this unstable chlorine from reverting back to salt. So the CYA may have two jobs, one for UV and another for stabilizing the chlorine in the cell.

Also, from what I understand, chlorine has two jobs. First is for sanitation which kills all of the nasty bugs in pool water. From what I have read, this does not require a very high residual to accomplish so even 60-80 ppm CYA can usually accomplish this fairly quickly or quick enough.

The second job of chlorine is oxidation which removes all of the other stuff such as sweat, suntan oil and algae which is a big one. Higher levels of chlorine are required to accomplish this. However, it is not necessary to remove this stuff in seconds or minutes but is sufficient to remove in hours or even days.

So it would seem that the high CL in the cell does much of the oxidation, which can be accomplished over hours or day, and the CL residual is responsible for killing the bugs, which can be accomplished with a low CL and a fairly high CYA.

The only anomaly to this whole theory is that my residual chlorine did not increase with an increase in CYA which I would have expected. I am willing to bump it up to 70 just to make sure there is not something else going on.

Sorry for the long thought experiment but I would like to fully understand the SWG process and as Waste has pointed out, so far, we have not seen a comprehensive explanation. This is my first attempt at one but I am sure it has some flaws.

[gwrace1]

I run my CYA at 35-40 PPM with 2.5 PPM free chlorine. Pump is set on low speed for a run time of 12 hours each night with Aquarite set at 40%. The system manages to maintain levels even after the heaviest rains. Pool has been crystal clear since startup. Just got a free pool fill up yesterday with 3 inches. Nice to see some rain during the summer in south Texas.

[waterbear]

After thinking about this a bit more, I wondered what the actual chlorine ppm would be leaving the cell given the production rate of the cell.

(Deleted long duplicative quote -- use links if needed Waterbear; don't requote the W...H...O...L...E L...O...N...G thing! PoolDoc)

If your hypotheses about the oxidation is correct then monitoring the CC at lower CYA and then again at higher CYA would be a telling factor!

[chem geek]

I started commenting on appropriate CYA levels in an SWG pool in the CYA / Pool Pilot Question thread and I was pointed to this thread which has been very informative (and has me eat crow).

Obviously with my technical interest I would like to get to the bottom of why higher CYA levels are needed in an SWG pool. Assuming the data on the breakdown of free unbound chlorine vs. chlorine tied up in CYA is correct, then the amount of extra work (time) the SWG needs to be on at 40 ppm CYA is only 12% higher than at 80 ppm CYA with 3 ppm FC (it's a 45% increase in work or time going from 80 to 20 ppm CYA). Since most people see a much bigger jump than that (to maintain the same FC level), either the chlorine breakdown data from sunlight is wrong or CYA is needed in the SWG process to improve its efficiency.

It may very well be that what was said earlier on this thread is what is happening. Namely, that rather high localized concentrations of chlorine (HOCl, OCl- from dissolved Cl2) are generated and that with CYA these high concentrations get tied up quickly before they have a chance to breakdown. I find this somewhat surprising as a decent flow rate should dilute the concentrations rather quickly, but physical flow rates are usually not nearly as fast as most chemical processes (more on that later in this post).

The best way to test this theory would be with a pool that is covered to minimize the breakdown from sunlight to remove that factor from the experiment (also try not to use the pool during the experiment). Since the chlorine demand would be lowered, the chlorine generator cell should be set to run less frequently (lower percentage) to maintain a constant FC level, but the cell should be run at the normal power level used in a typical uncovered pool. Start out with a CYA level of 40 ppm or below -- way below, such as 20 ppm, would be even better. Record what percentage of time the cell needs to be on to maintain a constant FC level (say, 3 ppm). Then add CYA to the recommended range of 80 ppm. Adjust and record the percentage of time the cell needs to be on to maintain a constant FC level. Report back to this thread your results.

The above experiment was sort of done by some members of this forum (and posters to this thread) but I don't believe a cover was used so the variability in sunlight is a problem since this is a huge factor in chlorine breakdown.

I am still puzzled by the prevention of algae in an 80 ppm CYA 3 ppm chlorine pool since that only produces 0.015 ppm HOCl at a pH of 7.5 which is enough for disinfection, but presumably not enough for preventing algae (in Ben's experience). Though some have said that the higher chlorine concentrations in the SWG cell may kill algae, that would not be true for any algae that adhered to the plaster. So maybe the SWG cell kills free-floating algae and that usually that is good enough.

A post from "chem geek" would be incomplete without some technical analysis to scare the bejesus out of 90% of the forum, so stop reading at this point if you're (rightfully) frightened by chemistry.

I actually have the rate constants for the conversion of HOCl into the chlorinated cyanurates (i.e. the take-up by CYA). My rough calculations show two things. First, that a level of CYA of around 50 ppm may be the concentration where the rate of take-up by CYA of incrementally generated chlorine in the cell is equal to the generation rate of this chlorine. This lends support to the theory that the CYA helps the chlorine generation by preventing the buildup of unbound chlorine (i.e. HOCl and OCl-). Second, the incremental concentration generated by the cell is rather small so this build-up theory only works if there is a very high local concentration at the generation site (the electric plate) and that is certainly plausible. However, this whole business about superchlorination is a bit overblown since the calculations show that it would take nearly 100 turnovers of the pool water to shock it with the equivalent of 10 ppm. On the other hand, a shock of 1 ppm takes only 10 turnovers and 0.1 ppm only one. Therefore I don't believe the theory of the "supershock" is the appropriate one because not enough water is exposed to 10 ppm or higher levels of chlorine, but free-floating algae even with Ben's chart is prevented by 0.05 ppm HOCl.

Richard