CYA for Salt Chlorinator Pools II

[Karin]

Hi folks....I'm new, but eager to be part of this 'wet ' wild' pool community. ;)

My present concern is about the best CYA level to maintain for a pool using an Aqua Rite Salt Chlorinator. I recently drained and refilled some of my pool in order to reduce the CYA from 100ppm to about 40ppm. I would really like to stay in this range, but for some reason the manual suggests 60-80ppm.

Does anyone else have a SWG system and what do you keep your CYA at? Why would a salt system need higher CYA?

Thanks in advance. Karin

[tonyl]

The higher CYA helps the chlorine stick around in your pool longer. That allows you to run with a lower output setting on the SWG. I've had one for several years with cya of 70-80 and 1-2ppm chlorine with great results. I only add bleach after very heavy rains. Hope this helps, Tony

[Karin]

Thanks Tony....that does help. How long do you run your SWCG each day? It seems that mine is way too eager to please and my chlorine level is up at 6ppm after running the pump/filter/Kreepy thing for about 6-8 hours each day.

I know I can adjust it downward and I do....but we just had a horrifying experience with yellow algae and now I'm nervous that the alien algae will return.

[gwrace1]

I run my CYA at 35-40PPM with a FC level of 2.5 PPM. Aquarite is set on 40% and runs for 12 hours each night with pump on low speed. Water is always crystal clear.

[tonyl]

Well for our pool my output is up a bit this year due to the hot hot summer so far here in the midwest. I'm at 50% and 8 hours per day. Water temp is 91.

Edit: forgot to mention it's more efficient to run with salt at the higher end (3400), your output is also dependent on the salt level.

[rbonin]

The high recommended CYA level by most salt water chlorinator manufacturers seems to be a mystery to most on this forum.

I abide by Ben's "best guess" chart of CYA vs. FC and so I keep my pool at 40ppm CYA and about 5ppm FC. My Polaris AquaClear cell runs 10 hours a day as my pool is in direct sun all day.

[mas985]

I am interested in everyone's settings as well, since there seem to be many different lines of thinking on this:

1) Use recommended CYA 60-80 and keep chlorine level per Ben's CYA table.
2) Use recommended CYA 60-80 and ignore Ben's CYA table and rely on cell to continuously shock water plus add bleach when necessary.
3) Use lower CYA level and Ben's CYA table. Recent studies indicate the CYA can contribute to dissolving plaster and shortening it's life.
4) Use lower CYA level, ignore Ben's CYA table and rely on cell to shock water.

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. I have calculated that with my current settings, the cell puts in about 2 ppm per day so I figure I am using up 2 ppm per day.

If anyone is interested, the aquarite/logic cell puts in 1.45 lbs/day @ 100% setting. This is equivalent to 1.45 gallons of 12.5% bleach.

I think if I upped the CYA with the current settings, the chlorine would probably rise but not as quickly as per Ben's table. I am trying to research this a bit more to see if I can predict a chlorine ppm level with different CYA levels. I would love to experment with different CYA levels but given the difficulty of reducing CYA, this is not an option.

[cwstnsko]

Agree with the previous post about following Ben's guidelines. I run my CYA at about 30 ppm. I adjust the SWG so that my FC is at about 5 ppm in the morning, and it drops no lower than 3 ppm by late afternoon in full AZ sun. I run my pump on low speed 24/7, SWG at about 30% on power level 1, salt is at 2900-3000.

[Karin]

I've heard that using my SWCG to shock will hasten the cell's life expectancy. I don't want to hasten its demise and will probably use good ole bleach on the occasions it may need some shocking--after heavy rains.

I can only run my pump at one speed. It can adequately turn my water in a 4-6 hour period, but I leave it on for 6-8 hours....it usually starts at 8am. Should I maybe have it start later in the morning so that it runs during most of the hot afternoon part of the day and keeps adding fresh chlorine that way?

Based on your comments, I'm leaning toward a CYA of around 50---close to Ben's recommendations and also Aqua Rite's. How's that for straddlin' the fence?

Tell me more about CYA staining??? I added some the other day through the skimmer---tried to dissolve it with some warm water first, but that stuff doesn't like to dissolve, and then slowly poured it into the skimmer while it gulped it down.

I presently have some yellowish stains in corners and the risers of the steps and along the walls in the deep end that appear to be recent. Since we just had it resurfaced with some Diamond Brite in March, I'm very concerned.

We had a bout of yellow algae about a week ago---which I think is gone. (Cleaned the filter daily.)
We did a partial water change to lower the CYA only to find out that the city water was full of phospates....plus I forgot to add a sequestering agent till after it had been filled. Then after going to 4 pool stores and getting 4 different CYA readings of : 15, 25, 30 and 60 all from the same sample, I added some CYA to bring it up to 40. Yes....I do need a life...

So now I'm wondering what the culprit is for the stains:

From the Yellow algae?
From the new water full of phosphates etc?
From the additional CYA?

Other than for the stains, the pool and water look great: clear and blue....sigh..I should be in there right now instead of typing about it....
FC 6.5
TC 6.5
PH 7.4
Hardness 200ppm
Alkalinity 100
CYA 40
Salt 3100

[Simmons99]

I've never heard of CYA staining - it could be due to metals or algae

[PatL34]

Karin & Others,

The 60 - 80 ppm CYA recommended by the SWCG manufacturers is because the cell instantaneously creates a high concentration of chlorine, and kills the chloramines as well.

Without the high CYA, the chlorine would be used up rapidly before it gets into the body of the pool.

With my SWCG, a 50 - 60 ppm CYA level appears to work fine. So 50 ppm you are using is in the ballpark.

Regarding the staining, it could well come from your fill water, but others with more experience can help you. Have you tried to brush this stain? To my knowledge CYA does not stain, otherwise we would be using something else.

Your water chemistry apart from the CYA is also in the ballpark, and I can well understand your frustration with 4 pool stores giving you different readings, so I take them with a pinch of salt (pun intended);) They do not know how to be consistent. Once you do it yourself the same identical way each time, you will be less frustrated:D

mas985, I would use option #2, based on what I said at the beginning. The cell should produce the chlorine regardless, and if not you can always aise the output. A bit of trial & error. I have not heard of CYA dissolving plaster pools. That's a new one to me. Will have to investigate.

Hope this helps.

Pat

[Karin]

Well, I went to the horse's mouth and called up Aqua Rite to ask them about their higher recommended CYA levels. After 10 minutes of listening to some vintage elevator music, the rep told me that the chlorine that a SWCG makes is different than the regular chlorine you just add to a pool.

That's why they want a higher CYA---to protect it from sunburn.

Was she just trying to get me to hang up, or does that sound plausible? Hmmm...

[tonyl]

I think you'll have to experiment a bit on your CYA levels. I keep mine within the recommended range due to full sun exposure. I think most pools would be fine with 50ppm and after all, stabilizer isn't cheap.

[waterbear]

Well, I went to the horse's mouth and called up Aqua Rite to ask them about their higher recommended CYA levels. After 10 minutes of listening to some vintage elevator music, the rep told me that the chlorine that a SWCG makes is different than the regular chlorine you just add to a pool.

That's why they want a higher CYA---to protect it from sunburn.

Was she just trying to get me to hang up, or does that sound plausible? Hmmm...
She told you the same thing that PatL34 stated above and the same thing that I have stated in other threads. There is a difference between manually chlorinating and producing chlorine by electrolysis. I have spoken to tech support at Goldline Controls, Poolsean, who is in this forum and works for AutoPilot Systems (Pool Pilot SWG's), and some of the dealer literature that I have access to from my job (the store I work at sell several brands of SWGs) all are saying the same thing. It is better to run the CYA level higher in a pool with a SWG. I run mine at 70 ppm with my Aqualogic PS-8 (Goldline Controls). I also test a lot of water all day and can access histories of test results. About half our customers have SWG's installed and there are several that have been running CYA at 30-50 ppm. I have notices that these people have not been able to maintain adequite FC levels, run their cells at very high outputs, and some have had problems with algae blooms, including Mustard Algae. I have not seen any of these problems in the ones running higher CYA levels and keeping the FC at about 3 ppm or slightly higher. It is a small sample, maybe about 50 or 60 customers but the trend I have seen is unmistakable!

[mas985]

Here is the CYA article about plaster and CYA:

http://findarticles.com/p/articles/mi_m0NTB/is_23_44/ai_n15932555

Although this probably applys more to very high CYA, this was why I was reluctant to go to higher CYA levels. I may end up going a bit higher anyway.

[waterbear]

Here is the CYA article about plaster and CYA:

http://findarticles.com/p/articles/mi_m0NTB/is_23_44/ai_n15932555

Although this probably applys more to very high CYA, this was why I was reluctant to go to higher CYA levels. I may end up going a bit higher anyway.
The sudy was done by ARCH chemicals! These are the same people who put copper into trichlor pucks. I am sure they are talking about CYA levels well above 100 ppm! There is also evidence that sulfates from using dry acid can affect the plaster so there can be multiple causes for damage to the plaster finish. The article did state that ARCH chemical recommeded switching between trichlor and other forms of chlorine to keep stabilzier levels from going too high. Does this mean that they are starting to read the Pool Forum? Seem like THAT piece of advice has been given on here for a very long time!:D

[dep78737]

Looks like the SWG manufacturers understand their products better than we do. It's in their best interest to suggest CYA/CL levels that will work in a vast majority of pools, otherwise people would be cursing them instead of singing their praises. Partially based on advice in this forum, my CYA was around 40 last year and I kept having to crank up the output. Raised it to 60 this year and now run at 40-50% instead of 60-70%. FC levels are more stable and acid use is down also, maybe since it's working less and maybe since the plaster is now 2 yrs old.

[waterbear]

If your cell output is down your pH should not drift up as quickly since you are manufacturing less sodium hydroxide! Another reason to run the cya a little higher! Actually, your results are pretty typical of the things I have seen with my customers when viewing their histories. The vase majority of them bring water in for testing weekly or bi weekly and the LaMotte software we use allows me to graph test results which makes spotting these trends easy to see. (Now you all know what I do when I get bored at work!) ;)

[bbb]

Great thread - thanks to everyone posting.

For what it's worth - I'm on my first year with a Pool Pilot, and *have* been able to keep FC in the pool even with CYA in the 30 range (28K gallon pool, 48 size cell, power level 2, 20% to 33% usage). I have been debating about where to keep my CYA levels, too. I have kept my new solar cover on at all times this year except for when we actually are using the pool. One theory is that my blue/silver solar cover has helped me maintain a higher FC level in the pool, since there is less sunlight hitting the water. I chose blue/silver because I wanted to preserve heat from my heat pump.

However, thanks to all these discussions, and the fact that we are now in full summer "school is out" mode - meaning more time with the cover off and a higher bather load) I bought 2 bottles of hth brand CYA at my local hardware store to raise the level to around 50-60. Interestingly, the hth bottle is starting to warn people of CYA levels -- it says 20-40 is ideal, and it also says that chlorine effectiveness is severely limited over 100.

One other boast - I used plenty of borax to stabilize my ph last year, and added more this spring. My ph has been constant at 7.2 all year! My theory - lots and lots of acidic rain in the Northeast this year. So I expect the ph to eventually drift up -- although we have rain in the forecast for each of the next 7 days...

Related boast - between the rain and the solar cover, I have not added one drop of water to the pool - on the contrary, I have had to drain inches off several times!

Final boast - 2nd year of bbb, perfect water every day this year - nothing added to pool except bleach (pre-SWG), polyquat (early spring, to make sure no algae before the cover came off), baking soda (to bring my alk up in early spring), and borax (pre-SWG, to bring ph up to 7.4, though it never moved)

last reading:
FC 3
CC 0
CYA 30 (will raise to around 60 for SWG, but no worries)
TA 90 (may raise with some baking soda, but no worries)
ph 7.2
salt 4400 ppm (overcalculated when I added it for the SWG; no worries)
calcium - doesn't matter - vinyl pool with titanium core heater

:)

[mas985]

Ok, I think I am convinced. I will try a CYA level closer to 60 ppm. It will be interesting to see how much the CL level rises with a CYA rise.

[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!:eek:;)

[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 (http://www.poolforum.com/pf2/showthread.php?p=27853#post27853) 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

[Poolsean]

Richard,

I can tell you from personal experience, I was the R & D director for AutoPilot at one time, that the concentration of chlorine within the cell is WAY in excess of 10 ppm. It was more along the line of 80 ppm, as determined by MANY dilutions of the water sample directly from BETWEEN the electrodes.

Remember, chlorine gas is produces with quickly gets absorbed by the water yielding a purer sodium hypochlorite. This certainly is sufficient to treat the water at a shock dose, regardless of the flow rate through the cell. However, we've found that by slowing down the water flow, you allow more "contact time" to process the water and rid the combined chlorines, and kill the "bad stuff". Basically, it increases the efficiency of the electrolytic process and killing effect.
Sorry I'm not a chemist to be able to present ionic formulas and osmosis reactions, but being a very hands on manager at AutoPilot, I've seen, repaired and resolved many salt generator issues as well as pool water chemistry related issues (with and without salt sytems). I'm no expert by any means, but I do have experience and a little (dangerous) knowledge to get by.

[chem geek]

Sean,

I wasn't clear with the amount of chlorine levels in the generator. I was talking about the normal operation that generates the incremental chlorine for the pool and referring to what some people were claiming was a natural superchlorination in the cell during this process. I was also referring to the entire cross-section of area that the water flows through and not just an area near the plates (which has incredibly high concentration of chlorine).

I believe what you are talking about is a superchlorination mode which runs the cell at a much higher power level. If you ran at this level during the duration of one pool turnover, then the pool itself would get to that 10 ppm level. Is this what you are talking about? I apologize for my ignorance about the mode you have for your cells.

Richard

[Poolsean]

No apologies, but thanks for "slappin me in the face" and making me aware that I sounded like a sales guy. That's the last thing I want to sound like here as it irritates Ben if I do.

The normal operation, when you inspect anyone's cell, is for the water to pass through and around the electrodes, or blades. Let's face it, the chlorine is only produced between the blades so the space around them does not get the same concentration of chlorine. However, it is quite high between the blades AND still quite high surrounding the blades too. Oddly, if you test the water coming out of the cell, you're prone to only get 1-5 ppm higher than what's tested in the body of the pool. You may see a similar result if you take a water sample from the closest return line to the pool equipment.
Now, the design of the cell is going to be rather different from manufacturer to manufacturer. If you inspect the various designs, you'll see different dimensions of blades, spacing between the blades, solid vs mesh, % ruthenium oxide coatings on the blades, and power (milliamps/sq cm) to the blades. So when you asked for more details, it's not easy...besides, most of this is proprietary information.

What I am taking about is not just under superchlorination mode, this is under normal operation. Besides, most Superchlorination mode does not increase the power to the cell, it simply extends the time the cell is energized. I believe there is only one manufacturer that claims to increase the output to 125%.
Most manufacturers cycle their cells on and off, during the day. Some run the cell continually, for a % of the pump run cycle (I'm not sure how this is determined without having to enter your physical pump run time). Some constantly energize the cell and fluctuate the power to the cell between a low output, up to a high output. Perhaps this last one is the model you're describing? Those are typical of Australian systems that also require higher salt levels to operate (4000 - 6000 ppm).

Hope this helps

[Tredge]

I've been fascinated by this discussion and I'm continually trying to understand.

Are you saying that because the water "between the blades" is 80+ppm chlorine while the water exiting the cell is only 1-2ppm higher than the pool itself that the high CYA acts as a transport of that high chlorine? Sort of a way to get it into the pool?

That logic makes sense to me.....what Doesnt make sense is when Ben's best guess chart comes into play.

The chlorine in the pool, regardless of where it came from, has the same chemical properties as any other chlorine.

In my experience, when I ran my SWG at 80ppm CYA, my chlorine maintained at a lower power setting....as suspected. However, Algae on the walls and in the pool ran out of control and I fought it all season.
Ever since I lowered my CYA to 35-40ppm, I've not had to shock my pool once and haven't had a hint of "slimy walls" or the onset of algae.

There could be other factors at play since my tests were over the course of 2 seasons...

[tphaggerty]

Yet my results are the exact opposite of Tredge. Running at about 30ppm CYA, I just couldn't keep my 30K IG pool "stable" (Aquarite). Every time I had any type of high bather load or lots of rain or a New York pollen "storm", the pool would get cloudy.

Now I am running at about 60 ppm CYA (I wonder if anyone can read the test more accurately than a 10ppm swing) and I have been able to scale back the SWG from 60 to 50 to now running at 40%. Water is clear and I am at about 4ppm FC. The pool is much more stable, it just stays clear and nice. It is possible that my results come from finally clearing out any residual "gunk" from opening, but I'm inclined to think it is due to running at or close to the SWG instructions.

Plus, my PH seems rock solid at 7.4/7.6.

[chem geek]

Sean,

Yes, you've got my point. The fact that some small area of the cell and therefore some small percentage of volume of water going through the cell is exposed to 80 ppm of chlorine is interesting, but doesn't mean that all or even most of the water is exposed to that level except through many, many turnovers of the pool water.

An analogy with what happens when you add liquid chlorine (or bleach) to the pool might be helpful. Someone else [EDIT](let's give credit to waste (http://www.poolforum.com/pf2/showpost.php?p=28171&postcount=23) where credit is due)[END-EDIT] mentioned this in this or another thread, but I'll describe the situation a little more deeply. When you add 6% bleach to your pool, you are adding chlorine in a concentration of 60,000 ppm. As the chlorine diffuses into the pool, at some point you will have the chlorine diffused into 1% of the total pool volume at which point the averge chlorine concentration in this volume of the pool will be 100 ppm assuming you added enough chlorine to raise the entire pool's chlorine level by 1 ppm. At a later point in time when the chlorine has mixed with 10% of the total pool volume, this volume has on average 10 ppm, etc. Now of course I'm oversimplifying since the mixing is not uniform (though I used the term "average" which covers up many sins) and there is not an absolute line of demarcation between pool water that has mixed with the chlorine vs. pool water that has not. But the principle is very much the same as what happens in the cell. Though some small amount of water is exposed to very high chlorine levels, it's still a small amount of total pool volume.

As an aside, the above "analysis" (and I use that term loosely since I was not at all precise in the above discussion) implies that it may be better to add liquid chlorine (or bleach) to the pool in a way that mixes rather quickly with a large volume of the pool water, but not with all of the pool water at once (which is impossible to do anyway). So adding the chlorine over a jet or into the skimmer may be better then putting it into one place in "calmer" water. I'm not sure how distributing it manually around the pool's entire edge would do, but personally I don't like to take that risk of splashing the 60,000 ppm chlorine onto my clothes!

Anyway, the bottom line is that I think that marketing a salt cell as somehow super-zapping your pool's water by using the high chlorine measurement example is telling the truth, but not the whole truth (I'm not accusing you -- you have been very forthright and honest in your discussions -- I'm talking about sales folks less knowledgeable than yourself). Unless there is good science to explain otherwise, what the salt cell does is not much different from what happens when you add liquid chlorine to your pool except that the cell is doing so continually over a longer period of time. It is possible that this is more optimal in producing the 10% volume / 10 ppm mix through the pool water. It is also true that the chlorine that is initially produced by the salt cell is unbound by CYA and therefore the true disinfecting chlorine concentration may be much higher, though the exposure time before this gets combined with CYA is rather short. Though the same thing occurs with liquid chlorine put into the pool, again the rapid nature of dumping chlorine in the pool is probably less optimal than the continous process of the salt cell.

Richard

[chem geek]

Are you saying that because the water "between the blades" is 80+ppm chlorine while the water exiting the cell is only 1-2ppm higher than the pool itself that the high CYA acts as a transport of that high chlorine? Sort of a way to get it into the pool?
:
:
In my experience, when I ran my SWG at 80ppm CYA, my chlorine maintained at a lower power setting....as suspected. However, Algae on the walls and in the pool ran out of control and I fought it all season.
Ever since I lowered my CYA to 35-40ppm, I've not had to shock my pool once and haven't had a hint of "slimy walls" or the onset of algae.

I don't believe the purpose of the high CYA is to "transport" the chlorine into the pool, but you are close to what I do believe is the purpose of the high CYA in making the salt cell more efficient. I'm going to try to explain this non-technically, but if I fail I apologize in advance.

The efficiency of the salt cell in generating chlorine is dependent on how much of this generated chlorine already exists near the generating plates. If there is already a lot of chlorine close to the plates that generate chlorine (due to its building up and not getting swept away fast enough), then the efficiency of the process drops and less chlorine is generated. The purpose of the CYA seems to be to combine with the chlorine that is generated and effectively "hide" this chlorine from the salt cell so that as far as the salt cell is concerned, the concentration of chlorine near its plates is not 80 ppm but something much less. The key to making this work is that the chemical reaction that combines the chlorine with the CYA to "hide" the chlorine needs to be faster than the production of the chlorine from the salt cell and it turns out that the critical point where this takes place is around the 60 ppm or so of CYA that is the minimum recommended by the manufacterer of the salt cell.

Your experience with algae is consistent with what I was saying in my earlier posts that Ben's chart is important to follow because the concentration of the disinfecting form of chlorine needs to be high enough throughout the pool, not just in pool water that passes through a salt cell. On the other hand, many people have reported no algae problems at higher CYA and Sean's experience is that those with lower CYA have had algae problems while those with high CYA have not, but that might have been due to the low-CYA cases having such low efficiency of salt cell chlorine generation that they couldn't produce enough FC to kill algae even at the lower level of CYA. At this point, I would say the jury is still out and my guess is that the continual dosing of chlorine from the salt cell is somewhat more efficient and does probably zap more bugs, oxidize more organics, and kill more *free-floating* algae than the manual dosing of liquid chlorine. That said, your experience shows that this greater efficiency of continual dosing from a salt cell is not enough and that you still need to have "Ben's chart" chlorine levels throughout your pool.

I do believe that it would be in the best interests of pool owners if the manufacturers of salt cells would at least consider the possibility that designing salt cells to operate at lower levels of CYA would be beneficial to "the rest of the pool" including pool surfaces where algae can develop (and bacteria can hang out in biofilms). All the salt cell makers have to do is simply increase the area of their plates over a larger volume and drop the power so that the local chlorine generation rates are lower since that will require a lower concentration of CYA to be able to "keep up" and "hide" the chlorine that is produced. And yes, this means that the salt cell may have to be bigger or, most likely, longer than it is today and that means more bulk and more piping to deal with this, but if it means a better situation for pool owners, then that's the right thing to do.

I just want to thank all of you that post on this and other threads. I have never experienced such a wealth and diversity of information and assitance in an environment that has remained civil. It's refreshing!

Richard

[tphaggerty]

Richard & Poolsean,

This is so great for those of us on the edge of understanding it all. Its like reading Scientific America - I don't understand most of it but it sure stretches the mind.

The ONE thing that hasn't been addressed in your analysis, Richard, is the fact (using the term loosely) that almost everyone using a SWG with fairly well balanced water reports CC at 0 all the time. This is not true (I believe) of those that use bleach to maintain normal FC, at some point they will get a buildup of CC and have to shock (which is another interesting thread you are involved in!). This would seem to support the idea that the SWG is "super-shocking" in the cell and it must be some larger percentage of the water that passes through the cell than you are calculating.

In thinking about this though, you did say that it might take a couple of days to "shock" all of the water in the pool, perhaps this is happening and is "good enough" to keep the CC reading at or close to zero. I am wondering why or whether this information is available in Austrailia where they have been using these systems for a lot longer than here (at least for non-commercial pools).

[chem geek]

A P.S. to Sean,

I seem to recall that the typical settings people are using with the salt chlorine generators is that they are on less than 50% of the time. Is it possible to have them on 100% of the time (when the pump is running), but operating at a much lower power level? Don't the lower power levels operate efficiently even at lower levels of CYA? If that is the case, then the simplest thing for owners of SWG to do would simply be to use the lowest power setting and increase their % on-time accordingly.

Of course, running at 100% means you have no leeway for the system to increase this if more chlorine is needed (due to increased chlorine demand), but if your system would instead just vary the power as needed (instead of the time) then this approach might work. What do you think?

Richard

[waterbear]

I wonder if some of the problems that SWG owners have encountered with algae at recommeded CYA levels could be to either:
1) running at the low end of the recommeded 1-3 ppm FC levels
2) not keeping a good check on the pH and allowing it to drift upward too much before lowering it?
I maintain my pool at 3 ppm FC and do not let my pH rise above 7.6 (okay, MAYBE 7.8) before I add acid.
Also since my SWG does not compensate for temperature (Goldine Aqualogic PS-8) I do ajust my cell output if I see the chlorine levels are dropping or rising. (my pool has run on as low as 5% output in cooler weather up to 15% recently with my FC going up to 4 ppm)
I do test my water at least weekly (full tests except CYA and CAL) and do OTO and pH testing at least once more duing the week. Perhaps this is the difference in why I have never had any problems with algae. I test a lot of water at the pool store and I see that many owners of SWGs let their pH climb to 8.0 or above and never really get it down to where it needs to be. I also see these people having algae problems in their pools. On the other hand I had several customers with very low CYA (30 ppm and below) that were running their cells at very high output and also had algae problems and cloudy water in their pools. In both cases these people reported back to me that their problems seemed to disappear when their CYA levels and pH were brought into line.

[chem geek]

Richard & Poolsean,
The ONE thing that hasn't been addressed in your analysis, Richard, is the fact (using the term loosely) that almost everyone using a SWG with fairly well balanced water reports CC at 0 all the time. This is not true (I believe) of those that use bleach to maintain normal FC, at some point they will get a buildup of CC and have to shock (which is another interesting thread you are involved in!). This would seem to support the idea that the SWG is "super-shocking" in the cell and it must be some larger percentage of the water that passes through the cell than you are calculating.

In thinking about this though, you did say that it might take a couple of days to "shock" all of the water in the pool, perhaps this is happening and is "good enough" to keep the CC reading at or close to zero. I am wondering why or whether this information is available in Austrailia where they have been using these systems for a lot longer than here (at least for non-commercial pools).
A properly maintained outdoor pool using manually dosed chlorine will also report a CC of 0 all the time. If the chlorine level gets too low, then [EDIT]when you later add some more chlorine[END-EDIT] you can build up chloramines, but if it is constantly maintained, then this is much less likely [EDIT]since breakpoint occurs "on the fly" when sufficient chlorine is present[END-EDIT]. Also, outdoor pools exposed to sunlight may have UV breakdown the chloramines [EDIT]and possibly chlorinated organics[END-EDIT]. If you think about it, if you were to manually drip liquid chlorine (remember this is 60,000 ppm chlorine concentration) into your pool near an output jet and did so more or less continuously whenever the pump was running, you are essesntially doing exactly the same thing that the SWG system is doing [EDIT]including exposing some volume of water to very high chlorine levels[END-EDIT]. This implies that for a manually dosed pool it *may* be better to add chlorine in small doses more frequently rather than wait to do it in larger doses every few days [EDIT](and maybe this slower dosing helps keep the CC at 0 by oxidizing CC albeit a small amount of pool water volume at a time).[END-EDIT]

[EDIT]From what I have read, chlorinated organics (aka Disinfection ByProducts or DBPs) are more likely to form at higher chlorine concentrations (which CYA keeps down low) and in particular they form even more when there are chloramines also present so an incomplete breakpoint is the worst situation to be in (i.e. to not have enough chlorine to meet bather loads). The "bad" chlorinated organics, known as THMs, form more at higher pH while at lower pH you get more non-volatile chlorinted organics that are not considered harmful to health, but would still show up in CC if they persisted.[END-EDIT]

To really test your theory, an analysis of indoor pools with and without SWG would be the place to start. Indoor pools have a much harder time avoiding CC since there is no sunlight to break them down nor is there good air circulation to sweep away the products of breakpoint chlorination (though I've never really been convinced that this is an issue since nitrogen and oxygen gasses are already pretty dominant in air so I doubt they get that much higher in concentration above an indoor pool's surface -- perhaps it's other gasses, such as carbon dioxide, that need to be swept away???).

Richard

[chem geek]

Evan (waterbear) also makes good points. If we really want to get to the bottom of this, we would need ALL of the relevant pool water chemistry information that all of you in these forums know is what is asked of you when you want people to help you with your water problems. Of particular relevance for this SWG algae discussion would be minimum FC level, CYA, and pH. The TDS is already roughly known and not a big factor nor is the CH or alkalinity (except that very low alkalinity can lead to large pH swings you might not be aware of) -- I'm not talking about water balance to protect plaster, but rather those components that affect disinfecting chlorine concentration.

Anyway, I don't want to bash SWG systems in any way. Even though I don't have one, I think they are great and the most important service they provide is that they automatically maintain FC levels. And then there's that silky feel of salt water...oooooooo, you've got me thinking about getting an SWG myself!

[waterbear]

Anyway, I don't want to bash SWG systems in any way. Even though I don't have one, I think they are great and the most important service they provide is that they automatically maintain FC levels. And then there's that silky feel of salt water...oooooooo, you've got me thinking about getting an SWG myself!

IF you do you won't reget it! Also, you might be able to clear up some of the issues raised here!;)

Seriously, IMHO, SWGs are the way to go! Pool maintenance becomes much easier (I only have to worry about keeping a jug or two of acid on hand. Every time I buy bleach for the pool 'just in case' it gets used in the laundry since I don't need it for the pool! I keep an extra bag or two of salt on hand for the water softener and If I need to bump up the salt in the pool a bit I use some of that unless I decide to spluge on bag of Morten or Diamond Crystal pool salt...learned my lesson about off brand pool salt with I got iron stains from one!)

[mas985]

There are a few things that I would be interested in understanding better. I have pretty much forgotten everything learned in Chem 101 & 102:mad:.

From what I understand about SWGs, after the chlorine gas is generated, it dissolves very quickly into water and creates HOCL and HCL.

Based upon the amps in the cell, how much chlorine gas is generated and does this match the advertised production of the cell (see below)?

Does some of the HOCL and HCL revert back to salt before exiting the cell, which might explain the higher levels in the cell?

How long does it take for some of the HOCL to break down to OCL- & H+?

How long does it take CYA to bind to the chlorine?


Next, I found this derivation on a university web site:

In an electrolysis of sodium chloride solution experiment a current of 7 A was passed for 1 minute

o Electrode equations:
(-) cathode 2H+ + 2e- ==> H2 and (+) anode 2Cl- -2e- ==> Cl2
o (a) Calculate the volume of chlorine gas produced.
Q = I x t, so Q = 7 x 1 x 60 = 420 C
420 C = 420 / 96500 = 0.00435 mol electrons
this will produce 0.00435 / 2 = 0.002176 mol Cl2 (two electrons/molecule)
vol = mol x molar volume = 0.002176 x 24000 = 52.23 cm3 of Cl2
o (b) What volume of hydrogen would be formed?
52.23 cm3 of H2 because two electrons transferred per molecule, same as chlorine.


First, is this formulation correct?

If so, a cell running for 24 hours would produce 4418658 cm3, 4419 liters or 14 kg (31 lbs). This is 21x the advertised production rate so if true, a majority of chlorine produced quickly reverts back to salt before returning to the pool.

Anyway, I have measured the water coming out of the return and, as Sean pointed out, it was only 2-3 ppm higher than the pool water. So my premise is that there may be some HOCL in very high concentrations in the cell but does not survive very long.

[chem geek]

Sean knows his cells better than I, but I'll take a stab at explaining things from a general chemistry perspective and I'll do so in-line in bold in your quoted post below.



From what I understand about SWGs, after the chlorine gas is generated, it dissolves very quickly into water and creates HOCL and HCL.
True.

Based upon the amps in the cell, how much chlorine gas is generated and does this match the advertised production of the cell (see below)?
I do not know, but suspect the production is close to the generation except for the "side-reactions" that can occur and inefficiency from the buildup of products (Cl2 and HOCl).

Does some of the HOCL and HCL revert back to salt before exiting the cell, which might explain the higher levels in the cell?
There is always an equilibrium that goes on, though the generation is very fast and things are out-of-equilibrium at various points in the system. First chlorine gas is produced. Then this gets converted to HOCl. Then this gets bound up with CYA. So there *is* a peak of concentration of HOCl in some region of the water, but this exact same thing also happens (minus the chlorine gas) when you add liquid chlorine or bleach to your pool.

How long does it take for some of the HOCL to break down to OCL- & H+?
This is an extraordinarily fast reaction, but I don't have the rate constant at hand (in general, protonation reactions with acids are extraodinarily fast).

How long does it take CYA to bind to the chlorine?
Now in this case I have the actual rate constant and the answer depends on the concentration of the different CYA species in the water. The reaction of CYAs taking up HOCl in pool water is as follows at 80 ppm CYA:

... rate ......... [CYA-]
7.4x10^4 * 4.64x10(-4) * [HOCl] = 34 * [HOCl]
... rate ......... [CYA(2-)]
2.2x10^7 * 7.98x10^(-8) * [HOCl] = 1.8 * [HOCl]

Though the reaction rate is dependent on the HOCl concentration, one can calculate the half-life of HOCl which is how long it takes for half of the HOCl to get converted as follows (I'll only use the faster of the two reactions which is the CYA- one):

Half-life of HOCl = -ln(0.5)/34 = 0.02 seconds

It is this rather slow reaction (in chemistry 0.02 seconds seems like an eternity) that is why such high CYA levels are required to make the salt cell more efficient. If CYA reacted faster, then lower levels would "take up" the HOCl fast enough to keep the cell operating efficiently. In fact, if you calculate how long water spends inside the salt cell at normal flow rates, you get a time that is only somewhat longer than the 0.02 seconds which is why the 80 ppm CYA concentration is needed (for today's sizes of cells and chlorine production rates). With a CYA of 20 ppm, the time rises to around 0.1 seconds which is longer than the time water spends inside the salt cell. This means that Cl2 and HOCl build up more inside the cell and that degrades the efficiency of the cell.


Next, I found this derivation on a university web site:

In an electrolysis of sodium chloride solution experiment a current of 7 A was passed for 1 minute

o Electrode equations:
(-) cathode 2H+ + 2e- ==> H2 and (+) anode 2Cl- -2e- ==> Cl2
o (a) Calculate the volume of chlorine gas produced.
Q = I x t, so Q = 7 x 1 x 60 = 420 C
420 C = 420 / 96500 = 0.00435 mol electrons
this will produce 0.00435 / 2 = 0.002176 mol Cl2 (two electrons/molecule)
vol = mol x molar volume = 0.002176 x 24000 = 52.23 cm3 of Cl2
o (b) What volume of hydrogen would be formed?
52.23 cm3 of H2 because two electrons transferred per molecule, same as chlorine.


First, is this formulation correct?
Yes, this computation is correct (some constants were rounded so the final answer isn't as accurate as implied, but its essentially correct). The 0.002176 moles of chlorine is equivalent to (0.002176 moles/minute)*(2*35.4532 g/mole)/(453.59237 grams/pound)*60(min/hr)*24(hours/day) = 0.49 (pounds/24hours)

If so, a cell running for 24 hours would produce 4418658 cm3, 4419 liters or 14 kg (31 lbs). This is 21x the advertised production rate so if true, a majority of chlorine produced quickly reverts back to salt before returning to the pool.


52.23 * 60 * 24 = 75211 cm3, not 4418658 that you calculated. It looks like you multiplied by an extra factor of 60 thinking that the above example was per second when it was done for one minute.

The electrolysis current was 7 Amps. One of the manufacturers I looked up said their cell current was 4.5 to 7.8 amps, presumably adjustable as a "power level". They claim they can produce up to 1.45 pounds per day of chlorine. Remember that we calculated 0.49 pounds per day of Cl2 above. I don't know how to reconcile that, but the cell certainly isn't producing more chlorine than they claim.


Anyway, I have measured the water coming out of the return and, as Sean pointed out, it was only 2-3 ppm higher than the pool water. So my premise is that there may be some HOCL in very high concentrations in the cell but does not survive very long.
There is most certainly some HOCl in very high concentrations in the cell and most of it gets bound up by CYA, but that is also what happens when you add liquid chlorine to the pool.

If instead there were huge amounts of HOCl produced over larger areas of the cell and if this did in fact break down back into chloride ion before getting into the pool, then the "super-chlorination" effect would be real. However, over the short lifetime of the water from the cell and into your pool, the water can tolerate incredibly high concentrations of HOCl without breaking down. Remember that 12.5% chlorine (which is around 11% available chlorine) is over 100,000 ppm in concentration and yet takes months to breakdown (the half-life under normal conditions is over 200 days) naturally (at normal temperature and *not* exposed to sunlight).

Now if you had a special kind of salt cell that had TWO reactions in it where it generated HOCl at one end of the cell and then intentionally destroyed HOCl at the other end of the cell, then this would produce a middle area with high levels of HOCl that could super-chlorinate, but it is my understanding that this is not how the salt cells operate. Instead, they operate more like you described with the electrode equations.

[EDIT]
The fact is that the computations based on current (amps) shows that the salt cell simply does not produce enough chlorine to cause the average chlorine level of the entire cell to be higher than what you are measuring at the outlet which is just a few ppm higher UNLESS some water in the cell is moving much more slowly (more on that below). We have a conflict between the claimed 1.45 pounds/24hour maximum rate and the 0.5 pounds/24hour rate calculated from the amperage, but let's just see what we get using some reasonable numbers. One user in an earlier post estimated 90 GPM flow rates and I'd say that's about the same as my pool as well so let's use that.
0.5(pounds/24hour) * 453.952337(g/pound) * 1000(mg/g) / 24(hours/24hour) / 60(minutes/hour) = 158(mg/minute)
90(gallons/minute) * 3.785(liters/gallon) = 341(liters/minute)
158(mg/minute)/341(liters/minute) = 0.46 ppm
So the increase in chlorine at the outlet would only be 0.46 ppm at maximum. Since people measure more than this, something is wrong. If the 1.45 pounds/24hr is correct and the amperage is wrong, then you would get around 1.5 ppm incrementally higher output which sounds closer to being what people are measuring.

So how is it that someone can measure the ppm inside a cell and find it to be so high, seemingly over a reasonably large volume of the cell? Ignoring problems with how you perform such a measurement, if there are parts of the cell with slowly moving water then this water can indeed have very high chlorine concentration, but that means that most water moving through the cell bypasses this high chlorine area so even though it seems like a good-sized volume of the cell has high chlorine (which is true), it is not true that this "superchlorination" is getting applied to large volumes of pool water.

Sean mentioned how slowing down the water made it possible to give such water enough time to be exposed to the superchlorination so all of that makes sense, but it also means that the amount of total pool water exposed to this superchlorination is very small. So my point about taking many turnovers to get much of the water through this superchlorination is still valid. And this is essentially the same situation that happens when you add liquid chlorine (or bleach) to the pool except that an SWG dones this much more slowly over an extended period of time and therefore is more efficient and better. I just don't like the claim that implies a superchlorination of your entire pool water over a short period of time -- say, a day or two -- and leaving out the fact that bugs or algae stuck to walls aren't super-chlorinated at all.
[END-EDIT]

[brent.roberts]

I've onlhy been running the SWG for a few weeks now and have observed
that the pool liner is "squeaky clean" ... the exact opposite of slimy.

I have been raising the CYA since discovering it all went away during the winter.
It is now in the 40-50 range
Keep the ph in 7.2 to 7.5 range.
CC always at or near 0
FC 3 to 5... still playing with settings to get a balance between run time and power.

We have an unprotected hill top site, so lots of debris. Every morning about 30 to 50 crickets, equal numbers of some other beetle like bug, and 15 or so young frogs ... mostly dead by morning.

The water is astonishingly clear, despite the debris.

Reading all this I'm inclined to keep moving the CYA upward.

[chem geek]

Sean,

I updated the end of my post
See Above (http://www.poolforum.com/pf2/showthread.php?p=28022#post28022)
to reflect what is consistent with your point of slower moving water in the cell getting longer contact time in a superchlorinated environment. We haven't heard from you so I hope I didn't offend. The theories about extra chlorine being generated and then breaking down just don't add up, but the theory of slower moving water at higher chlorine concentration does make sense. It's just that this doesn't actually superchlorinate all of the pool water quickly -- it would take weeks. You can either slow down the flow in a good part of the cell and have this part be exceptionally superchlorinated or you can have the water flow evenly through the entire cell at small chlorine boost, or a combination of the two, but you can't get the full benefits of both (superchlorination and full pool volume) at the same time.

Richard

[waterbear]

Richard,
I don't mean to offend you but it is obvious that you have never worked with or lived with a pool that has a SWG. Chlorine levels will change very slowly with changes in pump run time and cell output...sometimes it can take days for it to stabilize. While you are right that only the water passing through the plates is superchlorinated, in actual practice this seems to be enough to keep the CC at 0 with FC levels that are way below the best guess chart. Also, since algae will first start in the water column before it attaches to the sides and walls of the pool, once again, in actual practice, this seems to be sufficient to prevent algae blooms in normal operation. I view this in the same light as UV sanitizers and ozone generators. Any sanitation only takes place in the reaction chamber and not in the pool proper. The residual chlorine in the water takes care of the rest in all threee of these cases. Perhaps SWGs need to be looked at in the light of this as having 2 separate functions: To generate a residual FC level for the pool and to superchlorinate a small quantity of water in the reaction cell during cell operation to constantly destroy free floating algae and oxidize any nitrogenous compounds in much the same way an ozonator would. As far as superchlorinating all the water, the boost or superchlorinate functions on most units are, IMHO, not really good for this since it can take up to 24 hours to reach the high chlorine levels necessary. I feel it is much more effective to add sodium hypochlorite in the recommeded dosage all at once than to let it gradually climb up. If I understand breakpoint chlorination properly then many undsirable forms of combined chloramines would form at high chlorine levels that are below breakpoint and make breakpoint even more difficult to achieve thus the gradual rise in FC levels by running the celll 24/7 at 100% are not the optimum way to achieve it and may acutally hinder it.
I have only emperical evidence from my own experimentation with my pool but I can tell you this. If I maintain the cya in the 60-80 range and my FC at least 3 ppm and my pH 7.6 or below I do not have any CC nor does the pool ever get cloudy. I have never had an algae breakout since the SWG went online. I have experimented with the above factors and have found that
1. When my CYA is below 60 ppm I have to turn the output up to maintain a 3 ppm FC level.
2. When my pH is above 7.6 I start to show some CC on testing. When it reaches 8.0 my FC level starts to drop.
3. When my FC level is below 3 ppm I have experienced CC and cloudiness of the water.

It is a given that theory often does not hold up in field conditions because of other factors that are not taken into account.

Your thoughts on these points would be appreciated.

[chem geek]

Richard,
I don't mean to offend you
Perhaps SWGs need to be looked at in the light of this as having 2 separate functions: To generate a residual FC level for the pool and to superchlorinate a small quantity of water in the reaction cell during cell operation to constantly destroy free floating algae and oxidize any nitrogenous compounds in much the same way an ozonator would.
It is a given that theory often does not hold up in field condtions because of other factors that are not taken into account.
Your thoughts on these points would be appreciated.
Absolutely no offense taken at all. You are correct that I have zero experience with SWG systems, I agree completely with your points, and I'm the one who has to be careful not to offend. My educated guess as to what is going on is not inconsistent with the real-world results and yes, it's the theory that must match the real world, not the other way around.:) I was just dumbfounded with the initial claim that the salt cell has superchlorinated levels of chlorine AND that this was getting applied to all of the pool water (in a reasonable time).

There is one big difference between UV and ozone sanitizers vs. SWG as far as I understand them and that is that the former do in fact "do their work" on the entire volume of water that flows through their chambers so that after a few hours with one turnover of pool water, the majority (I forget the number and how to calculate it, but I think it's around 70%) of the water in the pool has been sterilized and oxidized at least once. This does not appear to be the case with the SWG and instead some fraction of the water goes through amounts of superchlorination. I could be dead-wrong about how the UV and ozone systems work so if anyone knows if they "slow down" some part of the water in their systems in order to more effectively disinfect and oxidize, please let us know.

Since putting in liquid chlorine (bleach) also superchlorinates a part of the pool water when it is introduced into the pool, then the incremental benefit of the SWG probably derives from its continual dosing which is more optimal. I wonder if people without SWG were to manually dose their pools more frequently (with smaller amounts of chlorine each time) and did so over pool jets at both ends of the pool if they, too, could operate at lower levels of chlorine safely. My guess is that they could, but I have no idea what the required FC level would be between "once a day" well-distributed manual dosing vs. the continual dosing of the SWG.

The 3 ppm FC level that you found to be required for your pool is still very safe from a disinfection point of view since 70 ppm CYA, 7.6 pH and 3 ppm FC gives 0.017 ppm HOCl which is above the 0.011 "minimum" that appears to be needed for disinfection (and that's in non-SWG pools and spas).

Again, I think SWG is great and just because I "bash" a specific claim doesn't mean I think the technology as a whole has no net benefit. On the contrary, I regret not having one installed when we put in our pool (it was not mentioned as an option by our pool contractor and I knew even less about pools then than I do now).

[waterbear]

Absolutely no offense taken at all. You are correct that I have zero experience with SWG systems, I agree completely with your points, and I'm the one who has to be careful not to offend. My educated guess as to what is going on is not inconsistent with the real-world results and yes, it's the theory that must match the real world, not the other way around.:) I was just dumbfounded with the initial claim that the salt cell has superchlorinated levels of chlorine AND that this was getting applied to all of the pool water (in a reasonable time).
You are right that it is only being applied to a fraction of the pool water but it is an ongoing process and I would believe the effect would become cumulative after a while...in much the same way UV sterilization would only kill the bacteria in the reaction tube but with each pass there would be less and less bacteria in the whole of the water. I know that UV light has to be in close proximity to the water for sterilization to occur from my experience with it in aquarium use and not all the water flowing through the tube receives enought intensity to completely kill the pathogens present. Ozone, also only works on the water in the reaction chamber. In fact, no residual ozone is supposed to be in the water when it enters the pool. Once again the benifit seems to be gained from an incremental effect. Plus ozone acutally depletes chlorine levels somewhat so a negative factor is introduced.
There is one big difference between UV and ozone sanitizers vs. SWG as far as I understand them and that is that the former do in fact "do their work" on the entire volume of water that flows through their chambers so that after a few hours with one turnover of pool water, the majority (I forget the number and how to calculate it, but I think it's around 70%) of the water in the pool has been sterilized and oxidized at least once. This does not appear to be the case with the SWG and instead some fraction of the water goes through amounts of superchlorination. I could be dead-wrong about how the UV and ozone systems work so if anyone knows if they "slow down" some part of the water in their systems in order to more effectively disinfect and oxidize, please let us know.

Since putting in liquid chlorine (bleach) also superchlorinates a part of the pool water when it is introduced into the pool, then the incremental benefit of the SWG probably derives from its continual dosing which is more optimal. I wonder if people without SWG were to manually dose their pools more frequently (with smaller amounts of chlorine each time) and did so over pool jets at both ends of the pool if they, too, could operate at lower levels of chlorine safely.
It would seem that systems that use ORP controllers and peristaltic pumps would achieve this but I don't believe that optimin ORP mv levels are reached with lower chlorine concentrations. And it is a well known fact that CYA disturbs ORP readings. Whether these actually have an effect on the actual sanitation I do not know. I would be intersting to compare pools with SWGs and peristaltic pumps at comparable FC levels and CYA levels and see if, in actual use, similar results would be obtained.
My guess is that they could, but I have no idea what the required FC level would be between "once a day" well-distributed manual dosing vs. the continual dosing of the SWG.

The 3 ppm FC level that you found to be required for your pool is still very safe from a disinfection point of view since 70 ppm CYA, 7.6 pH and 3 ppm FC gives 0.017 ppm HOCl which is above the 0.011 "minimum" that appears to be needed for disinfection (and that's in non-SWG pools and spas).

Again, I think SWG is great and just because I "bash" a specific claim doesn't mean I think the technology as a whole has no net benefit. On the contrary, I regret not having one installed when we put in our pool (it was not mentioned as an option by our pool contractor and I knew even less about pools then than I do now). A most interesting discourse!