A Theory About Rising pH in SWG Pools

Since you have a vinyl pool, you can't have corrosion in the "calcium carbonate" sense which is why you don't have your CH up at 300 or so as you would in a plaster/gunite pool. Corrosion of metal surfaces (especially stainless steel, galvanized metal, and copper) is unlikely to occur if you keep your pH above 7.0 at all times and I would just shoot for around 7.4-7.6 which is generally better on the eyes anyway. The purpose of TA in your vinyl pool isn't so much to prevent corrosion as it is to act as a pH buffer, but you don't need much of it to get this buffering effect and as you've found out, too much is not good either as it causes more CO2 outgassing (and pH rise).

Richard

Another update on my situation: Alkalinty is now at 50 ppm and pH has gone from 7.3 to 7.4 over the last 4 days without adding any muriatic acid; a vast improvement. We'll see how it goes from here but I'm very hopeful that my acid comsumption is going to go way down.

As to my original theory, from all the stories I've read here and elsewhere, I'm becoming covinced that it's at least somewhat valid; SWGs themselves are not the underlying cause of rising pH.

Kurt's post just had me get the idea that perhaps the hydrogen gas generation in the salt cell which has hydrogen gas bubbles coming out of the return jets may act like "aeration" and accelerate the outgassing of carbon dioxide. You know how dropping sugar crystals or other substances into carbonated beverages (including champagne) has lots of bubbles coming from these "nucleation sites"? Well, perhaps hydrogen gas bubbles act somewhat like nucleation sites for the dissolved carbon dioxide -- or they act as mini-atmospheres just as bubbling air might do in the pool water.

In fact, given how incredibly productive the TA lowering procedure is when one uses "tiny bubbles" from an air compressor, I really think that the hydrogen gas from the SWCG systems may simply be accelerating the carbon dioxide outgassing process (sorry for repeating myself, but I'm really excited about this, thus proving once and for all that I am a true nerd).

Richard

Richard,
Sounds plausible, but why doesn't everyone with an SWG see the constant upward presure on pH?

Has anyone discussed how a solar cover comes into play? The other change in my pool besides adding the fountain was NOT using the solar cover due to the hotter weather. With the cover on, there's a very limited surface area for the CO2 exchange. We should add cover usage to the variable list.

Quote:

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Originally Posted by KurtV

Richard,
Sounds plausible, but why doesn't everyone with an SWG see the constant upward presure on pH?

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Possibly due to lower TA and higher pH that some people have. Maybe some SWG's produce different sized bubbles -- larger bubbles would be less efficient than smaller bubbles as was borne out by the "nozzle" experiment to lower TA.

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Originally Posted by nater

We should add cover usage to the variable list.

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Yes, using a cover significantly reduces CO2 outgassing. Or course, with an SWCG system, are people told never to use a cover or to only cover part of their pool? Otherwise they could build up a large bubble of hydrogen gas under their cover. It's not "explosive" the way propane would be, but it certainly burns (if ignited by flame or a spark) when mixed with oxygen (remember the Hindenburg!).

Richard

Quote:

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Originally Posted by chem geek

Possibly due to lower TA and higher pH that some people have. Maybe some SWG's produce different sized bubbles -- larger bubbles would be less efficient than smaller bubbles as was borne out by the "nozzle" experiment to lower TA.

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I missed that, what was the nozzle experiment?

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Originally Posted by chem geek

Yes, using a cover significantly reduces CO2 outgassing. Or course, with an SWCG system, are people told never to use a cover or to only cover part of their pool? Otherwise they could build up a large bubble of hydrogen gas under their cover. It's not "explosive" the way propane would be, but it certainly burns (if ignited by flame or a spark) when mixed with oxygen (remember the Hindenburg!).

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I have a cover and run the generator with it covered often. If it builds up a bubble, I don't see it. I suspect the cover is porous to hydrogen and it isn't getting a chance to build up. Your speculation does suggest an experiment though. Holding a lighter or match above the bubble stream of my primary return should produce some combustion if hydrogen is being produced. It would not be visible to the eye, but should show up on a digital camera or digital video camera. Those image sensors are quite sensitive to IR. Would chlorine burn too? Would there be a characterisitc color?

This is a don't try this at home, mythbuster kind of experiment. I suspect there is potential for real danger, small potential, but real.

Actually I am more tempted to look on the underside of the cover for discoloration. Since I see significant pH rises, if it is due to chlorine gas in the bubble stream going into my pool when the SWG is on, then there should be fading or hardening of the material above the first return jet where most of the bubbles enter the pool. Perhaps I can check those ideas out over the weekend.

Quote:

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Originally Posted by Sentient

I missed that, what was the nozzle experiment?

Holding a lighter or match above the bubble stream of my primary return should produce some combustion if hydrogen is being produced. It would not be visible to the eye, but should show up on a digital camera or digital video camera. Those image sensors are quite sensitive to IR. Would chlorine burn too? Would there be a characterisitc color?

Actually I am more tempted to look on the underside of the cover for discoloration. Since I see significant pH rises, if it is due to chlorine gas in the bubble stream going into my pool when the SWG is on, then there should be fading or hardening of the material above the first return jet where most of the bubbles enter the pool. Perhaps I can check those ideas out over the weekend.

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Look at this thread for more info on the use of compressors, on nozzle size and bubble size, and other info related to aeration.

If you do the flame experiment with the pool uncovered, then you aren't going to do anything dangerous since the amount (rate) of hydrogen produced isn't particularly large. In fact, I used to do some water experiments at home producing oxygen/hydrogen (using carbon cores from batteries for the "plates") and also could smell some chlorine from it as well, especially when I added lots of salt. When I lit the hydrogen bubbles, you would just get some minor popping. I didn't notice any color.

Chlorine is not flammable or combustible (it's an oxidizer just like oxygen it can't "burn") though it can be combustible (generate lots of heat) when combined with other compounds such as ammonia.

I don't know what chlorine gas does to covers. It is certainly a corrosive oxidant, but I just don't know what, if anything, it would do to a solar cover. Let us know your results of all your experiments and observations, and please be careful in any event -- the use of gloves and goggles would be wise to be extra safe.

Richard

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Originally Posted by chem geek

Look at this thread for more info on the use of compressors, on nozzle size and bubble size, and other info related to aeration.
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I don't know what chlorine gas does to covers. It is certainly a corrosive oxidant, but I just don't know what, if anything, it would do to a solar cover. Let us know your results of all your experiments and observations, and please be careful in any event -- the use of gloves and goggles would be wise to be extra safe.

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Thanks for the link to the aeration post. I am trying to get my alkalinity down as others are. I want to see if it reduces my acid usage a bit.

I swam under the cover and see no discoloration at all. So either my bubbles are pretty much all hydrogen, or the cover is immune to the chlorine, at least in a visible way. So I would call that a non-result with no real information gained. I didn't have time to play with fire this weekend, perhaps tonight.

Mark

I go through about 1/2 gallon a week of muriatic acid on my 19,000 gal in ground pool, and I have an AquaPure 1400 SWG. I have a waterfall feature that I use rarely, so a majority of the time the pool just vacs & filters.

pool is relatively new, plaster bottom, and I check pH twice a week.

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Originally Posted by klharmon

I go through about 1/2 gallon a week of muriatic acid on my 19,000 gal in ground pool, and I have an AquaPure 1400 SWG. I have a waterfall feature that I use rarely, so a majority of the time the pool just vacs & filters.

pool is relatively new, plaster bottom, and I check pH twice a week.

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What's your TA, CYA and pH?

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Originally Posted by KurtV

Richard,
Updated data. I think the acid demad might be letting up a bit.

Date pH Alk. Acid addition
7/26 7.8 100 24 oz.
7/27 7.4 100
7/28 7.8 100 16 oz.
7/29 7.7 100 16 oz.
7/30 7.2 90 32 oz. (before testing)
7/31 Vacation
8/1 Vacation 16 oz.
8/2 Vacation
8/3 Vacation 16 oz.
8/4 7.8 90 28 oz.
8/5 7.6 80 16 oz.
8/6 7.6 80 16 oz.
8/7 7.4 80 8 oz.
8/8 7.6 70 16 oz.
8/9 7.6 70 16 oz.
8/10 7.5 70 16 oz.
8/11 7.3 70 8 oz.
8/12 7.4 70 16 oz.
8/13 7.6 70 16 oz.
8/14 Work trip
8/15 7.6 70 16 oz.
8/16 7.6 70 24 oz.
8/17 7.4 [IMG]file:///C:/DOCUME%7E1/CLV/LOCALS%7E1/Temp/moz-screenshot-2.jpg[/IMG][IMG]file:///C:/DOCUME%7E1/CLV/LOCALS%7E1/Temp/moz-screenshot-3.jpg[/IMG]65 16 oz.
8/18 7.4 60 8 oz.
9/19 7.5 60 20 oz.
8/20 7.2 60 8 oz.
8/21 7.4 60 8 oz.

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On another pool forum (poolcenter.com) in this thread via an unfortunately not-so-nice discussion, I calculated whether the SWG produces enough hydrogen gas bubble volume to have the small amount of carbon dioxide that would be allowed into it (via Henry's Law for equilibrium) to make the pH rise. It isn't enough and that's under ideal conditions with full equilibrium reached (TxPool says the kinetics don't allow for any effective transfer, but I'm not sure about that -- in any event, the volume just isn't enough).

So, I'm back to leaning towards the other explanation I originally started with in this post in this thread where I thought it might be undissolved chlorine gas bubbles. If this is the case, then one way to improve the situation could be to point the returns downward to give the bubbles a greater chance of dissolving more completely into the water. The lower TA effect that KurtV saw wasn't huge and could just be regular carbon dioxide outgassing without much change from the SWG itself -- that is, the SWG effect may mostly be chlorine gas escaping.

KurtV (or anyone else with an SWG that would like to experiment), if you are still experiencing a rising pH in your SWG pool, can you try pointing the returns downward and seeing if that makes any difference? Can you see the bubbles at night to see if they linger longer when the returns point downwards or if you notice any of them getting smaller in the water (dissolving)? Do you ever smell any fresh chlorine smell over the return where the bubbles come up compared to other parts of the pool where there are no bubbles?

If the chlorine outgassing is a primary cause for the incremental pH rise in SWG pools, then I would expect this effect to be worst in pools with short distances between the SWG and the pool returns and for 2-speed or variable speed pumps when the pump is on high speed but not on low speed. If pointing the returns downwards helps, then one has the trade off of rising pH vs. potentially worse cleaning of the pool water surface into the skimmer (which I think is better when the returns are not pointed downwards).

Richard

My pool has three returns, the third does not bubble at all, I will leave it pointing up and point the other two down. I have a cover, which I keep on most of the time when the pool is not in use and need to add 1Qt of acid a week. It will be interesting to see if this makes any difference. I guess it is finally time to start a log so I can figure out what really is going on versus my gut feel.

Mark

Mark,

That's REALLY interesting that one of your returns doesn't bubble at all. I assume all three are coming from the output of your SWG, right? You don't have any sort of bypass of flow with some going around your SWG to one return, right?

If the SWG output is going to all three returns, then is the one return with no visible bubbles at a farther distance away from your SWG? If so, then the theory of the chlorine bubbles needing more time to dissolve may very well be on the right track.

I would think that a pool cover would keep the chlorine in contact with the water longer so that it would eventually dissolve and not escape, but I suppose we'll find out.

I'm looking forward to the results of your experiment. Thanks!

Richard

Yes, the first return is about 15 feet from the pool equipment, the second about 25 and the third about 40 feet away. My presumption has been that the bubbles all escaped at the earlier ports, but it could be absorption. I will ask some family members to also give the area where the bubbles surface a sniff to see if they can smell chlorine. Many of the tiny bubbles remain in suspension, never surfacing. I can wipe them off of any surface underwater with my hands, including the cleaner hose which is 20 feet from the nearest return.

There is no bypass of any kind. I have only a single speed pump and the flow rate is pretty substantial.

All my returns, 1 - do not bubble, at least that I can tell, 2 - are pointed down because my cleaner instructions said to do that. I still take 3 pints (30K gallon plaster inground) every week.

Just an F.Y.I.

Thanks. This is just one of those areas with multiple factors. Lowering TA does help, but doesn't always help that much.

Not having any bubbles at all doesn't make that much sense since there should at least be hydrogen gas bubbles since I would expect the water to get close to fully saturated and for the hydrogen bubbles to outgas -- at least by the end of the day with the SWG still running. You might take a look at night with an interior pool light turned on (and, obviously, turning on the pump and SWG). Many times people don't see bubbles during the day, but can readily see them at night.

Richard

I'll have to keep thought in mind as I can't see squat right now thanks to Hurricane Ike and no power. If I ever get back in (running out of season now) I'll post back.