A Theory About Rising pH in SWG Pools

[chem geek]

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

[KurtV]

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.

[chem geek]

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

[KurtV]

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

[nater]

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.

[chem geek]

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

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.

We should add cover usage to the variable list.

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

[Sentient]

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.

I missed that, what was the nozzle experiment?

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!).

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.

[chem geek]

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.

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

[Sentient]

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.

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

[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.