A Theory About Rising pH in SWG Pools

[nater]

Sounds like a plan.

Just tested again, and pH is up 0.1 to 7.3, and Alk has dropped 20ish to 110 with only the jets "rippling" the surface. I'll turn on the fountain tonight, keep the SWC at 30% OP and track CL, ALK, and pH with temp over the next several days while not adding anything to the pool (barring any unpleasant surprises from Ernesto).

Question: With TA around 80, what's my lower limit on pH to limit any potential corrosion to my light ring or SWC cell?

Thanks.

[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.