Quote Originally Posted by chem geek
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2) Curing of plaster (technically, the concrete in plaster/gunite). When enough Calcium Hydroxide has been produced from the curing process to raise the Calcium Hardness level by 10 ppm (which is probably the minimum detectable amount in test kits), this causes a rise (with a TA of 100) of 0.91 in pH so that 43 ounces of Muriatic Acid are needed to restore back to a 7.5 pH. So, the rise in CH will be relatively slow, though if you see it rise about 10 ppm after cumulatively adding 5-6 cups of acid over time, then this may be the cause of the pH rise. I also would expect that this curing process wouldn't last (in significant amounts) much more than 1 year at the most and that most of the curing would occur in the first 3-6 months. Don't forget that backwashing DE and sand filters as well as splash-out will dilute the CH (and everything else) over time so may hide the rise in CH (at a CH of 300, a loss of 10 is a 3.3% dilution or 333 gallons per 10,000 gallons in a pool). On the other hand, evaporation and refill will tend to increase CH unless the fill water has no CH.

This is why I asked Pat whether he was continuing to see calcium hardness increase. I have not see any increase at all in the 8 or 9 months I've had my Diamond Brite finish installed. None. In fact, because my fill water has a CH of 50 ppm, I occasionally have to add calcium to replace that lost to splash out and backwashing. This can't be the source of my pH rise.

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4) Other factors. These include fill water that is high in pH. Bather sweat is probably not very alkaline and rain is normally acidic so these are not likely to increase the pH.
My fill water has a pH of about 8.0 but as I add little of that I probably isn't a significant contributor.

Kurt's data showed that at least part of the pH rise was due to the outgassing of CO2 since his TA kept dropping. Kurt, if you are willing, continue to let it drop to about 50 (see this post for an example of where that worked for someone else. That still provides some buffering, but would virtually slow to a crawl the CO2 outgassing (at a pH of 7.5 or above). Someone who has an SWCG system and experiences rising pH can also try the experiment I mention in #3 above to lower your power setting and increase your time on your SWCG. Let us know what happens if you do this! Oh yes, and Kurt, how old is your pool if it's plaster/gunite?
I suspect you're right about a significant portion of my pH rise coming from CO2 outgassing. I've been letting the alkalinity drop for several weeks after reading your other posts on the subject but haven't seen any benefit from it so far. I'll let it continue down to 50 ppm; I'll probably be there within a week.

Kurt, if you do conclude that your specific source of rising pH is the outgassing of CO2, then you might consider adding 50 ppm Borates (sodium tetraborate) to act as a substitute buffer replacing part of your carbonate buffer system. That would let you run with much, much lower TA readings. Before we get to that point, however, we need to see if you can get to a low TA level where the pH rise slows down significantly.
I've considered this also since reading Evan's posts on the subject. I'll probably wait until later this fall when the pool is unswimmably cool to start messing around with that.

Richard
I'll update my data this evening. I might actually be starting to see a bit of slackening in the rate of pH rise.

Is there starting to be some consensus here that SWGs themselves have gotten an undseserved reputation as the source for constant pH rise even if we can figure out what the real culprit is?

Pat,
You might be interested to know that I finally took the SWG plunge. I'm hoping my AquaRite will arrive in the next couple of days.