When TA was 60, pH would go from 7.3 to 8.0 in 1 week. At TA of 50, it goes from 7.3 to 8.0 in two weeks. I'm planning on bringing TA down to 40 to try to stabilize pH even more. But now I'm concerned about CSI.

How high can/should I raise CH to keep CSI above -0.6? Is there another way to keep CSI up?

Then with winter coming, as the temp drops, what's the best way to keep CSI above -0.6?

FC 3-5
pH 8.0
TA 50
CH 340
CYA 35
Borates 50
Salt 2090 (no SWG)
temp 83 degrees
pool built at least 6 years ago

Thanks!

You shouldn't be bringing the pH down so low since it isn't just higher TA that outgasses faster but lower pH as well. See this chart for how much a pool is over-carbonated at various TA and pH levels. You should never need to lower the pH below 7.5 and should target around 7.7 or 7.8 where it might become more stable (unless the plaster is new which has the pH rise independent of carbon dioxide outgassing). If you raise your CH to 500 ppm, then you should be in great shape even at a low TA level of 40 ppm. The CSI at 7.5 would be around -0.4 while at 7.8 it would be around -0.1. If you wanted to raise your CH to 600 ppm then that would be OK too and would add around 0.1 to the saturation index compared with 500 ppm.

As for the winter, as the temperature drops the pH naturally rises, but if the pH is kept at around 7.8 then even at 50ºF the CSI would be -0.4 and with the water cold any reactions with plaster would be much slower.

Thanks chem geek. I'm only bringing the pH that low to lower the TA- I'm trying to stabilize the pH. I would be a happy clam if the pH would stay stable at 7.8. I just don't like it going over 8.0. I thought adding borates would do the trick, but the pH seems to drift up faster after adding borates (I also added salt at the same time).

What would be the problem if my pool just likes to to have a pH around 8.0? I know FC needs to be higher to have the same effectiveness, but I can live with keeping the FC above 4 or 5. Would that be high enough?

With CYA in the water the active chlorine level is only 14% lower at a pH of 8.0 compared to a pH of 7.5 so having your FC level be about 20% higher than normal should more than make up for that.

The only real issue with the higher pH is the risk of metal staining pool surfaces. If you don't have metals in your water, then that's not an issue. Some municipal water districts have the pH of their tap water at around 8.0. There is also the issue of being able to measure such pH levels since the standard phenol red test won't let you figure out whether you are at 8.2 or higher. 7.8 is easier to see, but 8.0 is harder to tell.

My concern is that the pH rise isn't coming just from carbon dioxide outgassing because such rise should slow down significantly at higher pH and with such a low TA. There is something else apparently contributing to the pH rise so it may not settle at 8.0 and could go higher.

Thanks again for your words of wisdom. The pH rise does seem to slow down once it goes over 7.8, but I'm not sure if it is actually slowing down or if the test is not as accurate.

What do you think could be causing pH to rise?

Do you have any water features that aerate the water (like a waterfall, or fountain, etc.)? That would cause your pH to rise.

No water features- no waterfalls, no fountains. In fact, if I had to aerate, I would have to rig something up, since none of my returns have eyeballs. The returns are about 1 1/2 - 2 feet below the surface and seem to shoot pretty much straight out, level with the water surface. Two of them do create a barely noticeable ripple on the surface about 4-6 feet out from the wall. But I'm guessing that wouldn't be enough to aerate.

I am stumped. Any chance when your water is returning it is bringing little bubbles with it? If you are having cavitation somewhere in your system, that could cause a steady TAEDIT: I meant pH here rise. Maybe back down the intelflo flow rate some and see if that makes a difference?

Thanks for trying to solve my problem, linen. I'm not having a problem with rising TA, though. Just trying to get the pH more steady. I think I'm heading in the right direction. I will try bringing TA down to 40 and bring CH up to keep CSI in line and hope that does the trick.

Thanks again chem geek for all your insight. Your numerous posts have really helped me get a handle on how to handle our pool.

Sorry I meant ph rise, not TA rise. TA will actually go down if there is aeration or cavitation. Fixed in my post.

As I re-read this thread, particurly chemgeeks post, I think my cavitation idea is unlikely, since as he points out, you have low TA therefore not much potential for carbon dioxide outgassing. I think you idea of lowering TA is okay, but you are pretty low right now. I'm still stumped (pretty easy to do it to me)

LOL! I'm pretty easily stumped, too. Since this is my first year owning a pool (or does it own me?), I'm giving myself a break for not knowing evreything (like chem geek) yet.