My comments in bold in your quoted post below.

Quote Originally Posted by jereece
Thanks for your very interesing post. I want to learn more about this, because not too long ago I made a similar post about pH increasing and was told to increase my TA from 80 to 100 - 125. Here is a link to that post: http://www.poolforum.com/pf2/showthread.php?t=3715.
Thank you so much for including this link. The advice that was given to you was correct for the problem you described at that time because a large pH shift at the lower TA of 80 will get reduced (somewhat) at higher TA because higher TA buffers the water to make the pH move less when there is a "push" to make the pH move (in your case, from base to increase pH). TA can be confusing because having a low TA makes your pool more easily change in pH, but a high TA makes your pool more likely to rise in pH from outgassing carbon dioxide. With more info, a "balance" can be obtained though it will still result in a rise in pH, but will hopefully lower the amount of acid you need to add each week.

I do not have any water features that would increase aeration and I normally try to keep my return flow so that it does not splash. I have read on this forum that splashing can decrease TA, so I have always tried to keep return flow from splashing very much.
What you read on this forum is correct. Splashing, combined with adding acid, will decrease TA. When you splash, you can increase the transfer of carbon dioxide from the pool water droplets into the air and this process raises pH without changing alkalinity. Then, when you later add acid to compensate to restore the pH, you lower both pH and alkalinity. So it is the combination of splashing and adding acid that lowers alkalinity, but for practical purposes, you can think of splashing as lowering alkalinity which is what you've heard on this forum.

I believe I do see TA slowly decreasing. I have not taken notes on this, but my recollection is that it slowly decreases over time. I accounted this to the process you describe being accelerated when I add acid. Normally when my pH gets to 7.8 I will add acid to get it back down to 7.4 or so. But even when my TA was 80 (earlier this year), I still saw pH rising at about the same rate. After increasing the TA, I have not noticed pH rising slower.
In the post you linked to (above), you mention adjusting from a pH of 8.0 down to 7.4 once a week and that's a larger change than you are dealing with now which is from 7.8 to 7.4 (or 7.7 to 7.4 since you said a 0.2-0.3 pH change per week) so it does look like the higher TA is helping to reduce the pH bounce somewhat (which is expected). On the other hand, it may also be starting to cause an additional pH increase with outgassing -- this is why the change in alkalinity over time is important to note as this is how to answer what is going on.

Do you notice needing to add more acid per week now than you did when the TA was at 80?

So, are you saying that my TA will continue to decrease until it gets to a stable concentration and then pretty much stay there. And at that point, my pH should only rise about 0.1 units per week? If that's true, that would be great. I will pay more attention to my TA and see if I can notice where it gets to a stable point and if my pH increase slows.
Well, sort of, but not quite. At the recommended pH near 7.5 and TA near 80-120, a pool is always going to be out-of-balance and will want to outgas carbon dioxide. So there will always be some pressure for pH to increase. The idea is to find your pool's ideal sweet spot where the outgas effect is relatively small (TA is low enough) while at the same time you have sufficient buffer to minimize your changes in pH (TA is high enough). That's two competing factors. At the sweet spot, the amount of added acid each week is smaller but without too much of an annoying pH swing.

Very interesting info, but it seems to go against what I have read so far which is to keep my TA around 125.
With the additional info you provided that earlier on you had a TA of 80 and had a large increase in pH, then the advice is not inconsistent. You were clearly in a bad situation with the TA of 80 and you are in a somewhat better situation now with a TA of 125, but perhaps you might be in an even better situation with a different TA (maybe 100 or 110?) but we really haven't figured out what that should be yet.

Thanks again for the interesting info.

Jim
Jim,

Since you still had a very high tendency to increase pH even when the TA was low, I don't think the main problem you are having is outgassing of carbon dioxide. Yes, by setting the TA as high as you have you may have started to introduce a little more extra pH increase as would be seen by a more rapid (but still quite slow) drop in alkalinity and a small increase in the amount of acid you need to add. So perhaps your TA should be dropped a little bit (which will happen slowly over time if you don't add any bicarbonate of soda), but that will not solve the core problem you are having, namely that something is adding a lot of extra base to your pool.

Unless there are other chemicals you are adding to your pool besides chlorine and acid, this extra base being added to your pool must either be coming from the chlorine you are using or from something entering into your pool. One thing you could try, just for a few weeks, is to use a different source of bleach. Even though it's not part of the BBB system, if you go to the pool store and get the more concentrated 12% liquid chlorine, then at least I'm more certain of the amount of base that is in it. Just remember that you only need to add about half as much as you were before. At least this is something to try.

If anyone else has any ideas as to the possible source of this pH increase that isn't significantly affected by TA, please chime in.

Richard