Quote Originally Posted by Simmons99
TA will reduce when you add acid - get PH down to 7.2-7.4, test an hour later, then add baking soda to raise the TA to around 100. See if that works.

I believe (but others can verify) that with the TA that low - it will allow your PH to bounce around (go higher) so that you have to add acid more than necessary, which reduces the TA.

Also - are you aerating your pool with your PH below 7.2? If so - it will cause your PH to raise and your TA to lower.
You shouldn't worry too much about your pH bouncing around even at a lower TA of 70. This is plenty of buffering unless you use Tri-Chlor tablets or MPS regularly since both are very acidic. The bottom line is that if you don't see much of a pH change (and you can measure the extremes in pH about an hour after you add chlorine and then again just before you add chlorine the next time) then you're fine.

A lower TA of 70 will let you maintain a lower pH or even a normal 7.4-7.5 pH much more comfortably with less pH drift. You can refer to this graph or this table to see how lower pH and higher TA lead to more outgassing of carbon dioxide which is what causes pH to rise in most pools. Exactly how much of a rise is dependent on aeration and that's something I can't predict nor put into graphs or tables!

Richard