Quote Originally Posted by CarlD View Post
Richard (Chem_Geek) can give you a far more precise definition, but in short T/A is a measure of the BUFFERING of pH.
I think your definition is just fine. Total Alkalinity (TA) has TWO effects: one is pH buffering, but the other is as a source of rising pH itself (due to carbon dioxide outgassing). So ironically, a lower TA can result in greater pH stability when using hypochlorite sources of chlorine (e.g. chlorinating liquid, bleach, Cal-Hypo, lithium hypochlorite).

The easiest rule to follow is to simply adjust the TA until the pH tends to be stable. If your pH tends to rise over time, you can lower the TA (within reason) to reduce the rate of rise. If your pH tends to fall over time, you can raise the TA to reduce this rate of rise.

After you've settled on a reasonable TA that has the pH be relatively stable, then you can adjust the Calcium Hardness (CH), if needed, to have the saturation index be near zero to protect plaster surfaces. As noted by Carl, this index isn't as important in a vinyl pool so you normally don't worry about the CH (unless it's high since that can lead to scaling).

Richard