Thanks for the info.
I'll say this - the borates are working.....
Before borates - 0.2 pH unit rise per day and 20oz of acid to down adjust pH
After borates - 0.06 pH unit rise and about 60oz of acid to down adjust pH
0.2/0.06 = 3.33
60oz/20oz = 3.0
When borates are applied to water, they do not change the total amount of acid needed over time. The only thing they do is slow the pH rise. So, if your pH rise slows by a factor of 3, then the amount of acid needed will also go up by a factor of 3. In other words, adding borates doesn't reduce acid usage, you will use the same amount of acid both before and after borates. The only thing borates do is stretch out the frequency a bit. In your case, the change wasn't all that big, so I can see why you're disappointed with it.
But here's the problem - plaster pools (with bad plaster jobs), along with fill water, will add alkalinity to the pool and thus put greater pressure on the pH. If your plaster is still emitting CaOH then that will cause pH to rise faster. Also, if the carbonate is dissolving from the plaster, that will make it harder to keep down the TA. If you can figure out your CH rise based on your evaporation rates (what you would expect theoretically) then any excess in CH rise, not from what you've added externally with the calcium chloride, will be related to the plaster as that is really the only other source of CH. There really shouldn't be anymore CaOH coming out of your plaster now that it is 15 months old, but a really bad plaster job where nodules may be forming or cracks that might expose the underlying plaster would cause such emissions. I doubt that's the case for your pool unless you're keeping your CSI too negative, in which case that can cause plaster to dissolve (although quite slowly).
Thank you Matt,
Given your enlightening math explanation, maybe going up from 40 to 50-55 ppm borates could further slow the rise? What do you think? If overall acid usage is truly the same, then what do I have to lose?
There's definitely no cracks in my plaster and it is nice and smooth with no nodules forming. CSI right now is -0.14 to -0.22 per the calculator depending on pH.
I'm bad about putting my pool cover on, so here in dry CA my evaporation might be fairly high.