Low TA and High pH - what happened?

[rodrigo]

I've been following TFP methods for maintaining my pool for several years. I've had clear water and no real balancing problems, but I had something happen this week which has me befuddled.

Sunday afternoon I did my usual clean up on the pool (in ground fiberglass, 12.4K gallons, SWG, no water features). I vacuumed and backwashed, during which I normally turn off the SWG and heater. Unfortunately this time I forgot to turn 'em back on. I was out of town until yesterday (Thursday). I went out to check on things and noticed my mistake and had a brief panic attack. However, the water was still crystal clear and beautiful. I turned the SWG back on and ran the system overnight to get my FC back up.

I use a Taylor reagents to test, and almost always use FAS-DPD to check chlorine since my OTO comparator (sp?) only goes to 5.0 FC (I prefer my FC to be a bit higher).

My weekend test results (before I this whole episode went down):
pH 7.8
FC 6.5 (high according to some, but it's where I like mine to be)
CC 0
TA 90

Then yesterday evening (when I found my mistake and turned it back on after four days of no chlorine generation):
pH 7.6
FC 1.0
CC 0
TA 60 (a huge drop, and the very bottom range of where I want my TA)

Then this morning (after running the system all night with SWG @ 80% to get my FC back up):
pH 8.0
FC 8.5
CC 0
TA 60

Then again today at noontime:
pH 8.0
FC 8.0
CC 0
TA 60

I checked pH twice this morning because I couldn't believe it had shot up that high overnight. I understand that low TA will trigger pH swings... but wow. I never, ever have adjust my TA or pH upward. Maybe once or twice in the 10 years since we built the pool. With my SWG, my pH behaves very predictably. It creeps up to 7.8 over a month or so, I add acid to bring it down to 7.4, repeat. TA seems to move in lockstep.

Acting on instinct I added muriatic acid sufficient to reduce my pH to 7.6 when I as home at lunchtime today (I did not have time to retest afterward), and if my experience holds I'll probably find my TA down around 40 when I retest tonight.

My CYA level was 70 about two weeks ago. I know I should increase that with my SWG, but just haven't gotten around to adding any yet.

I've read some threads about using cycles of baking soda/acid to get both values back in range and I can manage that process. But I'm curious as to what triggered this? Was it because the pool had four days of no 'aeration' via the SWG cell? Something else?

 

[pabeader]

A SWG will cause the pH to rise. Not that much usually, but if you had it on high that might account for it.

 

[chem geek]

The question is what caused the TA to drop unless acid was added. When the SWG was off the chlorine dropped and that is net acidic, but not that much. I calculate that the pH would drop from 7.8 to 7.55 if there were no carbon dioxide outgassing but the TA should only have dropped from 90 to 86 ppm. Then when the SWG went back on and the FC went back up, the pH and TA should have gone back to where they started. While the pH is behaving pretty much as expected, the TA is not.