Sudden .3 pH drop - what caused it

[spidey07]

I'm using pucks and the pH has been rock solid at 7.7 for 6+ weeks now. Almost to the point of I felt redundant testing it every evening or two...look at the color, yep 7.7 as always. TA was 100 but I haven't tested it in 3+ weeks so I "assumed" my TA was good because of pH stability.

With unusualy cooler temps (low 70s highs, low 50s low) I put my solar cover on for 24 hours for a pool party on Sunday. Sunday had high bather load most of the day, no trouble with FC or clarity. I tested FC at 7 and pH was 7.4. That got my attention as pH has been so rock stable. Off to check TA = 70.

So my TA was probably why my pH went down (pucks are acidic). Bumped TA to 110 with baking soda which also raised pH to 7.5 so I'm going to let it sit there and see what happens.

I guess my question is why the sudden drop? Bathers? Sun not being on pool because of cover? There are two things that changed, so I'm trying to understand better. Solar cover on, heaviest bather load of the season were the changes not to mention radically cooler temperatures.

Thanks in advance.
pH 7.7 -> 7.4 -> 7.5 (baking soda addition)
TA 70 -> 110 (baking soda)
FC = 6-7
CYA=50
CH=90

-edit-
I should have known something was up when opening my eyes underwater the water wasn't "perfect"

 

[jblizzle]

Are the pucks in an automatic feeder? Did you run the pump more before/during/after the high bather load?

 

[spidey07]

Are the pucks in an automatic feeder? Did you run the pump more before/during/after the high bather load?

Yes, no change in that or the amount. One of those hayward inline chlorinator models. Pump runs 24x7.

 

[spidey07]

Go figure. Back to it's happy spot.

pH = 7.7
TA = 110-120 (115 targeted)
FC=7
CC=0

 

[jblizzle]

Had me stumped ... will be curious if it happens to be repeatable.

 

[spidey07]

Had me stumped ... will be curious if it happens to be repeatable.

Heh, wife said "The water feels so much better! What did you do!"

I fixed the pH. Just the 10 some pounds of baking soda got my ALK back in line while raising pH some.

It's like you guys always said, you'll get to know your water/pool and can tell something's up by the way it feels/looks.

 

[JamesW]

An important part of how the TA offsets pH drop is carbon dioxide offgassing. The cover would reduce offgassing (and) the subsequent pH rise.

 

[spidey07]

An important part of how the TA offsets pH drop is carbon dioxide offgassing. The cover would reduce offgassing the subsequent pH rise.

BINGO! Yep, that makes perfect sense. Carbonic acid.

I remember now from my reef aquarium keeping days, no outgas of CO2.

Mods - feel free to move to deep end. I'll just leave this here...

http://reefkeeping.com/issues/2005-03/rhf/#4

 

[spidey07]

Update. Had the cover on plenty and since raising TA pH is still rock solid at 7.7.