pH Instability - What TA to shoot for?

[sfbayjay]

Hi all. Confused here (again) - looking for some help and advice.

Drained and refilled my pool about a month ago to deal with high CYA. Before drain/refill I could keep pH dialed in adding relatively small doses of acid about once each week. TA during that timeframe was 110 ppm or so.

(Note in the graph, linked below, drain/refill was done on 5/12-13 - that's the big spike in pH and TA, which is due to high values in refill (city) water)

Refill water had high TA, around 200 ppm. So, I diligently have been adding acid to knock pH down to around 7.1, then letting it climb back up, then knocking it back down, etc.

Now, TA is sitting around 90 ppm, lower than before drain/refill, but I'm seeing very rapid rise of pH between acid additions. I'm confused - why isn't it showing similar behavior to before (slow upward drift) now that TA is back down to, and even below, pre-refill level?

Attaching a graph showing pH, TA, and acid additions both before and after drain/refill.

The only other differences in the water between before/after refill are:
CYA (dropped from about 175 to 35)
CH dropped from about 450 to 300
Temp up from around 79 to around 85

Can't think of anything else. What can I do to stabilize pH? Should I continue to lower alkalinity?

Thanks for any insights!

Can't seem to insert image of graph, so here's a link: PH/TA/Acid Chart

 

[mknauss]

What were you using for chlorination before you drained? If it was trichlor (pucks), those are acidic, thus they would have kept your pH rise in check for the most part.

 

[sfbayjay]

I've been using liquid chlorine all along. 10.5% in 2.5-gallon jugs. No pucks - prior owner used all pucks and I inherited his CYA problem as a result when I bought the house in Feb.

 

[mknauss]

I would assume you have more evaporation now than before you drained if that was before summer? Not sure how much evaporation you have in Florida - we have lots here in Nevada.

If you are adding more high TA water to combat the evaporation now than before that would increase your pH rise due to your high TA water.

If you are getting lots of rain that can also raise your pH as the rain drops are a form of aeration.

Hope you find what works.

Take care.

 

[sfbayjay]

Thanks for the ideas.

We get a fair amount of evaporation, but we also get a lot of rain. Rainwater has been more than making up for evaporation lately, so I don't think I'm adding TA. You can see constantly declining TA numbers in the graph I linked in my original post.

Rainwater, as I understand it, is typically acidic, so I would think it would hold pH more in check now that's it's started raining more. Our pool is under a screened enclosure, so the drops that do hit the pool have been made tiny by passing through the screen. It's interesting to watch - big fat drops outside and kind of a mist hitting the pool through the screen.

I'm stumped. I guess I'll continue to let the TA drop and see if things start to stabilize, but I can't figure out what is different about the chemistry now as opposed to before I drained and refilled.

 

[pooldv]

Let TA drop as low as 60. Rain water is not typically acidic or basic, it varies. Rain does tend to push pH up due to aeration of the water surface.

 

[sfbayjay]

Thanks for this - I'll continue to monitor pH trends and let TA slowly drop. We're getting some significant rain here now, so that in combination with acid additions should keep the TA trending downward. Hopefully pH will stabilize. I still can't figure out just what's different now versus before I drained, but if I can get pH to level out maybe that's not important to figure out.