Strange issue with Alkalinity and pH

I have a in ground pool, ~13,000 gallons, about 10 years old with pebble tech finish. It has a spa that spills into the main pool. I've always kept the alkalinity level between 80-100 for my SWG. During the last 2 months, we had some heavy rains that lowered the alkalinity to ~40. When checking the water chemistry, I saw that the pH was only 7.4. Also, since I usually add liquid acid once a week to keep the pH in the acceptable range, constantly adding acid probably help reduce the alkalinity also.

Hum...that is weird because I usually have to add liquid acid one a week to keep the pH in the proper range. So I ignored both values and waited another week. In the next test, the pH only rose to 7.6, and the alkalinity remained at 40.

So now my question: why would they recommend to have the alkalinity at a higher level, when it causes the pH to raise significantly in a week? According to the mfg of the SWG cell, the recommend range is 80-100.

Current readings today using a Taylor test kit (the one with the powder for CC count)
Water temp: 68 F
FC: 2.6
CC: 0
pH: 8+
TA: 90
CH: 425
CYA: 80
Phosphate: 0
Salt: 2200 (normally at 3000, but due to rains, was deluded somewhat. Okay because my cell is not connected during the winter months - I use a bypass and liquid chlorine during this period).

So come next Sunday the 15th when I check the water again, I expect the pH to be back up to 8 after adding acid to bring it down to 7.2.

Well, I wouldn't wait until Sunday. I would recommend lowering the pH ASAP and then bring your FC up with some liquid chlorine. Your SWG can't bring it up fast enough. You want your FC Between 6-9 for CYA of 80. Never below 5. Your pH and FC are more important than TA right now.

Thanks for the reply,
I always lower the pH to about 7.2, after adding acid, waiting about 4-5 hours, then check it again. Usually comes in at 7.2-7.4, but by the weekend again, it will be at 8. I know my FC is low, but that is okay since I also added liquid chlorine at the same time. I may have to insert my cell since the temperatures here in Southern CA have increased. Today, it was 80 degrees F. Water temp is approaching 70.

I wonder if adding borates would help stabilize pH?

I maintain 60 ppm borates along with keeping TA about 80. Even running the SWCG my pH stays between 7.6 - 7.8 without having to add any acid.

If you really want to attack the TA issue, add baking soda in small quantities until you reach your goal or pH stabilizes. It will raise your pH some, so I still recommend lowering the pH first.

Jim has an interesting idea. When you get your pH and TA stable, adding borates has many benefits. The process of adding them is a roller coaster ride that swings your pH high with borax and low with MA, so I would recommend waiting for now.
Here is a link to the process: http://www.poolforum.com/pf2/showthr...stant-to-Algae
If you decide to go for it, test part way through. I found that I reached 50ppm about half way through the recommended dosage.

See our last posts, but also: I reread your OP and I think your question was about why they recommended 80-100 TA. Jim can correct me if I'm wrong since I know only a little about SWGs. I believe the SWG system causes a certain amount of aeration/out-gassing which lowers TA a bit over time.

Quote:

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Originally Posted by FormerBromineUser

If you really want to attack the TA issue, add baking soda in small quantities until you reach your goal or pH stabilizes. It will raise your pH some, so I still recommend lowering the pH first.

Jim has an interesting idea. When you get your pH and TA stable, adding borates has many benefits. The process of adding them is a roller coaster ride that swings your pH high with borax and low with MA, so I would recommend waiting for now.
Here is a link to the process: http://www.poolforum.com/pf2/showthr...stant-to-Algae
If you decide to go for it, test part way through. I found that I reached 50ppm about half way through the recommended dosage.

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If you use boric acid (I buy it online) instead of borax, you don't have to add acid. Boric acid will lower pH a little, but I've not found this to be an issue for me.

Quote:

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Originally Posted by FormerBromineUser

See our last posts, but also: I reread your OP and I think your question was about why they recommended 80-100 TA. Jim can correct me if I'm wrong since I know only a little about SWGs. I believe the SWG system causes a certain amount of aeration/out-gassing which lowers TA a bit over time.

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I've not noticed an impact on TA running my SWCG. I wonder if the boric acid in the pool has a stabilizing effect here as well?

This is of course chem geeks territory but as you have a pebbletec/plaster finish protecting that from degradation is important. You are trying to get the pH down a little too far and maybe better shooting for pH7.5-7.7 which could decrease the outgassing of Co2 and stabilise your pH better. going lower increases the outgassing giving a bigger pH rise especially if you have a chlorinator which produces hydrogen bubbles which tend to cause the pH to drift on its own.
TA acts as a buffer to pH both in the upward drift and downward drift adding borates may help yet further as does your CYA level. but the important calcium saturation index is influenced quicker by pH than alkalinity. 80-100 is the industry standard for plaster type finishes to supply sufficient carbonates to protect the surface.

Quote:

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Originally Posted by Teapot

This is of course chem geeks territory but as you have a pebbletec/plaster finish protecting that from degradation is important. You are trying to get the pH down a little too far and maybe better shooting for pH7.5-7.7 which could decrease the outgassing of Co2 and stabilise your pH better. going lower increases the outgassing giving a bigger pH rise especially if you have a chlorinator which produces hydrogen bubbles which tend to cause the pH to drift on its own.
TA acts as a buffer to pH both in the upward drift and downward drift adding borates may help yet further as does your CYA level. but the important calcium saturation index is influenced quicker by pH than alkalinity. 80-100 is the industry standard for plaster type finishes to supply sufficient carbonates to protect the surface.

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Good post. :)

My only experience is with our vinyl liner pool, so I don't know much about taking care of other pool types.

Glad you chimed in here, Teapot! Do you agree with the adding of baking soda after the pH comes down from 8+?