Problems getting/keeping pH down

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ErniesPool

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May 22, 2013
127
Dallas, TX
I have an older plaster pool (in need of replastering) that I seem to be pouring acid into to get the pH down to 7.5 and keep it there. The alkalinity runs about 90, the pool is clear and I keep the chlorine balanced with 10% bleach. I know that water features can cause CO2 to be blown off, reducing the carbonic acid in the pool and raising the pH, but my pool only has one water feature. That is where the spa water gently flows over the lip of the spa and into the pool. No bubbling, no spray.

There are some spots in the pool where the plaster has been etched to the concrete but I wouldn't expect the concrete to be contributing to raising the pH any more than the plaster.

What could possible be the issue with getting the pH down? I have gone through almost a gallon of HCl in the last two weeks!
 
Why are you chasing a PH of 7.5? Any PH in the 7's is ok. Perhaps your pool likes 7.8? How much are you letting it rise before you add acid to lower it? Are you certain that when you add acid that you are hitting your target?

You could work to lower your TA down some to see if that will help it become more stable. You do that by adding acid to drop your PH to 7.2 and then aerating the water until it reaches 7.8. Repeat that cycle until your TA is where you want it. Have you run a full set of tests and calculated your CSI?
 
TA down to 50 is ok. Let the TA drop as you lower the pH.

Plaster pools often prefer to be in the 7.6 - 7.8 pH range.

What is your CSI?

What % acid do you use?

Half a gallon of acid a week is not unusual for plaster pools.
 
Responses: As for my targets, I'm basically following the suggested numbers provided by troublefreepools in the Calculator. 7.5 is suggested, but I'm happy with 7.6 or so.

- Nothing magic about 7.5 but I don't like it running at 8+ which is where I usually find it, and that's hard on the eyes. I may be mistaken but I seem to remember reading that the chlorine is less effective at high pH values.

- 31.45% acid

- CSI is -0.17, according to the Pool Calculator

- Yes, just today I added 2 cups and hit the target pH

I check it every 2-4 days and most of the time it is up close to 8 or above. The acid does bring it down but not for long.
 
What is the TA of your fill water?
 
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Mine did the same for a good while after I moved in to my current home. TA was super high when I bought the house so I began the process of adding acid to get the pH to 7.2 plus aeration to drop the TA. A few gallons of acid and a many weeks later I had TA less than 100. At that point, I was still using a good bit of acid to keep the pH in the 7.8 range so I let it keep going down to the current level of about 60. At this point, I add a cup or two a week at most to stay in the 7.5 to 7.8 range.
 

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pool_dufus said:
90
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So you have TA 90 in your pool and TA 90 in your fill water?

Even if you get your pool water down to around TA 60 you will need to periodically lower the pH and TA since your fill water will be raising your TA.

Your pH is more likely to be stable at pH 7.8 and TA 60 but will probably not stay there based on your environment.
 
My fill water has a TA of 110. If you are in the city limits, you are on a different reservoir than us in the burbs, but its still ground water that has gone through lots of limestone and likely has a high TA and a PH of 8+.
 

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