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Ph Constantly Drifting Upward


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We have a 24K gallon in ground pool (gunnite). It's about 8 years old and was totally drained, acid washed, and refilled this year due to staining (which really looked like plaster hydration!). The plaster is in good shape, the pool sits in full sun, and the water temperature often is quite warm (93° today). We have to add about a gallon of muriatic acid a week to keep the pH at 7.2-7.5. For example, today we are at free Cl of 3.6, pH of 8.0, alkalinity of 140 ppm, hardness of 250 ppm, CYA of 80 and a saturation index of 0.8. We have no water features. First question is why the pH constantly drifts up? I can only imagine that adding this much acid over the summer isn't going to be a good thing. Would addition of borates help stabilize the situation, or make it harder to bring the pH into range? BTW: had the same balance issues before draining. Using a Taylor 2006 test kit. TIA!

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Your TA is too high and that is why your pH tends to rise. There is no mystery here. TA is mostly a measure of bicarbonate in the water and at a given pH, more bicarbonate means more carbon dioxide in the water. More carbon dioxide means more outgassing and that causes the pH to rise. Pools are essentially over-carbonated to (ironically) provide a pH buffer and to protect plaster surfaces. See Lowering Total Alkalinity, though the procedure just accelerates what you are going through now -- it uses a lot more acid (with aeration at low pH) up-front in order to achieve better pH stability down-the-road. It's a pay-me-now vs. pay-me-later situation. There is no avoiding how much acid you need to add -- you can only choose when to add it -- a lot up-front over a short period of time or a little every so often over a long period of time.

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alkalinity of 140 ppm,

Here is your problem. Lower your TA to aroudn 70-80 ppm (and bump up the calcium to keep the water balanced for the plaster) and you wlll find your pH becomes much more stable. Adding borate will help but only after you get the TA in line. You might want to read this to understand what is going on but basically the higher the TA the faster you outgas CO2 and outgassing of CO2 is the main cause of pH rise in pools and spas (that do not have new, curing plaster).

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  • 1 month later...

An inch or two of 7.8 pH water is not enough to cause a problem, especially if you don't try and keep your pH low (i.e. don't lower it below 7.5 and don't worry if it's at 7.8). More likely the bigger issue is the TA in the fill water since that adds to whatever is in the pool so your TA may be rising or not dropping much when you add acid to maintain the pH.

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Do you find that your TA is dropping over time? If not, then it is probably being maintained (or even rising) from the evaporation and refill with water that has significant TA. Have you tested the TA of your fill water? If you wanted to, you can accelerate the process of lowering your TA by intentionally lowering the pH to around 7.0 and aerating the water and adding acid to keep the pH low as this will get the TA lower more quickly. Your pH will then rise more slowly with a lower TA of say 70 or 80 ppm. However, if your evaporation and refill keeps increasing your TA, you'll always be adding acid one way or another -- you can just choose to do it in bursts with the lowering TA procedure or adding it weekly as you are doing.

At least by your keeping a higher pH target and not going below 7.6 (in your case) that helps. There is more outgassing of carbon dioxide at lower pH so it's better to let a pool that tends to rise in pH stay above 7.5. Technically, you could operate a pool in the 8.0 or higher region, but there is greater risk for metal staining and you'd have to have other water parameters adjusted lower to prevent scaling. I think what you are doing right now is reasonable -- test your TA of the fill water and let us know what it is.

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