PH good, ALK dropping

Problems relating to pH and total alkalinity.
Increase ph, increase TA. Reduce pH, reduce TA.
pH chemistry advice and techniques for the pool.
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PH good, ALK dropping

Postby vadar007 » Sat 05 Apr, 2008 10:12

I have a 25000 gallon salt water pool. My PH is reading at 7.4 and I typically add 2 quarts Muratic acid each week to keep it in range. I have a water fall so there is ample aeration. My ALK has been dropping over the weeks and is now at 60ppm Should I worry about this and try to boost it or let it ride?

FC 1.5
CC 0
PH 7.4
ALK 60
CYA 60

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Postby chem geek » Sat 05 Apr, 2008 12:39

What you are experiencing is normal. The combination of aeration with acid addition results in a drop in TA. In fact, that is the procedure to make the TA drop except that one would also lower the pH (to around 7.0 or 7.2) to make the drop happen even faster. In your case, of course, you don't want that.

The aeration comes not only from your waterfall, but also from hydrogen gas bubbles produced by the SWG. Both of these effects cause carbon dioxide to be outgassed faster from the pool and that causes the pH to rise. It is not uncommon for SWG pools to have a rise in pH because of this. Having a lower TA, higher pH (and possibly higher CH to compensate) helps reduce this effect. Also, using 50 ppm Borates will help if there is some nascent algae growth as this will allow one to lower the SWG on-time thus reducing its aeration. This chart shows the relative rate of CO[sub]2[/sub] outgassing at different pH and levels of TA. The absolute rate is also a function of aeration (so is not readily quantifiable).

You should be setting your FC target at 3 ppm with your CYA currently at 60 ppm. 1.5 ppm is low and we've seen SWG pools get algae at that level, even at 2 ppm. The early stages of algae growth will not be visible and will instead appear as chlorine demand requiring a higher on-time for the SWG making the pH rise problem worse. A minimum FC that is 4.5% of the CYA level seems to be the critical point. Of course, this depends on the algae nutrient level in the pool, but the aforementioned rule seems to work well up to a phosphate level of 3000 ppb which is quite high. If your pool is exposed to a lot of sun, you may find that increasing the CYA to 70-80 ppm and having an FC target of 4 ppm actually results in less chlorine usage, though your situation at 60 ppm CYA isn't bad.

You did not post your Calcium Hardness (CH) reading. It sounds like you've got an in-ground pool that is probably plaster/gunite so needs calcium. Even with a CH of 300, your pool's water is suggestive of being corrosive to plaster with a Calcite Saturation Index (CSI) of about -0.7 (you can use The Pool Calculator to get a reasonable estimate of the CSI).

To reduce the rate of carbon dioxide outgassing which is causing the pH to rise, I would target a pH of around 7.7 instead of trying to go down to 7.5 or lower. Add acid when it gets to 7.9 and drop it no lower than 7.5 (or a narrower range around 7.7 if you don't mind adding less acid more frequently). If you want to keep the TA lower to reduce the outgassing, then to have a TA of 60 you should get your CH up to 400 ppm. A pH of 7.7, TA of 60, CH of 400 with your 3000 ppm salt results in a CSI of -0.3 at the temp of 71, but this will rise so at a temp of 85 it will be -0.2 (it's fine to be a little negative since you want to avoid scaling in the salt cell).

The above targets should lower your rate of pH rise and the amount of acid you cumulatively need to add, but over time the TA will slowly drop and you'll need to add some baking soda to raise it back to your target level.


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