Effect of low TA

[etreaster]

I have had tremendous difficulty maintaining a low pH since the day my pool was installed. This is the 5th summer for our pool and still fighting pH > 8 at times. When closing, I have it balanced perfect except the rising pH (TA 90-110). I keep dumping acid in through out winter when I can get to the cover to bring it down. For 2-3 years I have been getting scale in the winter (scale stops at the winter water line). So, this year I have been tracking pH and how much liquid MA I have been adding. I have added almost 7 gal since May 1 of MA. TA at opening was 90. Since I have been adding MA, I have watched the TA slowly come down. Now it is 30-40 and I am able to hold a pH 7.3-7.4 longer without adding anything. Evan after large rains my pH is holding whereas before after a rain it would go above 8.

So, I know everywhere it says 90-110 TA to keep pH fluctuations. But I am finding the opposite. So, is there any harm having a TA 30-40 while the pool is maintaining balance? I still have scale everywhere so trying to hold a normal to low pH is vital to keep it down.

Thanks
Eric

 

[chem geek]

You should not have your TA be that low in a plaster pool and you do not need to target such a low pH of 7.3-7.4. You may be able to have your TA be in the 50-70 ppm range and target a 7.7-7.8 pH with reasonable pH stability. That would be a lot better for your plaster and you could set your CH level higher to balance the Calcite Saturation Index (CSI) to be closer to zero. You can use Pool Math to calculate the CSI.

Where does "everywhere it says 90-110 TA to keep pH fluctuations"? We do not say that. The Recommended Levels in Pool School for non-SWCG says 70-90+ while for SWCG pools it says 60-80 ppm. We have ALWAYS said that TA is a SOURCE of rising pH because TA is a measure of how over-carbonate the pool is with respect to air. In the ABCs of Pool Water Chemistry again in Pool School says "At low TA levels, the PH tends to swing around wildly. At high TA levels, the PH tends to drift up." In the Pool School "how to" for Lower Total Alkalinity it says "There are two reasons to lower your total alkalinity (TA) right away, because you want to slow down the rate that the PH rises, or if high TA is contributing to a high calcium saturation index (CSI) which puts you at risk of calcium scaling." You cannot mix the advice from pool stores or some other websites with our advice here; that simply won't work.

Note that the pH rise from a newer pool will also come from the plaster curing (converting bicarbonate to carbonate as calcium carbonate in the plaster) or from calcium hydroxide being released from the plaster and lowering the TA will not help with this. Lowering the TA and targeting a higher pH only helps reduce the rate of carbon dioxide outgassing so the rate of pH rise from that alone.