Is SWCG scaling caused by "high" TA?

[Malcolm]

I have recently observed a phenomenon in my pool that leads me to propose the following hypothesis:

SWCG scaling is caused by relatively high TA levels, whether or not the pool's chemistry is in balance.

My pool is three years old. For the first two years I experienced the commonly reported problem of calcium flakes being discharged into the pool from my SWCG. During that entire time, I kept the pool's chemistry in balance. This last year, in an attempt to get the other commonly reported problem of constantly rising pH under control, I brought my TA down to about 70-80 ppm. During that time, I did not see any discharge of calcium flakes. A week ago, in order to repair a crack, the pool was mostly drained and refilled. Due to the chemistry of my fill water, after rebalancing, my water now has lower CH and higher TA than before. Flakes galore! To those of you who understand chemistry, is there some possible reaction going on in the SWCG with "high" TA levels that leads to scaling?

Before the drain and refill (no flakes):
FC: 4.6
CC: .4
pH: 7.8
TA: 80
CH: 400
CYA: 70
NaCl 3600
CSI: .05

After the drain, refill and rebalance (lots of flakes):
FC: 4.4
CC: 0
pH: 7.2 (hoping to drop the TA)
TA: 110
CH: 230
CYA: ? (waiting for it to dissolve)
NaCl: 3200
CSI: -.57

After a few days of the pH drifting higher (still flakes):
FC: 4.6
CC: 0
pH: 8.0
TA: 120
(the rest not retested, presumably the same)
CSI: .05

I'm not going to do much to drop the TA any more this year, since I have only about a month of the season left and, then, our rainy season will naturally bring the TA down before next year. But, I thought I'd let you all know about this in case it might spark some insights.

 

[chem geek]

There is another parameter used to determine the amount of possible scaling and that is called the Calcium Carbonate Precipitation Index (CCPI). Though two waters with similarly high Calcite Saturation Index (CSI) levels (as measured at the higher pH cathode) may both scale, the act of scaling itself decreases the pH and decreases the TA and CH locally. So with less pH buffering at a lower TA one can have less scale form, all else equal. In essence, it self-extinguishes with less total calcium carbonate formed. However, this effect is mostly for bulk pool water especially where water is still whereas water is constantly flowing through the salt cell.

What doesn't make much sense is why you got more scale even though your pH was much lower in scenario 2. I suspect that there is something else in your fill water that is contributing to more flaking. Perhaps there are more small particles to nucleate calcium carbonate to form at the cathode (and these had yet to filter out over time) or there is a different amount of magnesium or other ions that may be making the scale faster to occur (assuming CSI is already positive at the cathode).

 

[JamesW]

Did you regularly check the cell for scaling prior to the water change? Perhaps the cell was scaling but not releasing the scale during cell reversal.

The lower pH along with the higher Carbonate Alkalinity (about double) will create much more carbon dioxide in the water, which can off gas in the cell, making scaling more likely than the CSI would otherwise indicate.[Note: Edited below in a follow-up post]

The cyanuric acid at 70 ppm would have provided some buffering of the pH, it would reduce the amount of run time needed to maintain the FC, and it would have accounted for some of the TA (meaning that the carbonate alkalinity was about 56 ppm, which is about half of the later 110 ppm with no cyanuric acid) .

Overall, the best way to avoid scaling in the cell is to:

1) Maintain the pH in the 7.6 to 7.9 range.
2) Keep the TA as low as reasonably possible.
3) Keep the cyanuric acid between 60 and 70 ppm.
4) Add borates to 50 ppm (optional).

 

[Malcolm]

Did you regularly check the cell for scaling prior to the water change? Perhaps the cell was scaling but not releasing the scale during cell reversal.

Yes I had checked a couple of times, and there had been no built-up scale.

Overall, the best way to avoid scaling in the cell is to:

1) Maintain the pH in the 7.6 to 7.9 range.
2) Keep the TA as low as reasonably possible.
3) Keep the cyanuric acid between 60 and 70 ppm.
4) Add borates to 50 ppm (optional).

Other than 4, that pretty much describes the targets I aim for.

Thanks for the thoughts.

 

[JamesW]

Also, for gunite/plaster, you want to keep the CSI as close to zero as possible (without scaling the cell) to protect the plaster.

 

[JamesW]

The lower pH along with the higher Carbonate Alkalinity (about double) will create much more carbon dioxide in the water, which can off gas in the cell, making scaling more likely than the CSI would otherwise indicate.

[edit]The lower pH along with the higher Carbonate Alkalinity (about double) will create much more carbon dioxide in the water, which can off gas causing the pH to rise faster in general, but perhaps not in the cell.[end edit]

Is it possible that the pH was higher than you thought?

 

[Malcolm]

Is it possible that the pH was higher than you thought?

I don't think so. I tested the pH each day after I rebalanced and it was consistent with about a .2 rise per day from 7.2.