Can constant aerating be a problem?

[LindaE]

I bought a pool fountain for aesthetics, and it constantly aerates my pool when the pump is running. Will this continue to lower the alkalinity or will it stabalize after a while? I've had this fountain for a few months.
Test after being out of town a week( ALK was 130 before fountain)
FC 4
CC 0
TC 4
ALK 70
CA 120
PH 7.8
CYA 40
18,000 vinyl inground

Any feedback is appreciated. Thanks, Linda

[CarlD]

It doesn't actually lower T/A. It raises pH without raising T/A. There's a difference.

When you lower your pH to compensate, with acid, T/A will come down. When the aeration raises pH, T/A won't go back up. Normally, you when you raise pH with Borax or Soda Ash, T/A goes back up with it.

Simply compensate by adding baking soda when the T/A is lower than you want.

[chem geek]

What Carl said is true (as usual). Aeration leads to a rise in pH, not a change in TA until you add acid to lower (restore) the pH.

Yes, you should find that the rate at which the pH rises should slow down at lower TA and higher pH combinations. It won't stop until it gets to an equilibrium at extreme combinations such as a TA of 40 and a pH of 8.0, but you will likely find that the rise is barely noticeable with the numbers you posted (TA of 70, pH of 7.8). This chart (http://richardfalk.home.comcast.net/pool/CO2.htm) shows the relative rates of carbon dioxide outgassing which is what is probably causing your pH to rise. However, because you have significant aeration, you may see a pH rise even being in the green region in this chart.

You have several choices of what you can do in your situation:

1) You can do what Carl suggested and add baking soda when the TA is lower than you want. You will have to periodically add acid to lower the pH and add baking soda to raise the TA. However, since you have a vinyl pool and don't really need the calcium nor the TA at high levels for chemical balance (because you don't have plaster/gunite) you have other options (below). The TA level does keep your pH more stable from other sources (adding chlorine, adding acid or base), but if you use liquid chlorine or bleach, you probably don't get large pH swings anyway (compared to using Tri-Chlor tablets, for example).

2) You can let your TA continue to drop (after you add acid to lower the pH to your target) until the rise in pH is slow enough to be acceptable for you. However, the lower TA will make your pool more sensitive to pH swings from other sources. I wouldn't go below a TA of 50 for that reason.

3) You CAN let your TA drop below 50 if you use another pH buffer by adding 50 ppm Borates to your pool (which has other advantages such as reduced chlorine consumption and improved algae prevention). This is not considered mainstream in this forum (yet), but you can find more info at this thread (http://www.poolforum.com/pf2/showthread.php?t=4712).

4) You can operate your pool at a higher pH, such as the 7.8 you last measured. That is, you can stop trying to fight the rise in pH (at least, not as much). You may still need to have a lower than normal TA to be stable, but at a higher pH you don't need to go as low in TA as with option 2 (assuming you were targeting a pH of 7.5 for that option).

Richard

[LindaE]

Thank you for clearing all of that up for me. It explains why I have used so much acid for the last few months. Guess the beauty of the fountain is worth it. Linda--- (I converted from Baquacil early in summer and am enjoying the BBB and the beautiful clear water.)