A Theory About Rising pH in SWG Pools

[chem geek]

Richard,
Updated data. I think the acid demad might be letting up a bit.

Date pH Alk. Acid addition
7/26 7.8 100 24 oz.
7/27 7.4 100
7/28 7.8 100 16 oz.
7/29 7.7 100 16 oz.
7/30 7.2 90 32 oz. (before testing)
7/31 Vacation
8/1 Vacation 16 oz.
8/2 Vacation
8/3 Vacation 16 oz.
8/4 7.8 90 28 oz.
8/5 7.6 80 16 oz.
8/6 7.6 80 16 oz.
8/7 7.4 80 8 oz.
8/8 7.6 70 16 oz.
8/9 7.6 70 16 oz.
8/10 7.5 70 16 oz.
8/11 7.3 70 8 oz.
8/12 7.4 70 16 oz.
8/13 7.6 70 16 oz.
8/14 Work trip
8/15 7.6 70 16 oz.
8/16 7.6 70 24 oz.
8/17 7.4 [IMG]file:///C:/DOCUME%7E1/CLV/LOCALS%7E1/Temp/moz-screenshot-2.jpg[/IMG][IMG]file:///C:/DOCUME%7E1/CLV/LOCALS%7E1/Temp/moz-screenshot-3.jpg[/IMG]65 16 oz.
8/18 7.4 60 8 oz.
9/19 7.5 60 20 oz.
8/20 7.2 60 8 oz.
8/21 7.4 60 8 oz.

On another pool forum (poolcenter.com) in this thread via an unfortunately not-so-nice discussion, I calculated whether the SWG produces enough hydrogen gas bubble volume to have the small amount of carbon dioxide that would be allowed into it (via Henry's Law for equilibrium) to make the pH rise. It isn't enough and that's under ideal conditions with full equilibrium reached (TxPool says the kinetics don't allow for any effective transfer, but I'm not sure about that -- in any event, the volume just isn't enough).

So, I'm back to leaning towards the other explanation I originally started with in this post in this thread where I thought it might be undissolved chlorine gas bubbles. If this is the case, then one way to improve the situation could be to point the returns downward to give the bubbles a greater chance of dissolving more completely into the water. The lower TA effect that KurtV saw wasn't huge and could just be regular carbon dioxide outgassing without much change from the SWG itself -- that is, the SWG effect may mostly be chlorine gas escaping.

KurtV (or anyone else with an SWG that would like to experiment), if you are still experiencing a rising pH in your SWG pool, can you try pointing the returns downward and seeing if that makes any difference? Can you see the bubbles at night to see if they linger longer when the returns point downwards or if you notice any of them getting smaller in the water (dissolving)? Do you ever smell any fresh chlorine smell over the return where the bubbles come up compared to other parts of the pool where there are no bubbles?

If the chlorine outgassing is a primary cause for the incremental pH rise in SWG pools, then I would expect this effect to be worst in pools with short distances between the SWG and the pool returns and for 2-speed or variable speed pumps when the pump is on high speed but not on low speed. If pointing the returns downwards helps, then one has the trade off of rising pH vs. potentially worse cleaning of the pool water surface into the skimmer (which I think is better when the returns are not pointed downwards).

Richard