This is an often quoted misconception about SWG’s that is false. SWG’s are pH neutral and bleach injection is only slightly pH increasing (due to the excess caustic in all chlorinating liquids). As such, SWG pools do not require acid per se. What typically happens is pool owners get used to using solid chlorine sources which are generally acidic in nature (dichlor and trichlor are acidic) and so their pools are dosed with both chlorine AND a source of acid all the time. When they switch to an SWG or liquid chlorine dosing, they suddenly are no longer adding a source of acid all the time and they need muriatic acid to lower pH. They then falsely attribute the rise in pH to the installation of the SWG. There are plenty of examples of people that install SWGs and never need to add acid to control pH. It all depends on their source water alkalinity and the presence of aeration sources. SWGs contribute almost nothing to pH rise. One can easily prove this by simply covering a pool and running the SWG. Once evaporation is stopped and aeration is reduced, pH rise slows dramatically.
As is often said on this site and needs constant repetition - pH rise in swimming pool water is entirely dominated by CO2 outgassing.
I have heard multiple (anecdotal) explanations why an SWG increases pH (bubbles/aeration, chemical, etc). And I've read other posts, perhaps where you explained this elsewhere Matt, that SWGs do not contribute to pH rise. But in my own pool, I witness this phenomenon first hand, whatever the cause. I have a model TFP pool. I've added water, CYA, "liquid" chlorine, salt and MA. Absolutely nothing else, since the pool was resurfaced with pebble. There are no water features. I've never used solid chlorine sources of any kind. I experienced the expected pH rise from the new surface, while I ran the pool as non-swg, well tested and documented with my Taylor 2006 and the Pool Math app. Pretty constant, repeatable pH rise. I added salt and more CYA and turned on the SWG this spring and experience a very marked increase in pH rise, also well tested and documented. So much so that I had to dose with MA everyday, just to maintain high 7s. It was not a small amount of increase in pH rise.
I do have a fair amount of wind, moving the surface water. My returns point slightly down and only agitate the surface very slightly. I have high TA fill water. And I started the SWG in spring, so evaporation rate was on the rise. Based on your CO2 outgassing comment, all those could contribute to my pH rise. But that doesn't explain why all those conditions existed both before and after I turned on the SWG, and the increase in the rate of pH rise happened instantly after turning it on. Not gradually as you would expect if one of the aforementioned conditions were the cause (for example, the evaporation rate didn't suddenly, coincidentally increase the day I turned on the SWG).
So while I don't doubt the validity of the science of your comments, Matt, I have a pool that contradicts what you're telling us. Not to mention there seems to be a large contingent of TFPers that also experience this same pH rise with their SWGs. If it's not my SWG, then what is it?
Sorry to hijack this thread, but Matt is saying (I think) that a chlorine injection system will contribute to pH rise more than an SWG (his second sentence), and I'm finding that hard to believe. I can somewhat rationalize bringing this up in this thread, as it pertains to the pros and cons of SWG vs Stenner.