A Theory About Rising pH in SWG Pools

[chem geek]

nater,

Thank you for your very detailed set of numbers. Now that I have borates added to my spreadsheet, I can give you an idea of where I think you are at and where you can go from here.

Your CO2 outgassing rate number (at pH 7.2) is 27.8 which is very, very high so it's no wonder you are having to add so much acid and baking soda to your pool (at a pH of 7.5 it's a much more reasonable 12.8 which would probably be stable were it not for the fountain -- which is exactly what you have been seeing). In general, whenever you are not only adding acid, but also have to add baking soda to maintain TA, then you are seriously outgassing CO2 and should consider lowering TA and/or raising pH (unless you like spending money and time on acid and baking soda!). Even before the fountain, I would say you were somewhat lucky in not having to add so much acid and baking soda, but now with the fountain adding to the aeration, you can really see how "overly carbonated" your pool is.

If I assume you are at a baseline pH of 7.5 most of the time, then your acid and baking soda additions mean that you outgas 11.5% of your carbonate in your pool every 2 weeks and you have an additional source of base (pH rise) in your system since your additions would normally lower the pH to 7.36 and the TA to 130.6 over a 2 week period. It is as if you have an additional source of rising pH equivalent to 20.5 ounces of lye (sodium hydroxide) so perhaps this is from your chlorine generator. In fact, if your SWCG were to produce 5 ppm of chlorine over the 2 weeks that got outgassed as chlorine gas (i.e. didn't dissolve into the water to produce hypochlorite), then that would be consistent with the numbers that you gave.

So here's a plan that can be done in two steps to isolate the two sources of rising pH that you have -- CO2 outgassing and SWCG chlorine outgassing (assuming that's what's going on). First, follow Ben's Lowering Your Alkalinity Procedure to get your TA down to 80. Then we can see what happens. My guess is that you will still need to add acid (though less of it -- probably a little more than 6 cups every 2 weeks), but that you may not need to add baking soda anymore. I assume you will try to keep your pH at 7.5 (not 7.2).

Assuming the lower TA works for you (and you may need to get it lower than 80, but let's see where you are at there first), then you can see if you can lower the power level of your SWCG and have it run for a longer period of time (to make up for the lower power level) to see if this helps reduce the amount of wasted generated chlorine that may be escaping as chlorine gas and not getting dissolved into your pool as hypochlorite. If this works, then this will further lower how much acid you need to add to your pool.

Sound like a plan?

Richard