Interesting finding concerning pH...

[steveinaz]

For the last 16 days my SWCG system has been shut off (cell died, new system coming) and I've been chlorinating with Clorox. My pH has NOT moved from 7.5 this entire period. That has never happened. I typically have to add about 3 cups of muriatic a week to maintain good pH. This definitely shows how the SWCG affects pH, and it's a bit surprising given my fairly high TA:

Here's the numbers so far:

FC...2.5ppm (using clorox for now)
pH...7.5
CH...(ran out of reagent, so I couldn't test yet---likely around 300-350ppm)
TA...170
CYA...80ppm
Salt...2600ppm currently, but will be raised to 3400ppm this weekend for new SWCG

I'll be adding salt, and knocking down TA this weekend in prep for the pentair IC40. I just did about a 65% drain/refill 3 weeks ago.

[waterbear]

Not surprising at all if you understand a few things about TA, pH and SWCGs. I am more surprised that your TA did NOT rise during this period It's probably because the main source of aeration in your pool is the SWCG.

High TA will NOT prevent pH from rising, in fact it tends to allow pH to rise more quickly. What it will do is make any change in pH from the addition of acid or alkali more 'sluggish' which meant that you need to add more acid to lower the pH than if your TA was lower.

Your high TA means that any sources of aeration are going to cause CO2 to outgas at a faster rate than if the TA was lower (TA around 70 ppm is usually a good target TA for a pool with a SWCG). The faster outgassing of CO2 means faster pH rise.
SWCGs aerate the water because the product hydrogen bubbles in the cell. This aeration causes CO2 to outgass at a faster rate when the cell is producing chlorine and is the primary reason that SWCGs cause pH rise.

Three other things you can do to minimize or slow the pH rise from your SWCG:
1. make sure to keep your CYA at the recommended maximum of 80 ppm (this directly translates into less cell 'on' time which means less aeration which meand less outgassing of CO2 which means slower pH rise)
2. When you lower pH do not drop it below 7.6 since the lower you put the pH the faster it rises (because you convert more bicarbonate in the water-this is what TA really is- into carbonic acid--carbon dioxide dissolved in the water--so there is higher concentration of CO2 in the water to outgas faster). Don't drop the pH unitl it climbs above 7.8. This might seem like a very small window in which to keep your pH but it's really not that difficult to do with a good test kit.
3. Consider adding 50 ppm borate to to your water (easy to do with borax and acid). This puts a secondary borate pH buffer system in your water that works together with the TA (bicarbonate buffer) to keep the pH 'locked' between 7.8 to 7.8 for an extended time (Mine stays right at 7.7).

[chem geek]

Also, if you have short pipe runs from the SWCG to the returns in the pool, then if the chlorine gas doesn't all dissolve in the water and some of it outgasses instead, then that also contributes to the pH rising. If 40% of the chlorine generated by the SWCG were to outgas, then that would completely account for the amount of acid you had to add, but the TA would not drop in that case. If 0.7 pounds of carbon dioxide were to outgas, then that too would completely account for the amount of acid you had to add and in this case the TA would drop by 7 ppm. You may have both of these processes going on.

Note that your FC of 2.5 ppm is too low for your 80 ppm CYA. I'd get that up to around 6 ppm if you are manually dosing. When you get the SWCG back in operation, then the target would be 4 ppm assuming it can be maintained stably.

[steveinaz]

Note that your FC of 2.5 ppm is too low for your 80 ppm CYA. I'd get that up to around 6 ppm if you are manually dosing. When you get the SWCG back in operation, then the target would be 4 ppm assuming it can be maintained stably.

Yep, I'm aware--just doing the minimum as my SWCG gets installed Tuesday.