Split off of this topic. JasonLion
dschlic1, if I have my calculations correct, here is the process involved in the creation of chlorine from the SWCG:
4NaCl + 4H2O <> 4NaOH + 2Cl2 + 2H2 <> 2HOCl + 2NaOH + 2H2 + 2NaCl
4Sodium chloride + 4water <> 4Sodium hydroxide + 2chlorine gas + 2hydrogen gas <> 2hypochlorous acid + 2sodium hydroxide + 2hydrogen gas + 2salt.
You can also get some HCl (Hydrochloric acid) and OCl (the hypochlorite ion). As you'll notice, sodium hydroxide (aka lye or caustic soda) is produced in about the same amount as the hypochlorous acid. Sodium hydroxide is a very strong base, and, as such, it raises the pH.
The plaster can cause the pH to rise for a year or more, although it should be decreasing in effect now.
You should set out something to catch a sample of your rainwater so that you can test it and know what it is contributing to the water. You should also test your fill water to know its contribution.
Since you need to add acid and cyanuric acid on a regular basis, I would recommend an off-line chlorinator. Whenever you need to add cyanuric, you could just turn off your SWCG and put a few Trichlor tabs in the feeder and use that for a while. This will decrease your pH and add cyanuric acid.
I know that I will probably get some disagreement on using the Trichlor feeder, and that's fine. And, it is not the right idea for everyone. But, I think that it can be a good addition to a SWCG in some cases. Only you can determine if it would be beneficial to you. They are easy to install and use. Just make sure to put the feeder output tube past the cell. You would also have to be sure that it did not conflict with the acid pump.