I have a SWG. After this experiment the CL was at 0, so I ran the SWG for 8 hours and got the CL up to 7PPM. This also increased the PH from 7.4 to 8.4. Any idea why that is? I'm using about .4 gal of muratic acid a week to keep PH in check. TA is ~115
There are two effects. Most dominant is CO2 outgassing. With TA 115 you are at pH 7.4 quite a bit below the equilibrium pH where the amount of dissolved CO2 is in equilibrium with atmospheric CO2, which is somewhere above 8. The higher the TA, the higher the equilibrium pH, the further away from that equilibrium you are, the faster the rise towards equilibrium will be.
The other effect is that creating chlorine with a SWG increases pH, the following use of chlorine is an acidic process which reduces pH again, the whole chlorination cycle (creating chlorine and then using that exact amount) is pH-neutral. But when you increase the FC-level, i.e. you produce more chlorine than gets used (by UV-decay and by killing/oxidising "stuff" in the water), then pH will end up higher. Would you now let drop FC back down to zero again (which I don't suggest you should do), then pH will drop again a bit even though that drop might get overcompensated by further outgassing in the meantime.
Shouldn't the TA be going down over time given I'm dumping a gallon of acid in every 2 weeks?
If there is nothing increasing TA in the meantime, then yes. But usually you add TA via fill water to compensate evaporation losses.
I'm considering to get a CO2 automated system to keep the PH at the right level. These PH swings make it very difficult to keep it in balance and avoid calcium deposits. I thought with a SWG I would have very little chemical maintenance needed, not the case. I'm probably spending more on MA than I would be just adding CL directly.
In the same way that CO2 outgassing increases pH with no change in TA, CO2 injection (i.e. the opposite of outgassing) reduces pH with no change in TA.
From what you were saying, it sounds like you will need some MA to keep TA in check.
