Pink slime!

[waterbear]

I checked the SWCH plates and sensor yesterday -- first time ever since getting our pool last May! -- and the sensors were spotless, the plates had no build-up of any gunk that I could see.
Of course they were since you have been running a negative CSI for a while and have aggressive water that will dissolve calcium.
I'll check on that again since I'm running higher calcium now, to ensure the plates and sensor stay clean.
They should. If you check the CSI range for the parameters I gave you will find that it ranges from about -.3 to about +.24 which is balanced water! (This is with the TA at 70 ppm. At 80 ppm the water will have a bit more tendency to scale so you might want to think about dropping the TA by 10 ppm since , if you are not overdosing on acid, your TA should stay fairly stable for a while.)
And there was no pink slime to be seen anywhere in the pool this weekend. Lookin' good!

Use you acid demand test to get an idea of how much acid to add. I measure the acid into a plastic measuring cup that I put on a plastic dinner tray. If any spills I jut push the whole tray into the pool. One you get used to how much acid your pool takes you can eyeball it pretty close but you really should measure at first. If you don't want to measure acid then get a gallon jug and fill it with water and measure that to get your eyeballing of the acid more accurate. It's just practice.

[famousdavis]

Oddly, my pool's pH read 7.4 today -- same as it was three days ago! While I haven't been using the waterfall, I've never had the pH be stable for three days in a row. Weird! Pool temp is 79, no change there. Does calcium somehow stabilize pH, too?

My FC hit 4.5 today, so I'll readjust the SWCG back to its original setting (50%) and see if the FC stays the same.

[chem geek]

While I haven't been using the waterfall, I've never had the pH be stable for three days in a row.

A waterfall aerates the water increasing the rate of carbon dioxide outgassing which causes the pH to rise. No mystery here. Lowering the TA level reduces the over-carbonation of the pool so reduces the rate of pH rise, but reducing aeration sources also helps. The SWCG also aerates the water with its hydrogen gas bubbles and may also have some chlorine outgassing, both of which cause the pH to rise, if the pipe run between the SWCG and the pool is relatively short.

[famousdavis]

Thanks chem geek.

Taken what you and waterbear have written, I can conclude that the pH stability was borne from stopping my waterfall and by tamping down the ontime for my SWCG -- two causes for pH rise. This morning I opened the valve to the spa just a little so the water is replaced with filtered/chlorinated water, but it's just a water drip now, not a waterfall. ;-)

Forgot to mention that I used to measure muriatic acid until I read (somewhere here, maybe?) that adding the measuring step may cause more harm than good because it's an extra step that might lead to an accidental spillage of acid. So, I've just guesstimated my acid additions (with fairly consistent results - my pH would fall back to 7.2). I'll try guesstimating a smaller dose of acid from now on. Measuring acid is actually fairly easy by just looking at the acid container's liquid level, and calculating what a few cups poured out would look like. The thing I haven't done is perform an actual acid demand test to calculate with more precision just how much acid I should add. I'll do that the next time my pH needs downward adjusting, just so I can better estimate my acid demand needs.

Fine work, gentlemen!

[waterbear]

Thanks chem geek.

Taken what you and waterbear have written, I can conclude that the pH stability was borne from stopping my waterfall and by tamping down the ontime for my SWCG -- two causes for pH rise.

And the second reason is why the CYA should be at the maximim since it will mean that a lower on percentage will maintain the desired FC level. Lower on time means less aeration means slower pH rise.