My average FC loss during the day is about 2 PPM, however on Sunday I measured the pool in the afternoon and found ZERO FC. There were only two factors that I had changed since Saturday that may have contributed to it:
* Added 12 lbs calcium carbonate dihydrate to get CH from 200 to 300
* Accidentally left the heater on Spa mode (104 degrees) after using the hot tub on Saturday night. I switched the valves to spillover mode, so the heater was heating the entire pool.
My pump (and thus the heater) had only been running for 2.5 hours when I caught the issue (both the lack of FC and the heater running). Was that enough to zap out my chlorine?
Full set of numbers, which I took yesterday:
Since Sunday, it appears I'm back to the usual 2 PPM FC drop per day, even though it's been raining. 0 FC loss overnight.