Ran the overnight FC drop test, turned out my water was fine all along

Started by Alan Pierce · · 2 replies
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#1
Had been chasing what I thought was a slow FC bleed, losing about 1.5 ppm a day and figured algae starting somewhere I couldn't see. Finally sat down and did it properly, tested FC right at dusk, again first thing next morning before the sun hit the water. Lost less than 1 ppm overnight, which is just normal CYA-protected chlorine doing its job, not a biological demand. The real loss was happening during the day from UV, I just hadn't been testing at consistent times so it looked scarier than it was.

Feel a little silly for almost shocking a pool that didn't need it. Lesson for me was test timing matters as much as the number itself, forgive the chemistry lecture but an FC reading means nothing without knowing when you pulled the sample relative to sun exposure.

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#2
Not silly, that's just desert pool ownership 101 and most people never bother to isolate day loss from overnight loss. One thing though, sub 1ppm overnight only confirms CYA's doing its job if your level's actually where you think it is, worth checking that with a real test kit not strips since strips drift on CYA readings. My pool loses almost nothing overnight too, all the damage happens 10am to 6pm when sun's just cooking the water.
#3
Glad you isolated it before dumping shock in there, I almost did the exact same thing my first summer. Funny thing is with my salt system running the SWG on a timer during the day I barely see any FC drop at all even with three kids cannonballing in and out all afternoon, the cell just keeps topping it off as the sun eats it. Makes it hard for me to even notice day vs overnight loss the way yall are describing since mine never really dips low enough to chase. Anyway, good on you for testing at consistent times, thats half the battle.