Surprised how fast one thunderstorm ate through my FC

Started by Alan Pierce · · 2 replies
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#1
Had a good hard rain Tuesday night, maybe 20 minutes of real downpour, and I figured I'd just lose a little FC to dilution like always. Tested Wednesday morning and FC had dropped almost 3 ppm, way more than rainwater volume alone would explain. Took me a minute to remember that rain drags in all sorts of organic dust and pollen on the way down, plus whatever blew off the trees, so the chlorine isn't just getting diluted, it's actually got more to oxidize. Dosed back up with plain liquid chlorine and it held fine after. Just a good reminder that a big rain event isn't a one-variable problem, forgive the chemistry lecture, but FC and CYA math doesn't account for organic load spikes like that.

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#2
Same thing happened to me a couple summers back, big storm rolled off the ocean, maybe 25 minutes of hard rain, and I chalked up the FC drop to dilution too until I actually did the math. 20 minutes of downpour on a 12k gallon pool isn't adding enough water to tank your FC by 3ppm on its own, you're talking maybe a couple hundred gallons tops depending on your pool surface area. So Alan's right, it's the organic load, pollen and dust and whatever washed off the pool deck and landscaping around it.

What I started doing after that is just plan on a shock dose anytime we get a real gully washer, not a token top off. Myrtle Beach gets these pop up storms all summer and I used to just nudge the chlorine back up a little and call it done, then wonder why it crashed again by the next afternoon. Now I treat it like I would after a big pool party, hit it harder than feels necessary and test again a few hours later. Liquid chlorine like you used is the right call too, no CYA riding along with it when your stabilizer is already fine.
#3
Before piling onto the organic load theory, what's your CYA sitting at right now Alan, if it's already low that 3ppm drop could just be chlorine burning off faster with less stabilizer riding along, rain or no rain.