Waiting an hour after dosing before testing, small change big difference

Started by Alan Pierce · · 2 replies
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#1
Small lesson but it saved me some head scratching. I'd dose liquid chlorine in the evening then test right away and get numbers that never quite lined up with what I expected, FC reading lower than the math said it should be. Turns out I just wasn't giving it time to mix through 20k gallons, pump running or not there's dead spots near the steps and shallow end that take a bit to circulate. Started waiting close to an hour after dosing before I pull my sample and the FAS-DPD numbers finally track with what I put in. Feels obvious writing it out but for months I just assumed my math was off or the bleach was weaker than labeled. Neither, I was just impatient with my own test kit.

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#2
Bleach being weaker than labeled isn't a myth though, that part I'd push back on. Liquid chlorine degrades with heat and time, and my pool store fill water plus Arizona garage storage means my jugs are already down some percent before I even open them. So it's not either/or, you had a mixing problem AND the label was probably optimistic. I dose, wait, then test, same as you found out the hard way, but I still buy less at a time in summer because a jug sitting in a hot garage for a month is not the same strength as one bought yesterday.
#3
Had a version of this too, dosed FC one evening, tested maybe fifteen minutes later out of impatience, and the number was so far off from my math I nearly logged it as a bad batch. Waited and retested closer to forty five minutes and it lined up fine. My pool's screened in and small at 12k gallons so I didn't think dead spots were a thing for me, but even with decent circulation it just needs time to distribute, not a mixing bowl.

Karen's point about storage is fair though, that's a separate variable from Alan's timing issue. I buy jugs a gallon or two at a time from the same place and go through them within a week or so, so I can't really speak to the hot garage degradation anecdotally, n=1 on my end since Florida humidity is its own problem but at least my stuff isn't sitting for a month. Worth separating the two issues like Karen said, timing your test is free and fixes one kind of confusion, buying fresher or in smaller batches fixes a different one.