Caught my FAS-DPD reagent going bad before it threw off my FC numbers

Started by Alan Pierce · · 2 replies
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#1
Small one but felt good. Been tracking FC loss overnight and the numbers stopped making sense, losing like 0.5 ppm more than my CYA level should allow for given the sun we've had. Pulled out a fresh bottle of the FAS-DPD titrant next to my opened one and sure enough the old bottle was reading low, meaning my actual FC was higher than I thought all week. That reagent had been open about 14 months, past what I'd consider reliable even capped tight in the garage.

Lesson is I should've cross checked sooner instead of trusting a number that didn't fit the FC/CYA relationship I've watched for years. If your losses suddenly look weird and nothing else changed, don't rule out the reagent itself before you chase algae or lock or whatever the pool store wants to sell you.

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#2
Fourteen months is generous, honestly. I toss my FAS-DPD at the one year mark no matter what, garage heat out here cooks reagents faster than people think even capped tight. Good catch on the FC/CYA relationship not fitting, that's exactly the kind of gut check that saves you from chasing ghosts, but I'd push the replacement window in a lot sooner than you're planning to.
#3
Karen, that's a fair correction and honestly Georgia garage heat in July probably isn't that far off from Texas, so I take the point about pushing the window in. What I'd add though is the R-0871 titrant is the one that degrades fastest of the bunch, faster than the R-0870 or the DPD powder even, because it's the one doing the actual oxidation-reduction counting and it's sensitive to light and heat both. So it's not just a flat one year rule across your whole kit, I'd watch that specific bottle closer than the others and maybe date it the day you open it instead of the day you bought it.

The other thing I do now is keep the FC/CYA ratio as a standing sanity check rather than something I only think about when numbers look off. Once you've watched your own pool long enough you get a feel for what a given CYA level should be protecting overnight, and when the math stops fitting that's usually the first sign something upstream is wrong, reagent or otherwise, before I'd ever reach for a pool store diagnosis.