Caught a slow FC drift before it turned into anything, thanks to trend data

Started by Priya Raman · · 5 replies
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#1
Small one but felt good. Been logging daily and noticed my FC was drifting down about 0.3 a day for four days straight, nothing dramatic, pH and TA both steady the whole time. Ondilo flagged the trend before I would've noticed just spot testing weekly. Pulled my Taylor K-2006 to confirm and yeah, FC was at 3.1 when it should've been sitting closer to 4.5.

Turned out to be CYA creeping up (was at 45, crept to about 58) eating into my effective chlorine even though raw FC numbers looked okay-ish on paper. Bumped up dosing to compensate while I figure out a partial drain. Anecdotally this is the second season the trend graph has caught something before my gut would have. n=1 but I'll take it, screened pool means I don't get the obvious visual cues other people probably rely on.

5 Replies

#2
CYA at 58 creeping from 45 over a season is pretty normal if you're using any trichlor or even just splashout replacement with tap water that has some in it, but yeah it'll quietly raise your FC target without anything looking "wrong" on the test itself. That's the sneaky part, 3.1 FC isn't alarming in isolation, it only looks low once you know it should be chasing a higher CYA number.

Good catch either way. Partial drain is the right call rather than just dosing forever, chasing CYA up with dilution is the only real fix short of time.
#3
Yeah CYA creep is such a quiet problem, mine did the same thing last year going from like 40 to 60 just from topping off during a hot stretch with the kids cannonballing all summer and splashing half the pool out. Ended up doing a partial drain too, way easier than trying to out-dose it forever.
#4
Alan, are you dosing liquid chlorine or tabs day to day? Just asking because if it's tabs that explains the creep on its own without any splashout being involved, mine only moves CYA when I've had to shock with dichlor for something. Trying to figure out if my top-off water itself is the culprit or if it's coming from whatever I'm using to maintain FC day to day, since I don't get much evaporation loss being screened in.
#5
If you're running trichlor tabs day to day, that IS your CYA creep, no need to hunt for a mystery culprit, I've watched mine climb every summer for 30 years just from the tabs alone.. never bothered with fancy sensors, just do a partial drain every couple seasons when it gets up around 70 and call it done.
#6
Priya since your going liquid it wouldnt be tabs, so id look at your top off water first honestly. We're on well water and never checked it for CYA until my creep last year, turned out ours had a decent amount in it already so every time I topped off after the kids splashed half the shallow end out I was basically adding CYA without meaning to. Worth testing your fill water once with the K-2006 just to rule it out before you assume its a mystery.