Realized my test time of day was skewing my numbers all season

Started by Priya Raman · · 3 replies
Back to the forum
Reply
#1
Small one but felt good catching it. I always tested FC in the morning before work, 4.5 pretty steady, pH 7.6, figured that was my baseline. Started leaving the Ondilo logging overnight out of curiosity and it showed FC dropping almost a full point by late afternoon on the hot days, then bouncing back some by morning. Screen cage means basically zero debris so I never suspected sun/heat swings that big, just wasn't testing when it mattered. Started doing a second manual test around 4pm a couple times a week to confirm the Ondilo trend against the Taylor kit and yeah, real drop, not sensor drift. Doesn't change my dosing much since CYA's sitting at 45 and doing its job, but good to actually know what's happening between my morning tests instead of assuming flat all day.

3 Replies

#2
that full point swing on the hot days doesn't surprise me at all, phoenix sun just eats chlorine way faster than you'd think even with barely any debris getting in. i only test once in the morning too and always wondered if i was missing the real story mid afternoon.

my CYA is sitting around 40 so probably similar buffering to yours, but our summer temps here are brutal enough that i bet my afternoon dip is even worse than what you saw. might actually steal your idea and run a manual test around 4pm a few times just to see what im missing, curious if it bounces back by morning like yours did or if its still lagging when i wake up.
#3
Seen that same afternoon dip on my own pool, 14k gallon gunite with CYA around 60, I'll test 5 in the morning and be down near 3 by the time I'm home from route in July, screen cage or not doesn't matter, the sun's just doing work all day.
#4
Same thing happened to me last summer, I was only testing before the kids even got in the pool so id get 3.5 and think we were solid all day, then by the time they were doing cannonballs at 5pm the FC was basically gone and I couldnt figure out why the water looked a little hazy some evenings. Texas sun in July does the same eating-chlorine thing yall are describing, its not your cage or your CYA doing anything wrong, its just heat and UV working overtime. Since I got the salt system it self doses more evenly through the day so that dip isnt near as bad for me anymore, but before that I was definitely one of those flat-baseline people getting surprised every afternoon.