Chlorine losses I don't understand

[AndyPTx]

Guys forgive me if this is elsewhere. I've found elements and ideas but not quite an answer. And if this belongs elsewhere please move it.

Having been talking to others about, and helping with their pools with, what I have read (learnt?) here, I realized I had become slack with my own.
So I decided to thoroughly check my pool for a fresh summer. Looked lovely! But, as I was saying to swamp owners as we ended their SLAMs ...
"*Looks* clear...?!..."

Boring intro over. Long now! (edited HARD, I promise)

24ft round agp, sand filter, 2sp 1.5hp pump (for spd2 for vacuuming)

CH untested recently (vinyl)
TA 90
CYA 40 - 50
pH 7.2 if that can be trusted with...
FC11
TF-100

Initially tested over 24 hours - chlorine dropped around 16ppm or thereabouts.
Crud.
So I immediately dosed it up to 20ppm (plus a little extra for depletion) - for my CYA of at-or-near 50.
I retested, redosed, maintained over 20ppm consistently, OCLTd to check it wasn't ridiculous UV losses, saw OCLTs drop 8 to 10 or more, CC readings 'negligible' - I really never saw any convincing re-pinking. Maybe possibly trace? I think I just WANTED to see CCs to try to explain the FC losses.

Trying to not waffle...(!)
Retested chlorine several times a day at first, then am and pm.
Every 24hrs, FC losses ~16-20ppm, sometimes more.
Every 'overnight' - an hour or more after sun on pool, til maybe 10am but pool shaded til at least 1pm - every 'overnight/total shade' period lost +-8ppm.
Every daytime period lost more than that.

24'AGP means ~8ppm from a 10% gallon?
I have needed to add at least 2 gallons a day for 2 months or more to stay above 20ppm shock level.
And sometimes 3 gallons. Nearly 24ppm loss in a day.

Crystal water, scrubbed and brushed, steps out and hosed and cleaned and bleached, poured bleach into the nooks that water fills and left it there 'soaking' for hours before putting them back.

Sorry if I've missed an explanation for this in the threads.

Then...
There IS an end to the chlorine losses, but does it really explain it?

I use large *clear* plastic sheeting as a cover sometimes - I get LITERALLY about 90 minutes of full sun on this pool - which pegs the miserable temperature down to barely mid-80s. In Texas. In Summer.

Sun Aug18th I covered the pool to hold what warmth it had from its 90 minutes of sun, and stop it radiating off and being blown away in overnight breezes.
I got in Monday and it immediately felt, ... different.
And Tuesday? Dang it actually felt nice! High 80s already! Skin temperature!

But also?
FC losses absolutely nosedived just as fast.
Immediately.
I went from 2 gallons (16ppm) just to keep up/stay ahead, to 1 gallon because after so long I plain didn't dare NOT add!!
I OCLTd it and got zero.
I finally saw the mythical 4ppm/day loss happen in MY pool.
And since FCs were still in the 20s and now suddenly it wasn't using any? ......

I haven't added chlorine in 3? 4? days.
It still read FC10 today (well 2 days ago - I've taken time editing this). 6 yesterday and I added a gallon.

So.
Why the Crud is it losing ~16ppm and needing 2 gallons a day for weeks and weeks, and suddenly 'NOTHING' when I cover it in a clear plastic sheet. When the CYA is about 50, and it's shaded for 20 hours a day.

??
Over to you geniuses... please?

 

[jsygrls]

I use large *clear* plastic sheeting as a cover sometimes

No genius here but I had to ask. Have you or can you complete your slam, without this cover and seen how it goes, its seems its the only item you haven't addressed cleaning to this point, Unless I missed it.

 

[AndyPTx]

I'm sorry if I spoke unclearly! (My absolute curse). I have only just put the cover on.

The pool has been uncovered for the entirety of this interminable SLAM.
Coincidentally or otherwise, it was when I put the cover on that the chlorine losses 'magically' disappeared.

Nothing else about the pool's use, or my treatment of it, changed. Nor it's appearance. Perfectly clear. I look down at the pollen on the floor and bump my nose on the water's surface because I don't see it.

Daily testing, brushing, vacuuming of the pollen that always lands in and swirls into a patch in the middle - perfectly cleaned out one day, circle of pollen in the morning. Totally surrounded by trees - pecans and sycamore are the ones I know, any others I have no idea. The pool is totally shaded til about 1.30, shaded again by about 5.30, and 90 min in there is full sun.

Consistently needing to add 2 gallons of 10% every 24hrs (FC loss maybe <8ppm 6pm-10am, but usually >8ppm 10am-6pm). Almost always staying in FC20s - a morning gallon (8ppm) and an evening (another 8). Low 20s to hi 20s to low .....

And then I put the clear plastic sheet 'cover' on it and the chlorine loss dropped to zero between 6pm and 9/10am, and 4 in 24hrs.


I have long been afraid of whether the pollen is an algae, and whether it lands in or is spat out the filter, or, ... or, ....
But I put a cheap little canister on my manual vacuum and finally got to really see it when I vacuumed it up. Definitely floaty seeds of some kind.
And when I put the cover on there was magically none on the floor. Of course.

 

[AndyPTx]

I'm sorry if I spoke unclearly! (My absolute curse). I have only just put the cover on.

The pool has been uncovered for the entirety of this interminable SLAM.
Coincidentally or otherwise, it was when I put the cover on that the chlorine losses 'magically' disappeared.

Nothing else about the pool's use, or my treatment of it, changed. Nor it's appearance. Perfectly clear. I look down at the pollen on the floor and bump my nose on the water's surface because I don't see it.

Daily testing, brushing, vacuuming of the pollen that always lands in and swirls into a patch in the middle - perfectly cleaned out one day, circle of pollen in the morning. Totally surrounded by trees - pecans and sycamore are the ones I know, any others I have no idea. The pool is totally shaded til about 1.30, shaded again by about 5.30, and 90 min in there is full sun.

Consistently needing to add 2 gallons of 10% every 24hrs (FC loss maybe <8ppm 6pm-10am, but usually >8ppm 10am-6pm). Almost always staying in FC20s - a morning gallon (8ppm) and an evening (another 8). Low 20s to hi 20s to low .....

And then I put the clear plastic sheet 'cover' on it and the chlorine loss dropped to zero between 6pm and 9/10am, and 4 in 24hrs.


I have long been afraid of whether the pollen is an algae, and whether it lands in or is spat out the filter, or, ... or, ....
But I put a cheap little canister on my manual vacuum and finally got to really see it when I vacuumed it up. Definitely floaty seeds of some kind.
And when I put the cover on there was magically none on the floor. Of course.

By 'floaty seeds' I mean they float in the air and land in the pool. I guess they're so feathery they get sudden and sink. Some are in the skimmer sock.

 

[AndyPTx]

*sodden!