pool chemistry quandary

[302harrison]

I have a question about the minimum level of free chlorine. My target range is 7-9 and minimum 5. I usually bring my FC up to 7 and it loses FC down to 3 or 3.5 by the end of the day. I can add enough to bring it up to 9 or even higher, and it still loses down to 3 or 3.5. If I happen to miss a day, I'll check the FC level in a bit of a panic thinking it must be 0, but it has only fallen to 2 or so. My pool is in full sun pretty much all day and temps here are in the 90's. I have repeatedly done an overnight chlorine loss test, and it always comes out fine.

Is this because the sunlight eats up any chlorine above a certain level? Am I okay letting it fall below the recommended minimum of 5? I don't know how to keep it that way without raising it really high and losing a ton of extra chlorine each day. Why does it eat the chlorine so much slower once it reaches 3 or so? Part of this is just curiosity because the pool does seem to be doing fine. Thanks for any clarification!


FC 3.5 (just added chlorine to bring it to 7)
PH 7.8
TA 60
CYA 60
CH 200

 

[Newdude]

First things first. Overnight Chlorine Loss Test

Then we know we have time to fiddle with anything else. Unexpected FC loss, especially at 60 CYA in SC, is almost always algae.

Am I okay letting it fall below the recommended minimum of 5?

No. Minimum sounds way too OK-ish for me. Call it Swampville instead, and you'll naturally give it the respect it deserves.

'lalalalalalala *record scratch* I'm dancing in the outskirts of Swampville'

hitting min or even going below doesn't guarantee algae. But each occurance is a chance, and with enough chances, it's all but guaranteed. Ya know ?

 

[302harrison]

I will do another chlorine loss test tonight, but it always passes. Also, the chlorine consumption doesn't increase unless I add more chlorine. It is using the same amount every day. I would think if there were algae, it would eat more chlorine every day. I think it's sun related. Just wondering why once it gets down to 3 the chlorine consumption slows way down, like from 4 a day to 1 or so for that day.

 

[duraleigh]

Two things consume chlorine.

1. UV rays, but you have that buffered fairly well with a CYA of 60 ppm.

2. Organics (algae) in your pool water

The inescapable conclusion is you need to SLAM the pool, regardless of your other inconsistencies.

 

[Newdude]

1-a.

Maybe the 60 isn't the 60 you think it is.
(Ask me how I know )

 

[302harrison]

Pool passed the OCLT. Result was .5.

 

[302harrison]

Combined chloramine was also .5 and pool is sparkling.

 

[Newdude]

Get a good CYA test mid day in the overhead sun.

 

[302harrison]

I did the CYA test in bright sunlight and used the test regent for comparison. Still got 60. I raised it to 70 and that has helped my chorine consumption lower a bit to where it was in the spring. It is now lowering to a minimum of 4 each day, which is closer to the target minimum of 5. Thanks for the help!

 

[Newdude]

It is now lowering to a minimum of 4 each day, which is closer to the target minimum of 5

It's still below min. Aka Swampville. You need to do whatever you need to do to remain free and clear of min at your next test interval. Gonna be busy for 1.5 days and be late testing tomorrow ? Add a 1.5 dose, plus a little wiggle room based upon your recent loss.

In the peak season you may need to dose over target level to withstand the loss and remain above min. So be it.

Minimum is lava.

Dancing the line at minimum doesn't guarantee algae. But each occurance is a chance, and given enough chances, it's all but guaranteed. YMMV if it takes 2 chances or 9 to swamp you.

 

[302harrison]

The only thing is, it seems to come back to around the same spot even if I add more chlorine. It feels like I’m just wasting that chlorine to the sun.