FC dropping quickly during day, not at night

[scottfy99]

Just tested which is 12 hours from my last test.
FC 0
CC 0
PH 7.4
TA 100
CH 160
CYA 35
My FC dropped from 13.5 to 0 in 12 hours. I've done a SLAM and passed the OCLT the past 2 nights. The night before FC didn't drop, but during the day it dropped from 17 to 1. My pool does receive full sun most of the day. I drained my pool 4 times last week to get the CYA lower. It was extremely high. Should I increase CYA? To me that's a huge loss in FC during the day. Water looks great. Thanks

 

[Jimrahbe]

scott,

I'll have to let some of our chemistry guys chime in on this one, but I don't think it is possible for FC to go from 13 to 0 in 12 hours and still pass the OCLT...

Something is just not right...

Just like you, I'll be interested in what our Experts have to say..

Thanks,

Jim R.

 

[jblizzle]

Only way that should be possible is if your CYA is very low, close to 0.

How confident are you that the CYA is 35?
Are you running the pump for the OCLT?
Are you running the UV in your signature all the time?

 

[scottfy99]

I'm confident about the CYA. It is between 30 and 35. I did add CYA after tonight's test to raise CYA up to 40 per the app. I use a Taylor 2006-C kit. My pump and UV run 24 hours every day.

 

[kywildcats]

I'm no expert by any means, but this sounds like a testing error. Are you sure you're using the right reagents when doing the test? It may seem like a dumb question, but something is way off.

 

[scottfy99]

I'm positive. I do the exact same process every time. I even did one extra night of SLAM and OCLT because of the large daytime drop in Monday and no drop on Sunday night. I thought maybe something was off, but double checked everything last night and today. I had the same result.

 

[jblizzle]

Are you always testing the CC also?

 

[scottfy99]

I do if any FC shows. If it doesn't turn pink after 2 dippers then I do not test for CC.

- - - Updated - - -

I use 10 ml samples for FC.

 

[jblizzle]

Well perhaps you should try the CC test even if the FC is 0. At least once just to see if it is spiking, although I doubt it.

 

[scottfy99]

So after I added the 2 dippers and it didn't turn pink then I performed the CC and it still shows 0.

 

[scottfy99]

Also, should I only add enough bleach tonight to reach recommended level of 5 ppm?

 

[kabutotx]

Have you added any algaecide lately or in the past?

 

[scottfy99]

Not this summer. I did when I closed last year. Thanks.

 

[kabutotx]

Do you know which one? I was wondering if bromines got in the pool?

 

[scottfy99]

I did add Pink Treat in April because of pink mold. It has bromine. Although last week I drained my pool 4 times to the bottom step and refilled.

 

[jblizzle]

So, there is still some bromine in the water. I still do not have a good grasp on what implications that has.

 

[scottfy99]

I would think if bromine affects the water that much, then it should affect it at all times, not only during the day.
I added 2.5 gal of bleach last night. The app said if I would add that I should get a FC of 8. This morning my FC is 8.5 and CC is 0. Whenever the app suggests amounts to adjust, the result is usually very close. Great app! I'll update once I get home and test again. Thanks for everyone's help!

 

[pookiesunshine]

i would test FC at night about 30 to 60 minutes after bleach addition to be sure you are reaching your target. then, if you are still passing the OCLT but also losing massive amount of FC during the day, i would bump up the CYA slowly until the daytime FC loss is reduced. but i would not add big doses of CYA, certainly no more than 10ppm at a time, maybe 5ppm increments as you just did.

 

[jblizzle]

Well bromine cannot be protected from the Sun by The cya in the same way that chlorine can. So it sure seems that could be what is going on here.

 

[scottfy99]

I drained the pool to the bottom steps 4 times last week because of the high CYA levels. I've also added water throughout the summer because of evaporation. I would have thought the bromine would be diluted quite a bit by now. Is there a way to combat what little bromine that was added in April? Thank you.