FC drop

[dabuddy]

Hey guys, member me? I did the Bacqua-goo conversion a couple of weeks ago, went on vacation and now back and maintaining my crystal clear pool. I have a question about my FC consumption. I understand that FC is only consumed by sunlight or organic matter.

Sunday when I returned from vacation my CC was 0.5 (6 PM) and yesterday (Monday 8 PM) my CC wasto 1.0 so I brought my pool to shock level last night (FC 16.5 - took reading about 1/2 hour after adding bleach). ph 7.2, CYA 60.

This morning when I woke up I did a reading and my FC was 18.0 (actually went up!) CC was 0.2 (very very light pink; I don't think I needed a full drop to bring it back to clear so I estimate 0.2). pH 7.2. When I got home today from work I took a reading and my FC dropped to 8.0 and CC was 0.5. Swimmer load today was 3 teens for 2 hours and they didn't splash enough to raise the pH.My question/concern, with a CYA of 60 and FC of 18.0 this morning is it normal to drop 10 ppm today over a 12 hour period?

What should my actions be, wait, shock, other?

I just added 4 lbs, 12 oz of Borax just now to try to get pH up from 7.2 to 7.5.

 

[frustratedpoolmom]

I would say that's probably within normal limits....best way to check is to do an overnight comparison. Record tonights result, and compare it in the morning, see if your FC is holding in the dark....

 

[TizMe]

The following are some quotes taken from other threads about FC loss:

Sunlight losses are a percentage, so you lose more the higher the initial level. Disinfection loses are constant relative to the starting level, so it doesn't matter what you starting level is. For most pools sunlight loses predominate, but a heavy bather load could make the disinfection losses significant. There are some more complex effects if your level is low enough to allow algae to get started.

Chemgeek will probably correct me if I am wrong but first order extinction rates are based primarily upon the percentage of chlorine. So for example at a CYA of 30 ppm, I lose about 50% of the chlorine during the day. It doesn't seem to matter much what the level starts at but the extinction rate of 50% is pretty constant.

However, what I have found is that if you double the CYA, the extinction rate goes down by more than 50% so you come out ahead with higher CYA.

 

[JasonLion]

Chlorine loss to sunlight should have been 30-40%, call it 6 ppm. But you also had six person hours of swimming, which is a significant bather load. That could easily account for the other 4 ppm lost.

 

[dabuddy]

Thanks guys for the quick and clear responses. I'm still learning but I get it...

 

[dabuddy]

ph looks to be about 7.7 now.

 

[dabuddy]

Readings:

10 PM last night:
FC - 10.5
CC - 0.5 (very, very light pink though)
TC - 11.0
pH - 7.7

7 AM this morning:
FC - 10.5
CC - 0.5 (very, very light pink though)
TC - 11.0
pH - 7.7

This is my baseline for finding out how much FC I lose today due to sunlight and bather load.

 

[TizMe]

I think a CC of 0.5 can be ignored because sometimes even though the water appears clear with a pink haze sometimes that extra drop will make it actually turn clear. I always miss that extra drop when I'm testing!
Also IF after adding your drops you let it sit a few minutes and thrn looked at it again it will turn slightly pink

 

[chem geek]

If in doubt, you can use a 25 ml water sample instead of 10 ml and then each drop of titrant will represent 0.2 ppm instead of 0.5 ppm.

 

[dabuddy]

Cool, next time I do the test I'll use a 25ml sample.

In case anyone is wondering, my readings at 8:30 PM today after 2 swimmers for 2 hours and mostly overcast all day (sun came out mid-afternoon):

FC - 4.5
CC - 0.5 (very, very light pink though)
TC - 5.0
pH - 7.6

 

[Butterfly]

Cool, next time I do the test I'll use a 25ml sample.

In case anyone is wondering, my readings at 8:30 PM today after 2 swimmers for 2 hours and mostly overcast all day (sun came out mid-afternoon):

FC - 4.5
CC - 0.5 (very, very light pink though)
TC - 5.0
pH - 7.6

Okay, unless I missed something, your CYA is still 60? Then your MINIMUM FC should be 5 and you just reported 4.5ppm. :lol: With a CYA of 60, your Min is 5 & Target is 7 to 9, so you are going to have to add enough chlorine each evening to keep you above your minimum FC between doses! You may have to shoot for 10 or 12, depending on your pool's daily use (organics, bather load, sun, etc.) so that you do not drop too low. :lol:

It is not a good idea to get too close to your Min FC for your CYA, it can bite you when you least expect it! (Don't ask me how I know this )

So, what did you increase your FC to tonight?

 

[dabuddy]

Yup, CYA is still 60. i get FULL sun all day. I was shooting for 10 FC tonight. Since I don't want to drop below 5 perhaps I can yank the kids out a 1/2 hour earlier tomorrow! :lol:

I added 2.25 gallons according to the PoolCalculator to get it from 4.5 to 10. With your advice I will now get it to 12. I just added 3/4 more gallons. I ran out of R-0871 so I now am guessing I ordered replacement Sunday night so I should get it soon.

Thanks a bunch for your advice!

 

[Butterfly]

So glad you added some more! Sure don't want you to yank the kids out early! :lol:

 

[The Mermaid Queen]

perhaps I can yank the kids out a 1/2 hour earlier tomorrow! :lol:

yea, good luck with THAT one!!