CYA and FC?

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VOLmaniac

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Apr 24, 2013
20
Hi all,

I have a question regarding CYA level and FC. My pool is in direct sunlight for most of the day. I have opened the pool with success thanks to this site. My CYA is around 45. (I say around, because I'm not sure when you actually claim the black dot is no longer visible. It gets very faint at 45ish, but I can still barely see it if I squint very hard and focus on it from the beginning. If I have my wife look at it at 45 but not focus on it while filling the tube, she can not see it at 45.)

Anyway, my FC will typically fall about 5 or 6 during a full day of sunlight. This is with good numbers on everything else using the TF 100 kit. MY CC is 0. Is this much of a fall in FC typical, or should I consider a higher CYA level? I'm nervous on raising my CYA level due to the cautions I've read hear about issues with a CYA level too high.
 
Post up your full numbers for better help. If you are Going with 10 Fc lossing 5 fc per day would be about right for 45 cya. Might be good to raise it to 60 to see if that helps. I would do an overnight chlorine loss test before raising it just to be certain nothing is it your water which would be harder to kill with a higher CYA
 
Thanks. I have done the overnight test, and it indicates no FC loss. My pool is crystal clear, and I am not really having any issues right now. I just a new pool owner and want to make sure this FC drop in direct sunlight is the norm for the BBB method or if I need to raise my CYA level. I guess I'm also making sure I'm getting an accurate CYA result when testing since the CYA test seems to be a more subjective test. My numbers are (FC was before adding bleach to raise it):

FC - 3
CC - 0
ph - 7.6
TA - 110
CH - 70
CYA - 45
 
5-6 ppm is a bit excessive for your CYA of 45. Average is normally about 2-3 ppm assuming very little swimmer load. YOu might have some invisible organics still in your pool.

If you have not yet performed the OCLT, I would suggest doing that. Report the results
 
duraleigh said:
5-6 ppm is a bit excessive for your CYA of 45. Average is normally about 2-3 ppm assuming very little swimmer load. YOu might have some invisible organics still in your pool.

If you have not yet performed the OCLT, I would suggest doing that. Report the results
Click to expand...

I have, and the FC holds with no drop. I suspect I may be reading the CYA level incorrectly in fear of getting it too high. Is there a guide regarding the test for CYA?
 
JasonLion said:
Remember that the amount of chlorine you lose to sunlight depends on how high the FC level is to start with. Losing 5-6 out of 9-12 is normal. Losing 5-6 out of 7 means something is wrong.
Click to expand...


Oh, ok. That makes more sense then, maybe. I've been adding enough to get it to 8 to 10 range anticipating the 5-6 drop throughout the day. I wasn't comfortable leaving it around 6 or 7 because of the drop I was seeing. Maybe I should try setting it to 6 or so to see if that helps.

It doesn't make sense to me why the drop would be more when your FC is in the 9-12 range, but I will just accept that I guess. (The way my mind works, it's hard to accpet things with understanding the reasoning.)
 
Where are you located?

I feel like I am experiencing the exact same thing as you. CYA of 45ppm and to keep the FC > 4ppm, I am having to add 5-6ppm everyday. Passed the OCLT multiple times.
 

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VOL,

I believe the normal FC losses are on a percentage basis, so losing 5 ppm when you started with 10 is a 50% drop, but losing 5ppm if you started with 7 is about a 70% loss. So, if the 50% loss stays consistent and you start with 6, you *should* only lose 3 ppm or so, unless something else is in there trying to grow.
 
I have been doing the same as well, starting at 12.0 every morning, down to 8.0 in evening but with swg producing 2 as well, so 6ppm loss altogether. CYA about 45-50. Even though I passed all 3 shock criteria several days ago, including a good oclt, this has been making me think something is "lurking". very reassuring that all is probably well.
 
VOLmaniac said:
JasonLion said:
Remember that the amount of chlorine you lose to sunlight depends on how high the FC level is to start with. Losing 5-6 out of 9-12 is normal. Losing 5-6 out of 7 means something is wrong.
Click to expand...


Oh, ok. That makes more sense then, maybe. I've been adding enough to get it to 8 to 10 range anticipating the 5-6 drop throughout the day. I wasn't comfortable leaving it around 6 or 7 because of the drop I was seeing. Maybe I should try setting it to 6 or so to see if that helps.

It doesn't make sense to me why the drop would be more when your FC is in the 9-12 range, but I will just accept that I guess. (The way my mind works, it's hard to accpet things with understanding the reasoning.)
Click to expand...

You lose a percentage rather than an absolute number.
 
Pauls234 said:
VOL,

I believe the normal FC losses are on a percentage basis, so losing 5 ppm when you started with 10 is a 50% drop, but losing 5ppm if you started with 7 is about a 70% loss. So, if the 50% loss stays consistent and you start with 6, you *should* only lose 3 ppm or so, unless something else is in there trying to grow.
Click to expand...

Thanks, that makes sense then.
 
The loss from sunlight varies a lot based on the number of hours of direct sunlight on the pool, the time of year, and latitude. This post shows a table with some percentages where 50 ppm CYA would be around a 40% loss so having a roughly 50% loss at 45 ppm would not be completely out of line. This is why in very sunny areas some people run their pools with a higher CYA level, but of course that's a bit risky since if you do get the FC level too low and algae starts to grow it takes quite a bit more chlorine to shock the pool.
 
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