Losing FC in direct sunlight

[doublewide6]

FC 9
PH 7.6
CYA 65
CH 80
salt 2300
CC .5

If my pool is covered all day it is fine, but when I pull the solar cover and leave the pool exposed to the sun I'm losing a whole lot of FC. Yesterday morning 7AM FC was 14.5, in late evening it was 9 FC. The SWG ran 12hrs. at 70% and it is oversized. I preformed the OCLT test and it passed with no problem. I had algae about 3 weeks ago, and I slammed the pool, and kept it at slam level for several days and passed the OCLT test then. Water looks nice and clear right now. My guess is algae, but why am I passing the OCLT? Could there be anything chemically that would cause this?

 

[cowboycasey]

the only thing i see is your CYA is a little low at 65 ( probably 60), maybe take it to 70 or 80 and see if it will hold better. also your FC is high for a swg, you should be around 7 (pool school is down so I am not 100% positive on that number) with your FC number up over 10 it will lose more during the daylight hours.

 

[chem geek]

For an SWG, the FC minimum is roughly 5% of the CYA level so 4 ppm at 80 ppm CYA. This is a minimum so at the low point of the day in the morning just before the SWG turns on. You likely need to target higher so that it never gets below the minimum.

 

[doublewide6]

Okay thanks for the help. I am going to pull the cover off today and watch the numbers closely. I believe 3-10 FC is what is recommended by tfp with SWG and Vinyl. Thanks

 

[Griswald]

With the now 17 consecutive days over 90 degrees we are having, I had to uncover the pool (water temp hit 96 on top) and noticed a big drop in my FC as well. I doubled my SWG output time from 20% to 40% yesterday after discovering that I was hovering around the 2.0 FC mark. I quickly added 3/4 gallon of bleach as directed by pool pal and am at 6.5 FC as of 9:00 last night.

I will check it when I get home from work, but I can only hope that it stays above 3.0 mark. Also really low on water, 2 more days forecast above 90.

My numbers are very close to the OP's except I have 3000 ppm salt.