Does UV use up FC like Ozone will?

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After the fill, I just kept track of the CYA based on how much dichlor was added and when we were in the area I wanted (35) I did the drop test for CYA and it was right about there (filled the tube all the way up before the black dot disappeared). I am due to use dichlor this week for the once a month "make up for lost CYA" but maybe I'll test the CYA and see where it's at. Possibly it has gotten lower and needs to be bumped back up. I hate to actually do that test each week because of it using 7ml of the solution each time but clearly it's worth it.

Tho I feel like the rate of FC use was high even when we had sky high CYA (before you taught me about how dichlor adds and adds and then the FC has to be higher because of it).
 
You shouldn't need to test CYA that often, but we're trying to diagnose a higher than normal chorine demand so testing it would be reasonable and you're at the point anyway (i.e. before you'll add more CYA to bump it back up).

If the FC percentage loss was high even when the CYA was high, well that's mysterious unless the active chlorine level was so low that pathogens could grow.
 
Ok so I tested today, 25 hours after last test. Prior test had FC at 1.2, today 0.4 so a 66% loss (interesting since in 12 hours I lost ~40%).

CYA is < 30 (could still faintly see the black dot right at the top of the mark).

For right now, I've added 5oz of 8.25% bleach (~+6.6ppm FC) and am going to let that sit there today figuring it can clean up whatever from the FC being kind of low. Then tonite, I'm going to put the FC at say 4.0ppm (unless it's higher already from what I added this morning) and then test tomorrow morning so I'll have an overnite loss rate with a higher FC.

THEN I'll put some dichlor in to get the CYA level back up (I figure enough to raise it like 6 or 7 since I don't think I'm too much below 30 and the target is 30 - 40), test the CYA and FC so I know the start and then test FC again in the morning. I might have to delay this last test as we'll likely be in the tub tomorrow nite so I don't want the batherload to affect things.

Sound like a good plan?
 
I'd add enough CYA to get you to close to 40. That's a good value for a manually chlorinated pool at our latitude/climate. 6-7 ppm would be fine, then retest a week after it dissolves to verify. Add a bit more if needed later. I ran my pool last summer at about 30 ppm CYA and was going through more chlorine for sure than now when I have it at 40.
 
This is a covered spa, but I agree that 40 ppm CYA would be a better target especially since we know it will slowly drop over the month. Still, the chlorine demand is awfully high. Note however that (1-0.4)2 = 1-0.64 so a 40% 12-hour loss does correctly translate into a 64% 24-hour loss.
 
Ok haven't quite gotten to do all the testing I wanted yet but....

Last nite we took the FC up to 5.2ppm with bleach (actual read, pool math said it should have been 6.3ppm so we have weak bleach/more than 500 gallons). That was 10:30pm last nite. Today at 11:30am so 13 hours, the FC was 3.6ppm so a 31% loss. Less than with the lower FC to start but not great I guess.

Going to bump up the CYA with dichlor today and see if that helps during the week. Have some things going on so it might take a few days to get to a no batherload ONCLT.

Also, when we did the weekly full testing today, the TA is up to 80 (we took it down to 50 on 7/18 before we added borates) so I want to post a thread about that to see if I need to care right now.
 
Tested today at 8:45am. Last test was yesterday at 3:15pm so 17.5 hours ago. FC went from 14.6 to 10.0 so a loss of 32% in 17.5 hours. Slightly better I guess but chemgeek I wasn't sure what your formula was from before:

(1-0.4)^2

To see what 32% in 17.5 hours turned into for 24 hours. Either way, it's still higher than you'd expect even with the 37ppm CYA so it looks like the tub just eats more chlorine.
 

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