Please bear with me.....I'm still trying to figure stuff out...
I'm wondering if my new bottle of FAS-DPD titrating reagent is suspect, or if I really am losing as much FC to sunlight as it seems from the test results. Yes, my CYA is quite low right now (I know it's under 30 from the test, but not sure how much lower...the black dot is definitely starting to get hard to see at the 30ppm point, but I can still see it). This past week I shocked for yellow algae, and I didn't raise the CYA first because I liked the idea of a lower target FC level and keeping the titration tests tolerable. But as it turns out, that logic may have been very flawed.....
Here's what's got me confused a bit. I noticed that when I got home from work and tested the FC level, it was always quite a bit lower than what I would have expected. The FC drop overnight has been small (2-3 ppm at first, 1 ppm now), but the FC loss during the day has been huge. As an example, I'd start the day with 18 ppm FC, and I would have my wife dump 2 gallons in during the day at different times (which I believe would equate to 20 ppm for my pool), and the FC would read 14-16 ppm after I got home from work (a 22-24 ppm loss!). I would have expected it to be higher because in the few weeks prior to doing the shock, I was maintaining a level of around 5 ppm of FC, and I wasn't having too hard of a time doing it. The sunlight was perhaps killing off 2-3 ppm per day maximum it seemed. That doesn't seem right if I'm really losing this much during the day while doing the shock. I didn't think the algae was consuming too much during the day if I'm not losing a whole bunch of FC at night. But perhaps I'm wrong there, too.
So is it a percentage kind of thing, in where the more FC you have in the pool the more you'll lose to sunlight? Or does it appear that my reagent may be suspect? Is the algae consuming more during the day than at night? I certainly knew that I'd be losing FC during the day with my low CYA level and the July sun, but this just seemed much more than I would have expected.
Any and all help would be much appreciated. I'm off to get some CYA now....I promise!!
BTW, going into this my pH was 7.4 (higher now, I'm sure), TA 80, and CH 350.