First of all, thanks to TFP, I have had a truly TFP all summer long!
Background: I've been faithfully measuring FC levels all summer with no problems. A few unexpected readings that may be the result of two successive inaccuracies in opposite directions, but for the most part I've gotten to where I can predict what the reading will be with decent accuracy. I've noticed that as the temperature has decreased, so has the FC dissipation rate. For the last few weeks, I've averaged about 48 oz/day of bleach to maintain FC around 5 with CYA of 70. Since 32 oz of bleach raises the FC level ~0.5 ppm, this translates to a depletion rate of FC of about 1.5 ppm/day, which seems very reasonable (and quite a relief compared to the 3+ ppm/day during the hotter weeks of the summer). For much of the summer I was measuring every night, but lately I've been confident enough to only measure every 2-3 days. I've meticulously kept a spreadsheet of my pool stats all summer long.
Quandary: Lines 355-363 of attached spreadsheet [attachment=0:1e8t3clc]pool stats.xls[/attachment:1e8t3clc]
This period of 5 days indicates FC depletion of only 0.3 ppm/day on average, which is acceptable overnight FC loss in 24 hours! When I got the reading on line 360, I figured maybe the previous reading had been low and this one was high, but even if the line 355 reading was low by 0.5 ppm and the line 363 reading is high by 0.5, that would still be only 0.5 ppm/day of FC loss over this 5-day period! I didn't change anything in my testing method. I might not be that taken aback if the water were only 50 degrees, but at 70? Can anyone think of a possible explanation?