Daily Sun Affects on FC


Well-known member
Jun 4, 2008
Near Knoxville TN
Has anyone plotted FC during the course of the day to see what kind of trend line it is from sun up to sunset?

Also how about cloudy days versus bright sun?

And one more, at the end of the day is there a "coast down" drop in FC at sunset? The reason is I've been doing my testing about 1 hr before sunset and I was wondering if that matters much. I would suspect that as long as it's about the same time each evening that would be OK since adding 1-96 oz jug of Ultra 6% (calculated about 2 ppm FC addition) has kept me at 8-8.5 FC, 0-CC at the end of the day when I test. I've been this way since shocking the pool last week, YEA!. (CYA is ~80 and Full sun all day).


LifeTime Supporter
Platinum Supporter
TFP Expert
May 7, 2007
Silver Spring, MD
FC levels fall over the course of the day, faster when there is direct sunlight on the pool and slower when there is indirect sunlight.

If you are only adding chlorine once a day the only number you need to be concerned with is the FC level when you test right before adding chlorine. As long as that number is at or above the minimum FC level for your CYA level you will be fine. When adding chlorine once per day, the lowest FC level is always right before you add chlorine.

When adjusting a SWG, tablet feeder, or Liquidator you want to test about an hour before sunset, though an hour or two earlier or later than that is not going to cause a problem.