OCLT test, how long, the night hours can very


Mod Squad
LifeTime Supporter
Jan 17, 2012
Evans, Georgia
I do think the cover should be off to allow any CCs to leave. Your water "breaths" and the cover traps the gases. I wouldn't want that to alter your OCLT..... but I;m just guessing if it does or not?

Maddie 🌻


Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
Laughlin, NV
At night there is no UV so no CC is removed from the water during that time. So the cover will not make any difference. The pump does need to remain on over night.

Texas Splash

Mod Squad
TFP Expert
LifeTime Supporter
what i am trying to find out is, if you run a OCLT and test after 8 hr, will it be the same if I would have run for 12hr
If I am understanding your scenario/question correctly, the answer would be probably no. That's because after about 10-12 hours in the summer, most places probably already have sun over the horizon on the water, so the FC will begin to fall.


Well-known member
May 30, 2012
Montville NJ
There are two things (generally) that lower your FC - sunlight, and organic material in the water.

The point of the OCLT is that there is no sunlight, so if your FC goes down, it is due to organic material.

If your OCLT starts or end when the sun is up, then you have not eliminated the effects of the sun from the equation. Make the duration as long as possible while stopping and starting in the dark.


Gold Supporter
Jul 9, 2019
Winnipeg, Manitoba
Some of the guys I meet through work live so far north that the sun never really sets in the summer. I have no idea if anyone has a pool that far north but it would make for a challenging OCLT methinks.