OCLT timing question


Well-known member
Jul 23, 2020
I know the instructions are to test after the sun goes down and in the morning before the sun hits the pool. Is there a recommended amount of time between the tests? I’m up at all hours and I’m sure I’d want to test it if I’m up early but I want to make sure I’ve gone at least the minimum recommend amount of time. I’m thinking at this time of year, sun is down about 9pm so would 4 or 5am be long enough (7 to 8 hours)? If more, I’d need to start at dusk and hold off until sunrise plus a bit for the sun to hit the pool. That might get me about 2 to 3 additional hours.


Silver Supporter
In The Industry
Aug 15, 2017
Spring Valley, NY
6 plus is minimum but go the longest possible before the sun comes up. For OCLT the sun should NOT hit the pool. Get it before sunrise....


Gold Supporter
Jun 28, 2017
Fresno, CA
Pool Size
The key is to eliminate FC loss from sunlight and that will isolate any FC loss that then can be attributed to organic contaminants. Its a diagnosis tool so the more dark time the better but avoid the sunlight at all costs.


Bronze Supporter
When I do an OCLT I typically do the evening test around 6 pm, when the sun is off the pool completely (lots of trees) and the next morning about 730 or so when it's light but no sun on the pool yet. About 12 hours or so.

I'd say arrange the time so there's at least 8 to 10 hours between tests. 6 hours isn't enough.


Mod Squad
TFP Expert
Platinum Supporter
LifeTime Supporter
In The Industry
Apr 1, 2007
Sebring, Florida
There is some over thinking going on here. UV from sunlight negates the test.....we agree on that. So, how much nighttime do we need to call the valid? I don't know.

Is ten hours enough? Surely
Is twelve hours better? Why?
Is six hours enough? If the loss is absolute zero, it certainly could be. If the loss is 1 ppm, maybe not.

So, what's the "exact" answer? Overnight.

Add to that a dose of common sense and you have a valid test.


Gold Supporter
Bronze Supporter
LifeTime Supporter
TFP Guide
Sep 23, 2015
Brisbane, Australia.
Pool Size
Salt Water Generator
So sun down to sun up. If your too early at sun down it’s cold beverage time. If your too early at sun up it’s coffee time. That’s all good for retiree’s but if you have to get to work a half hour each side that would statistically still be valid as long as your not pre testing before sundown and post testing after sun up.
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