OCLT Question

brajgreg

Well-known member
Apr 28, 2012
80
Houston, TX
I did the OCLT last night just to make sure there wasn't something in my pool that I can't see (the pool is only a few weeks old to me as I just bought this house). I had taken the chlorine up to shock levels for my CYA (CYA = 45-50), and when I measured it last night it was at 17 (down from 20 earlier in the day).

Specifically, I did the 10 mL test on my Taylor K2006 kit, and it took 33 or MAYBE 34 drops last night to come clear. I hand stir the test. This morning it took me 30 or MAYBE 31 drops to get totally clear. That's either a 1 or a 2 ppm drop, but it's a pretty high FC level so I didn't know if that affects it. Is the 1.0 ppm rule hard and fast?

Also, on the OCLT in general, it says sometime after sundown is when you should test. Does this mean pretty shortly after sundown or midnight? I did my test at like 9:00 last night, but I was just wondering. And are you supposed to leave the pump running or does it really matter?

Thanks!
 

JasonLion

TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
37,879
Silver Spring, MD
The pump doesn't matter as long as you run it enough to be sure the chlorine is mixed in initially and a little before you test in the morning.

Early evening, as long as it is more than about 20 minutes after sunset, is fine.

The test is valid even when the FC level is fairly high as long as you are reasonably careful with your test technique, hold the titrant vial vertically, allow the drops to fall off naturally as opposed to "spraying" them out, and so on.

Your results are right near the line of acceptable. I suspect that the chlorine found something small to clean up, which is now gone. However there is a chance that there is some algae lurking somewhere hidden, such as inside a light niche or ladder.
 

Butterfly

TFP Expert
LifeTime Supporter
May 30, 2007
10,045
South Carolina
Also, on the OCLT in general, it says sometime after sundown is when you should test. Does this mean pretty shortly after sundown or midnight?
Well, as long as you start the test after the sun is off the pool at evening/night and before the sun hits it again in the morning. You are simply ruling out sunlight eating any chlorine.
 

brajgreg

Well-known member
Apr 28, 2012
80
Houston, TX
Correct, but that can be a little vague, don't you think (sorry if I sound like a contrarian :oops: )? I mean, let's say the sun goes down at 8:15, but you don't test it until midnight because you work and don't get home until late maybe?? Sun comes up at 6:30, so maybe you lose 1.2 ppm if you test at 8:30 but you only lose 0.8 if you test at midnight. It didn't exactly apply to my situation, but I was just wondering in general.

But maybe I'm not understanding the full reason for the test. If you have zero algae, should you lose zero FC overnight?
 

JasonLion

TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
37,879
Silver Spring, MD
If you don't have any algae the reading should come out exactly the same the next morning. However, there is always a tiny bit of non-repeatability in the test process, so we allow a difference of up to 1.0 to allow for testing errors.

While it is possible in theory to have something using up only 1 ppm of chlorine overnight, that situation is actually extremely rare. Usually you are either not going to lose any chlorine or you are going to lose several ppm of chlorine.

Your case is border line, so either your testing procedure was a little off, or there is one of those rare things that only use a tiny bit of chlorine (like algae hiding in a light niche).
 
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