OCLT ? What is that Test ?


Mod Squad
LifeTime Supporter
TFP Expert
Apr 18, 2010
East Texas
Test right at dark and again right at first light or before to see if anything besides swimmers or sunshine is eatin' your chlorine...............i.e. algae.

Overnight chlorine loss test


LifeTime Supporter
TFP Expert
Nov 5, 2008
Overnight Chlorine Loss Test -- bring pool up to a certain FC after dark, test again to be sure of true level after circulating well, then retest before sun hits the pool. If FC has dropped overnight, you must still have organics in the pool. Must not run SWG during this test.


LifeTime Supporter
Jun 27, 2012
Lafayette, Ca / USA
In additon to that previous question above, is ANY drop normal?? e.g., What if my FC is say 7.4 at dusk -- what would a "normal" drop be - or should it be 7.4 in the morning ?


Well-known member
Jul 3, 2010
cody21 said:
In additon to that previous question above, is ANY drop normal?? e.g., What if my FC is say 7.4 at dusk -- what would a "normal" drop be - or should it be 7.4 in the morning ?
You're testing FC and CC. Those two added together give you your TC number. The OCLT test is passed when you have a loss of 1ppm or less of FC and a CC reading of .5 or less. The third criteria is crystal clear water.

Here's what I do when my water is clear but I suspect something is off. I add Chlorine to the upper part of my guidelines for my CYA. Mine is 4 to 6ppm for a CYA of 50. With the filter running I add my Chlorine when the sun dips below the trees and is completely off the pool. I test my FC level one to two hours later. I like to give the Chlorine a chance to sanitize anything introduced by swimming that day. Next day I check before the sun hits the pool.

If my night time test is 5ppm FC and my morning test is 4ppm or more, I passed the FC part. If my CC test is .5 or less, I passed the CC part. If my water is still crystal clear, that's a passing grade.

All my tests are done using the 10 ml sample size. There's no need to use the 25 ml size unless you like doing so. The 10 ml sample is precise enough and save on testing agents.

If your water's not clear and you are shocking, there's no reason to perform the OCLT until your water is clear
Not trying to revive an old thread but I have a follow up question and still have not figured out how to post a new question - could be my out of date web browser.

Anyway, for the OCLT, do I do this with the pump on or off? I was told a long time ago to run the pump at night. Can't recall the reason, but the pump timer is set to kick on about 10:00 p.m. and the spa overflows into the pool making a very pleasant and relaxing sound right outside my bedroom (perhaps that was the reason). So for an OCLT, do I need to adjust my pump schedule or not?


TFP Guide
Jun 6, 2012
Central PA
I believe most are in the process of completing a "SLAM" when they do the OCLT, so their pump would be running all night. (It's best to run it 24/7 during the SLAM process.) You'll need to run the pump, for sure, for at least a half hour when you add your chlorine at night to make sure of your starting FC, and at least a half hour in the A.M. to make sure the water is well mixed. If your pump runs all night, you're good to go, just be sure to retest the morning value before the sun hits it.
Checked first thing this morning. Sun had been up for 20 minutes before me but the only shade my pool sees is early morning (pool in back yard and house faces slightly south of east so no direct sunlight for at least another hour). CL is still >5ppm and FC is still 4.5ppm, CC is 0.5 so no change at all in 8 hours.

(1) if pool passes the OCLT that means do NOT slam?
(2) with my numbers, is SLAMing going to do anything other than waste chlorine? (i.e. is there any preventative benefit to a SLAM)


Silver Supporter
Gold Supporter
Aug 14, 2016
Rochester, NY
Looks like you performed both OTO and Fas-DPD tests. No need to perform both. fas-DPD is the preferred test.

With no loss in FC and CC of 0.5 you are good to go, assuming water is clear. SLAM at this point would do nothing except use more of your chlorine supply.
Thanks RK. I suspected as much but it is good to verify. My water is absolutely crystal clear. You can toss a dime in the pool and tell if it landed heads or tails up.

I did perform both CL test. Why does the TF-100 ship with an OTO kit? I have been trying to figure that out since the FAS-DPD is so much more accurate and reading the posts on this forum it appears many if not most pool owners are running CL well above the OTO test limits. I have enough trouble distinguishing between the colors up to 5 but anything over 5ppm in the OTO kit is just dark yellow. Do most members just dump the OTO kit and do the FAS-DPD daily or do you just test weekly or do you keep some distilled water around and mix 50/50 to get a gauge using the OTO?

So far I am happy with the new diet of liquid chlorine, I have gotten into a routine of come home, grab a sample and add chlorine before going inside. I am sure over time I will figure out how much to add daily and not worry too much about it. Honestly, the biggest benefit I see to liquid chlorine is that I actually check and put it in. Using tabs, it was so easy to fill the floater and go about my business until I just happened to think about it or looked outside and notice it was floating high in the water, so my CL was probably bouncing like ball.


LifeTime Supporter
TFP Guide
Jan 18, 2015
Bunbury, WA, Australia
The OTO gives a quick and rough -'yep everything looks pretty good' sort of idea. The FAS-DPD gives the precision and your right -many just use the FAS-DPD test.

As to how often to test its a personal preference and depends on the season but when you are just starting out it's a good idea to test more often to learn how your pool 'behaves'.