OCLT & What it takes to pass...

bedelliott · Aug 2, 2023

Hello wonderfully helpful pool community,
I understand that the guidance for passing the OCLT says a loss of 1.0 or less is ok, but why is that - how can you lose 1.0 FC if there's no sun/algae on/in the pool?
My next question is about my particular situation. You can see the pic of my pool, it's about 3.5' on one end and tapers to about 9', and the floor drain in the deep end does not work. I have 2 returns, each is about 4' from either side of my skimmer, and they both point towards the ends of the pool and a bit downward, especially the one in the deep end, in hopes of getting a bit more circulation with it. I suspect my deep end doesn't circulate perfectly because of the broken drain/lack of suction, but I run my Polaris daily for 2 hrs to help the circulation. The question is... can my weak circulation in the deep end be impacting my OCLT if I run my OCLT with the pump ON but the Polaris is OFF? I take my water sample very consistently, elbow deep from the same place in the pool, which is on the side opposite the skimmer, about 1/3 of the way from the shallow end to the deep end, so it's maybe 4-4.5' deep there. I'm curious because my overnight FC loss is consistently around .6-1, and yes, I do both tests when no sun is up. 2 weeks ago I removed the drain cover, vacuumed it out very well then took 10% bleach down there in a few baggies and slowly poured it in, where it sat for 3-4 days before my Polaris eventually found it & sucked it all out (it stayed there given its density over that of water), so my drain hole should be extremely clean.
Maybe tonight I'll make sure the pool is thoroughly mixed (by running the pump & Polaris for a couple hours) before taking the water sample, then do the OCLT with the pump off to see if that reveals anything different.
Any thoughts on if this is passing the OCLT and how a broken drain plays into it?
Thank you so much!

wireform · Aug 2, 2023

Your nightly loss is exactly what the sanitization of chlorine is supposed to do. Obviously there's some work left for it to do overnight and that is what it's doing. Are you using the pool late in the evening or are you close to something in the neighborhood that can have stuff airborne and dropping in the pool. If those are consistent readings I wouldn't worry. Can you post all your levels so we can see what you have.

Bperry · Aug 2, 2023

bedelliott said:
Hello wonderfully helpful pool community,
I understand that the guidance for passing the OCLT says a loss of 1.0 or less is ok, but why is that - how can you lose 1.0 FC if there's no sun/algae on/in the pool?
My next question is about my particular situation. You can see the pic of my pool, it's about 3.5' on one end and tapers to about 9', and the floor drain in the deep end does not work. I have 2 returns, each is about 4' from either side of my skimmer, and they both point towards the ends of the pool and a bit downward, especially the one in the deep end, in hopes of getting a bit more circulation with it. I suspect my deep end doesn't circulate perfectly because of the broken drain/lack of suction, but I run my Polaris daily for 2 hrs to help the circulation. The question is... can my weak circulation in the deep end be impacting my OCLT if I run my OCLT with the pump ON but the Polaris is OFF? I take my water sample very consistently, elbow deep from the same place in the pool, which is on the side opposite the skimmer, about 1/3 of the way from the shallow end to the deep end, so it's maybe 4-4.5' deep there. I'm curious because my overnight FC loss is consistently around .6-1, and yes, I do both tests when no sun is up. 2 weeks ago I removed the drain cover, vacuumed it out very well then took 10% bleach down there in a few baggies and slowly poured it in, where it sat for 3-4 days before my Polaris eventually found it & sucked it all out (it stayed there given its density over that of water), so my drain hole should be extremely clean.
Maybe tonight I'll make sure the pool is thoroughly mixed (by running the pump & Polaris for a couple hours) before taking the water sample, then do the OCLT with the pump off to see if that reveals anything different.
Any thoughts on if this is passing the OCLT and how a broken drain plays into it?
Thank you so much!

The test tolerance is +\- 1 drop and so the tolerance ends up being 0.5 anyway and so the 1ppm drop takes that into account.

The circulation won’t have any effect. But you should be using the 10ml test as it’s close enough.

bedelliott · Aug 2, 2023

Thanks y'all!
Wireform, we're not using the pool for 4-5 hours leading up to when I take the water sample, and my yard is surrounded by trees and I get a few (cedar elm) small leaves in it each night, but probably <10 this time of year, and no pollen or other airbornes that I've ever seen. Given this, could my FC still be doing some sanitization that creates the FC loss?
Bperry, I just reread the guidance and it says FC <=1.0, it doesn't mention drops. Why wouldn't circulation have an effect, esp given the density of bleach vs water?
I forgot to mention that I have a Liquidator, so it's slowly dosing bleach all day, but my pump shuts off at 5pm (3-3.5 hrs before sunset) and when I run my OCLT with the pump on I close the Liquidator valve, so no bleach can get into my pool.
Here are my levels:
CYA 80
pH 7.5
TA 80
CH 190

wireform · Aug 2, 2023

You would need to start the OCLT With the pump running at least 30 minutes before and it has to be running again in the morning for a minimum 30 minutes before sampling the water.

Bperry · Aug 2, 2023

bedelliott said:
Thanks y'all!
Wireform, we're not using the pool for 4-5 hours leading up to when I take the water sample, and my yard is surrounded by trees and I get a few (cedar elm) small leaves in it each night, but probably <10 this time of year, and no pollen or other airbornes that I've ever seen. Given this, could my FC still be doing some sanitization that creates the FC loss?
Bperry, I just reread the guidance and it says FC <=1.0, it doesn't mention drops. Why wouldn't circulation have an effect, esp given the density of bleach vs water?
I forgot to mention that I have a Liquidator, so it's slowly dosing bleach all day, but my pump shuts off at 5pm (3-3.5 hrs before sunset) and when I run my OCLT with the pump on I close the Liquidator valve, so no bleach can get into my pool.
Here are my levels:
CYA 80
pH 7.5
TA 80
CH 190

I meant the tolerance of the chlorine test itself, not the OCLT. If the OCLT pass/fail criteria was 0ppm, very few people would ever pass it given the chlorine test has a separate tolerance. Experience seems to have shown that <= 1ppm is a good indication that algae is gone.

duraleigh · Aug 2, 2023

The OCLT is a homegrown original test developed by TFP.

From memory, I think chem geek and Jason lion were, by far, the two most influential members in it's development. Like others have said, the 1 ppm loss in the dark took into account the vagaries of having an open body of water still subject to contaminants but minimally so.