OCLT - does duration of the night matter?
- Thread starter novice123
- Start date
- Jun 1, 2018
- 15,974
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- SWG Type
- Hayward Aqua Rite (T-15)
At least 8 hours.
You want it to be dark for both tests. After sunset & before sunrise.
I am not sure of anywhere that it’s dark for 16 hours except Alaska certain times of the year.
You want it to be dark for both tests. After sunset & before sunrise.
I am not sure of anywhere that it’s dark for 16 hours except Alaska certain times of the year.
Actually a December night in London lasts almost 17 hours 
and it's longer as you go up North within the UK.
and it's longer as you go up North within the UK.
- Mar 5, 2020
- 3,221
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- 66000
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- Astral Viron V35
I assume that's a heated indoor pool? I'd call it a day after 12 hours. If you have a UV-system, make sure to turn it off during the OCLT.
Yep indoor as in an enclosure that’s retractable. Oh man I forgot to turn the uv off! It’s been 2 hours 50 mins since sundown when I took the chlorine reading. Would it mess with the results if I turn the uv off now? There’s another 5h 25 mins left till sunrise.
- May 3, 2014
- 62,657
- Pool Size
- 6000
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- Fiberglass
- Chlorine
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- SWG Type
- Pentair Intellichlor IC-40
Turn it off. Test FC in the morning. If UV is working, you will fail the OCLT.
- Jun 1, 2018
- 15,974
- Pool Size
- 26000
- Surface
- Vinyl
- Chlorine
- Salt Water Generator
- SWG Type
- Hayward Aqua Rite (T-15)
Interesting!Actually a December night in London lasts almost 17 hours
Can you explain this? Why would you fail the OCLT if the UV is working?
- May 3, 2014
- 62,657
- Pool Size
- 6000
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- Fiberglass
- Chlorine
- Salt Water Generator
- SWG Type
- Pentair Intellichlor IC-40
- Mar 5, 2020
- 3,221
- Pool Size
- 66000
- Surface
- Plaster
- Chlorine
- Salt Water Generator
- SWG Type
- Astral Viron V35
Because the UV would consume FC. That's why we do OCLTs over night. But that doesn't help much if you add artificial UV.
Depends on how much UV a UV system actually emits. That's what Marty was referring to. If the OP still passes the OCLT, then the UV system did bugger all.
Depends on how much UV a UV system actually emits. That's what Marty was referring to. If the OP still passes the OCLT, then the UV system did bugger all.
I failed the test 
went down by 1.7 not sure if it’s due to leaving the UV on for three hours during the test before I turned it off. what’s confusing is I read the wavelength of the uv is different to that of the sun and hence it doesn’t eat the chlorine up?
Current chlorine level is at 11.7 .pH has gone up to 8.15 and Cya is 33. But water is crystal clear
What should I do now?
went down by 1.7 not sure if it’s due to leaving the UV on for three hours during the test before I turned it off. what’s confusing is I read the wavelength of the uv is different to that of the sun and hence it doesn’t eat the chlorine up?Current chlorine level is at 11.7 .pH has gone up to 8.15 and Cya is 33. But water is crystal clear
What should I do now?
Also I still haven’t turned the heater on as apparently heat also consumes chlorine? Or is that not true?
- May 3, 2014
- 62,657
- Pool Size
- 6000
- Surface
- Fiberglass
- Chlorine
- Salt Water Generator
- SWG Type
- Pentair Intellichlor IC-40
your testing equipment likely will not provide the accuracy you need. Up to you. If the water is clear and no debris on the floor in the morning, maintain FC target range and see how it goes.
No.
Or is it because bacteria breeds in warmer environments and hence more chlorine is used up?
Also forgive me is this is a dumb thought but when someone fails the test by a slight margin could it be due to the Crud that’s inside the sand filter?
- May 3, 2014
- 62,657
- Pool Size
- 6000
- Surface
- Fiberglass
- Chlorine
- Salt Water Generator
- SWG Type
- Pentair Intellichlor IC-40
If your FC does not drop below minimum, you do not get any bacteria or algae.
The sand filter is fully immersed in the chlorine laden water. So acts the same as the pool. If the organics are neutralized, no consumption of chlorine.
- Mar 5, 2020
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- 66000
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- Plaster
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- Astral Viron V35
I wouldn't phrase it as "heat consumes FC".
Chlorine is constantly oxidising things and the reaction rates of those processes speed up with increasing temperature. That's why the OCLT criterion is not 0 ppm, because even with no algae and no UV there will be some loss. If you go to the extreme of very high FC, very high temperature and an extra long night of 16 hours, then you might have higher natural losses that are border line with the standard criterion. Plus the resolution of your test. We usually recommend FAS-DPD testing, which I know can be hard to source in the UK. And being above FC 8ppm, I believe you have to do a diluted test with the PoolLab, which costs more test accuracy.
My suggestion would be to drop your FC below 8ppm, but above target FC for CYA 40 (round up). Do the OCLT at a level where you don't have to do a diluted test with your PoolLab.
- Mar 5, 2020
- 3,221
- Pool Size
- 66000
- Surface
- Plaster
- Chlorine
- Salt Water Generator
- SWG Type
- Astral Viron V35
Does the diluted test reduce the accuracy?
They do have reagent tablets for high range chlorine: is that a better way?
Thanks I’ll have a look!
Is that because we never reach the point of making the pool water sterile and hence there are germs/bacteria that are always being dealt with by the chlorine? Or would this be the case even if the pool was 100% bacteria/virus/germ free?
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