Losing Free Chlorine

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surferboricua

Active member
Apr 25, 2022
27
Ocala/FL
Pool Size
8700
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Greetings,

Recently had a party at home and had a lot of bathers jump in the water. Free Chlorine the day before was a 11.5ppm. I knew it was high for my 90 CYA level, but knowing was going to have a high bather load, I did not adjust anything. Came the next few days, and the FC levels have been consistently going down from 11.5 to 9, 8, 7, 6, and now 4.5ppm since Saturday. I had not changed anything on chemistry or pump/SWCG output, except for today where I bumped up the output on my SWCG. Based on reading I think I should do a Overnight Chlorine Loss Test, and see what I come up with tomorrow morning. Any other suggestions?

Chemistry Levels:
FC 4.5
CC 0
pH 7.6
TA 90
CYA 90
CH 450
 
Run your pump 24/7 during OCLT.

Overnight Chlorine Loss Test

"If performing the OCLT independently as a diagnostic test, it is vital the pump be running at least 30 minutes before the grabbing the nighttime and morning water samples for testing in order to ensure the water is properly mixed."
 
I took my evening chlorine test while pump was running. I scheduled it to power on 1 hour before I do my morning Chlorine test so water will have circulated for an hour. Let me know if that wont do, you still think I should run 24/7 for OCLT only?
 
surferboricua said:
I took my evening chlorine test while pump was running. I scheduled it to power on 1 hour before I did my morning Chlorine test so water will have circulated for an hour. Let me know if that wont do, you still think I should run 24/7 for OCLT only?
Click to expand...
You should be good!!! Good luck, fingers crossed!
 
So in preparation for a SLAM, I went ahead and lowered pH to 7.2, and did the CYA test. I had tested CYA before, but this time every time I redid it seem like it was more 100 CYA rather than 90... Now I am thinking this is probably the reason why I got in this situation where I need a SLAM. I am thinking my next steps are to replace some of the water, to get the CYA levels down, before I do the SLAM.
 

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To figure out how high your CYA is, mix equal amounts of tap and pool water.

Use that water to fill to first line in CYA test, then reagent to second line. Multiply your result of CYA test by 2. That will help you know your real CYA level and how much to drain.
 
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PoolStored said:
To figure out how high your CYA is, mix equal amounts of tap and pool water.

Use that water to fill to first line in CYA test, then reagent to second line. Multiply your result of CYA test by 2. That will help you know your real CYA level and how much to drain.
Click to expand...

Makes sense, as the tube doesn't read more than 100. Once I get that CYA level, how do I know how much to drain?
 
PoolStored said:
it is 1:1.

If you have CYA of 100, drain 50% to get to 50.
Click to expand...
So here it is, tested CYA twice. Once with 50/50 pool and tap water got 40 which multiplied by 2 get me a CYA of 80. The other test I did with all pool water, and after tried a few times, landed on the same 80 CYA reading. Now I have to say that this time I had better sun lighting, than when I tested earlier. So being that before I had consistently gotten a CYA of 90, I want to say the truth lies between 80 and 90 CYA.

With that said, do I still want to drain, or SLAM assuming a 90 CYA level?
 
It's your call. The trouble with SLAMming at 90 CYA is that your target FC level is 35, which will require a TON of chlorine.

90 CYA actually isn't out of line for SWG owners for daily use per the FC/CYA Levels. So if your pool were clear and not needing a SLAM, reduction of CYA probably wouldn't be needed. But while you're SLAMming it will require a lot more liquid chlorine.

So it's a decision to make. Do you lower the CYA by replacing water (maybe about half to get down to 40-50 CYA), so that the SLAM will be easier, knowing that you'll want to bump up your CYA again when you're done to at least 70? Or do you just use more chlorine to SLAM the pool now, knowing when it's done you can let your SWG run at 90 CYA without too much trouble?

It's hard for us to answer and likely depends on how easily the water is exchanged for you, and how expensive chlorine is in your area. 8.7k pool going from 5.5ppm to 35ppm of FC will take a little over two gallons of LC, and then you'll need to maintain that higher level through whatever is eaten up by the sun, which may be difficult to do without a lot of additions.
 
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I have 6 gallons of 10% Sodium Hypochlorite. I think I do want to get my CYA down some, but not really looking forward to dumping half my pool water at the moment. Considering I was having no problems up until having a big party with lots of people in and out the pool. I am inclining more towards SLAMming assuming a 90 CYA. And if the SLAM goes well, then gradually replace pool water until a 70 CYA.
 
CYA does degrade over time too. I'm told about 5-10ppm in a month if it's hot, which in Florida probably isn't going to be an issue to achieve.

Get the pH down first - once it's above 10ppm of FC testing pH is going to be inaccurate as the dye gets bleached out. When your pH is down to 7.2, add the liquid chlorine and get the SLAM going! Hopefully it's fast. If you are failing OCLT with clear water, be sure to check anywhere algae could be hiding, like skimmer throats/weir doors/foam behind weir door, light niches, ladders, etc.
 
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I did drop pH to 7.2 this morning, in preparation for the SLAM. I am hoping SLAM goes quick since the water is as clear as it's always been, and CC at 0. However, did not pass OCLT.

I set my pump to manual for 24 hour operation, and dumped 3 gallons of 7.5% Sodium Hypochlorite, per PoolMath for 90 CYA. SLAM started.

Will post an update later.
 
OK, test and raise often, scrub away, and check those hidden areas well. They're often culprits. The number of times a user has been frustrated about OCLT failing with crystal clear water for days on end, just to take out a light and have a flood of algae come out, or find the back of their weir door foam is slimy and green, is really high. And if they didn't find and take care of that, the algae would just keep coming back from those spots as FC went back below SLAM levels.
 
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