FC Drop - Is this normal?

edif30

Gold Supporter
Jun 16, 2017
52
Thorofare, NJ
I have been a TFP supporter and following the great advice on this forum. I am back for another scenario.

This pool season has been great. Only adding in liquid 12.5% bleach daily. Typically my CYA was 45 and my daily chlorine dump was around 32oz to 64oz depending on how much sun was out. Disclaimer - My pool gets 100% direct sunlight most of the day. At night around 7pm I would add my chlorine. Most times my FC was around 5 or 6ppm. 24 hours later I do it all over again.

Then I started to notice that the FC was dipping lower into the 4-5ppm range. I then had to start adding in more chlorine. I started to watch more closely and decided to do an OCLT. Sure enough I was losing more than the 1-1.5ppm of FC overnight. I decided to SLAM. Recently I added in some stabilizer to get the CYA up to 60. I think I added a tad bit more than desired and upon testing my CYA was 65. If rounding is suggested lets just say its 70. Which isn't a nightmare but definitely outside the range.

That being said I started the SLAM. Took 2 days. Passed the OCLT and then monitored the daily FC loss. From 8am (Sun was already peaking into the pool directly until 10pm last night when it was dark, the pool lost a total of 13ppm. 8am the FC was 33. 10pm the FC is 20ppm. Again, the pool gets direct sunlight all day and while its great that I do not need a heater (my avg temp from Jun to Sept is 86 degrees), I'm sure if the FC dips low enough to start a bloom, its gonna happen fast. That being said, I just passed the OCLT. With a CYA of 70, should I really be losing 13ppm even with direct sunlight?

Any help is appreciated.
 
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edif30

Gold Supporter
Jun 16, 2017
52
Thorofare, NJ
Just an update. I just checked FC and it is at 19. So, from 10pm last night until now which is 10:32am, I lost 1ppm. It's also very cloudy, not any sun, and heavy rain.
 

CrystalRiver

Well-known member
Jun 19, 2020
236
Massachusetts
That is an extreme daily loss. 2-4 ppm per day is considered normal.

Have you tested the chlorine at least 30 minutes after adding, to make sure you are getting the effect you expect from your additions? How old are your reagents in your test kit?
 

edif30

Gold Supporter
Jun 16, 2017
52
Thorofare, NJ
That is an extreme daily loss. 2-4 ppm per day is considered normal.

Have you tested the chlorine at least 30 minutes after adding, to make sure you are getting the effect you expect from your additions? How old are your reagents in your test kit?
Yes, I actually wait longer. Usually an hour. But like I said. From 33 in the morning 8am to 10pm it shot down to 20ppm FC. Its now 10:40am and the FC is 19ppm. Sun is not out though. I have brand new DPD powder with a best date of 5/2021.
 

edif30

Gold Supporter
Jun 16, 2017
52
Thorofare, NJ
When you have very high levels of FC, you are going to experience losses far more rapidly than when at maintenance target ranges.
This is interesting. Is there any documentation or something that explains why chlorine breaks down faster when the concentration is higher? I'm mainly curious of the scientific reasoning.
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
32,984
Laughlin, NV
Is there any documentation or something that explains why chlorine breaks down faster when the concentration is higher? I'm mainly curious of the scientific reasoning.

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Chasarms

Silver Supporter
May 8, 2020
347
Dardenne Prairie, MO
This is interesting. Is there any documentation or something that explains why chlorine breaks down faster when the concentration is higher? I'm mainly curious of the scientific reasoning.
There are MANY folks here at TFP that understand it in much greater detail than I, but as I understand it, it would be because of the FC/CYA relationship.

FC chemically binds with CYA to form a molecule that is far less susceptible to UV than it otherwise would be. At any given time, the vast majority of FC is bound, and a small percentage is available to do the dirty work. This is the reasoning behind the FC/CYA chart. For any given CYA, there is an appropriate FC target to keep things in balance.

If FC is well over target for a given CYA, much more of it (as a percentage) is going to be floating around unbound as there is no available CYA for it to bind to. So it is highly susceptible to UV and will be destroyed by it more quickly. It really no different than trying to maintain 3-4 ppm of FC with no CYA in the water.
 
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edif30

Gold Supporter
Jun 16, 2017
52
Thorofare, NJ

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