FC at 0 CYA doesn't decay as expected

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davethomaspilot

Bronze Supporter
Aug 30, 2015
97
Apex,NC
I've never added stabilizer to my pool and I can't measure any CYA using the Taylor test kit.

Last week I measured ORP and FC using the Taylor FAS-DPD test before, and after, the pool was exposed to one hour of direct, overhead sunlight. I saw no difference in either ORP or FC during that hour!

Today, is a sunny, low humidity day in NC. I just did 6 FAS-DPD drop tests over the hour when the sun was directly overhead. Again, I see no measurable difference in either FC or ORP.

I do have .3 ppm of copper in the pool, even though I turned the ionizer off a week ago. Is it possible the copper protects the FC like CYA does?
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When I was only using copper ionization between weekly shock treatments, I could see the copper PPM decay when the ionizer setting wasn't high enough. It would decay to near zero in a few days, when the ionizer was off completely. However, now the ionizer has been off for a week with very little, if any, decay in copper PPM.

Maybe the copper gets depleted when it does the disinfection and now that there's FC, the "load" on the copper is lower? Is there another explanation why the copper could be observed decaying with a 1/2 life of one to two days, while now I can see no decay in a week?

Water temp 79

CYA stabilizer never added, dot clearly visible in Taylor test which implies < 30 ppm.
ORP 695
FC 1.2
PH 7.4
TA 80
CH 380
copper .3 ppm
iron 0 ppm

Any ideas why I don't see anywhere near the expected FC decay rate?
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Copper doesn't protect the chlorine from sunlight, though it may reduce chlorine consumption a bit due to it's algaestatic action.

Copper level will never decay unless water is removed from the pool by some method other than evaporation and replaced or it is deposited on pool surfaces or bathers. You'd be a millionaire if you could find an easy way to get rid of it.

It could be that your chlorine loss is within the resolution of the test. UV degradation of chlorine is proportional to the chorine level, so with only 1.2ppm, the loss over a 10 minute period or even an hour is going to be very, very small.
 
It could be that your chlorine loss is within the resolution of the test. UV degradation of chlorine is proportional to the chorine level, so with only 1.2ppm, the loss over a 10 minute period or even an hour is going to be very, very small
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According to the chart, the loss in 1 hour should be 50%, or a loss of .6 ppm. So, 1.2 ppm would decay to .6 ppm. That's well within the resolution of the test.

- - - Updated - - -

Also, the copper level as measured by the LaMotte copper test kit definitely decayed, a half life of a couple of days.

I can't reproduce that now, but over the previous months I observed the continous decay of copper as measured by the LaMotte copper test kit.

If it didn't decay, I couldn't have left the ionizer on for months without the copper level getting too high. It took a continuous setting of between 2 and 3 to keep copper around .3 ppm.

Higher settings resulted in copper getting to .5 ppm, but I could turn the ionizer of and watch the copper decay over a few days, at least as measured by the LaMotte Copper test kit.
 
Where are you pulling your test from...the surface? Elbow deep? Is the the pump running the whole time? UV will affect the water at the surface much faster. If I were you I'd scoop out some pool water into a shallow Tupperware and then test hour to hour.
 
I was pulling the water from as deep as I could reach.

Pump was running the full time.

If the data on FC decay rate is based on something like a shallow tupperware dish, or a jar with lots of surface area per unit volume of water, then maybe it makes sense. If the surface water, or only a shallow layer of water gets chlorine depletion due to sunlight, then it's a non-problem, at least for me.
 
davethomaspilot said:
According to the chart, the loss in 1 hour should be 50%, or a loss of .6 ppm. So, 1.2 ppm would decay to .6 ppm. That's well within the resolution of the test.
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The problem is, you have CYA in your water, you simply can't measure it. The melamine CYA test will not work with CYA levels below 20ppm. Even a small amount of CYA, say 10ppm, is more than enough raise the half-life by quite a lot. In fact, if you look at the chart, the major inflection point of all the half-life versus CYA curves occurs between 10-20ppm. If your CYA is 10ppm (unmeasurable by the Taylor test) and your FC is 1ppm, then your FC half life is over 4 hours.

As for Cu loss, it is likely due to FC. Chlorine is a strong oxidizer and will cause copper to precipitate out of pool water. Once the copper precipitates out, it will not show up on simple strip test.
 
Just did another measurement. 3:07 (2:07 standard time) in NC, full sun.

I now see 1.0 ppm. So a decay of .2 ppm FC in three hours (12:00 noon to 2:00 pm standard time) of full sun during the most direct sunlight I think the pool will see daily.

Maybe a bit more overhead around June 21, but not much.

That's much, much lower than the chart says I should expect.

I suspect the chart is based on theory, or glass jar results, not pool FC with pump running.
 
So, JofulNoise, a small amount of CYA in the pool would explain the results. And, I agree, I wouldn't be able to measure it.

Regardless of why, the actual 1/2 life I'm seeing suggests I need not add any CYA, would you agree?

I've tried swimming with my eyes open to see if I get any irritation. Nothing I can detect. I'm also leaving a swimming suit in 24/7 and haven't seen any bleaching yet.
 

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Also, JoyFulNoise, I WAS doing the shock thing once weekly (perhaps more often occasionally). So, maybe that was what was driving the copper ppm down?

By the way, my pool builder says this is what to expect (copper ppm decay), and all the copper pools he's set up do the same thing when maintained by running .3 ppm via the ionizer and shocking weekly.
 
davethomaspilot said:
So, JofulNoise, a small amount of CYA in the pool would explain the results. And, I agree, I wouldn't be able to measure it.

Regardless of why, the actual 1/2 life I'm seeing suggests I need not add any CYA, would you agree?

I've tried swimming with my eyes open to see if I get any irritation. Nothing I can detect. I'm also leaving a swimming suit in 24/7 and haven't seen any bleaching yet.
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It depends, what FC are you trying to maintain again?


davethomaspilot said:
Also, JoyFulNoise, I WAS doing the shock thing once weekly (perhaps more often occasionally). So, maybe that was what was driving the copper ppm down?

By the way, my pool builder says this is what to expect (copper ppm decay), and all the copper pools he's set up do the same thing when maintained by running .3 ppm via the ionizer and shocking weekly.
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If you were shocking weekly, then yes, you would be spiking your chlorine to levels well above the point at which the cupric ion (Cu2+) would begin to become unstable in water. It's entirely possible for metals to scale out into places where you won't see them or, as your PB so brilliantly surmised, your PebbleTec coating is perfect because it will "hide" the staining...At the low FC levels that are more consistently maintained by your IntelliChem system, I would expect to see less copper ion loss. Just remember to keep your ionizer off so that you're not building up excess copper.

Also, one thing to remember is that with low FC comes low oxidation rates of bather waste. So with your pool setup, you'd be more likely to see higher CC's and more persistent CC levels as bather load increases.
 
It depends, what FC are you trying to maintain again?
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At the 1-2 ppm I'm seeing now, chlorine use is so small, it's not a factor in keeping it high (the 2 ppm) given the relatively low CYA. I

turn the Intellichem off to see the FC decay. When Intellichem is on, it maintains at least .9 ppm, based on what I've observed so far. I plan to take FAS-DPD FC readings at the min ORP set point to see how well the FC is regulated at whatever CYA I actually have.

From what I've read here, the 2 ppm is likely "shock level", so I'm trying to see if I can observe the issues associated with having two much FC when the FC is on the high end. So, far, I can't.

Is there an objective way to determine whether the FC is causing things like skin irritation and bathing suit decay?
 
davethomaspilot said:
At the 1-2 ppm I'm seeing now, chlorine use is so small, it's not a factor in keeping it high (the 2 ppm) given the relatively low CYA. I

turn the Intellichem off to see the FC decay. When Intellichem is on, it maintains at least .9 ppm, based on what I've observed so far. I plan to take FAS-DPD FC readings at the min ORP set point to see how well the FC is regulated at whatever CYA I actually have.

From what I've read here, the 2 ppm is likely "shock level", so I'm trying to see if I can observe the issues associated with having two much FC when the FC is on the high end. So, far, I can't.

Is there an objective way to determine whether the FC is causing things like skin irritation and bathing suit decay?
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Shock level, at least at TFP, is considered 40% of CYA. So, if your CYA is 20ppm (lowest detectable limit), then your shock level would be 8ppm FC. So if you want to observe issues with high FC, you need to go MUCH higher.

FC rarely causes skin irritation. Skin irritations are almost always caused by pH being way off (often too high) or CC's being too high or both. However, if you want a simple test, try this test that was thought up by one of our TFP Guides, cowboycasey -

fill your bathtub with 40 gallons of warm water (80 degrees) put 1/2 pound of salt in and 1 tablespoon of regular bleach you use in laundry and mix... You have just made a saltwater pool chlorinated at 2ppm with 3500 ppm salt
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Water chlorinated to 2ppm without any CYA would be considered, by all measures, very harsh. If that level of FC causes you or your clothes issues, then you have your answer.
 
Whether I need to go much higher on FC "to get into trouble" depends on the CYA guess All I can determine with the test kit is that it's below 30 ppm.

But, if I'm not seeing any issues with clothes or irritation, it's not too high, right?
 
davethomaspilot said:
Whether I need to go much higher on FC "to get into trouble" depends on the CYA guess All I can determine with the test kit is that it's below 30 ppm.

But, if I'm not seeing any issues with clothes or irritation, it's not too high, right?
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That's only a very crude estimate at best. If there is a reasonable amount of CYA in a pool, say 30ppm or more, then a person can swim in water all the way up to shock level without experiencing any noticeable discomfort or bleaching of swim wear. This is because CYA is a very strong chlorine buffer. If you look at chem geek's charts in the second post of that Pool Water Chemistry thread, you can see how the relative amounts of hypochlorous acid, hypochlorite and chlorine bound to CYA vary with pH at 30ppm CYA. The fourth chart shows clearly that something like 99% of the chlorine in water is bound to CYA where it will have no interaction with your skin, hair, swim wear, etc. It's only the HOCl/OCl- components that cause bleaching or irritation and those will remain quite low when CYA is present.
 
I shocked about weekly when I was using only the copper ionizer + ozone. FC always measured zero.

I used "Chlorinating Shock" that was 68% Calcium hypochlorite and 32% "other ingredients".

I used three cups weekly, sometimes more frequently like after significant rain.
 
When I say "FC always measured zero", that was always a day or longer after the shock treatment and using the less accurate FC test in the Taylor 2005 kit.

I just recently got the FAS-DPD test kit so I now have the equivalent of the K-2006.

Never any issues.
 
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