Cyanuric Acid PPM Drop and Iron

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smallpooldad

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Aug 6, 2008
429
Honolulu
Both my (10k gals) and my neighbours (20k gals) pools are maintained at the same levels.

Both our pools are crystal clear, with no algae. Both have SWG.

Because we live on a hill/mountain at a 1000 ft above East Honolulu there is a fair amount of iron deposited into our pools, via trade winds averaging 16 mph, which stain the walls and the surrounding concrete. This comes from the volcanic soil and once a year we use the Citric Acid Method to remove it.

The CyA ppm will drop from 30 to 23/25 over a 3 month period in both pools. This has always baffled us as it is supposed to be constant. He backwashes once every two months and I once a month.

I ran across this link concering the degradation of CyA caused by ferrous sulfate. I realize the solution is not the same and they are using gamma rays, versus high UV rays, or are they interchangeable, but could this have something to do with our larger than normal usage?

The link:

http://www.sciencedirect.com/science?_o ... 3eb62b928f

I am not saying it is the reason but there could be a link. Furthemore I also realize that the answer would most probably only be answered by a person with an advanced level of chemistry. So while I hope to get a reply I will also understand if no person wishes to attempt these rather complicated equations.

The pool numbers for his pool look like this, I use borates and have higher calcium,. He does not use borates because of his dogs.


Initial
Measured pH 7.5
Total Alkalinity (ppm CaCO3) 60
Free Chlorine (ppm Cl2) 5.0
Cyanuric Acid (ppm CYA) 30
Calcium Hardness (ppm CaCO3) 575
Total Dissolved Solids (ppm) 3,610
Total Sulfate (ppm SO42-) 0
Total Borate (ppm Boron) 0.0
Total Ammonia (ppm Nitrogen) 0.0
U.S. Gallons 10,000
Temperature (oF) 84

Total Chloride (ppm NaCl) 3500
Carbonate Alkalinity (ppm CaCO3) 49.6
Langelier Saturation Index (LSI) -0.01
% HOCl (vs. Total Free Chlorine) 1.5%
OCl- (as ppm Cl2) 0.097
HOCl (as ppm Cl2) 0.077
Calcite Saturation Level (CSL) 0.69
Calcite Saturation Index (CSI) -0.16


Aloha.
 
The amount of ferrous sulfate in that article was 7-15 ppm which is pretty high for metal content in a pool (more normally in a 0-2 ppm range). In any event, it would appear that the iron may act as a catalyst for CYA degradation, but the degradation itself is still likely to be from oxidation by chlorine since that is known to occur, albeit slowly. In a pool, it's not from hydroxyl radicals to any great extent. I don't know if iron would catalyze oxidation of CYA by chlorine.

We just don't know enough to determine exactly why CYA drops in every case, but I do suspect that for some situations it is from a slow oxidation from chlorine (Wojtowicz writes about that). An interesting theory, though, especially in your situation.

I'm tracking the CYA level in my own pool which has almost no dilution at all since there is minimal splash-out and no backwashing. We'll see in about a month how it turns out comparing start of season at 30 ppm (after adding CYA and retesting a week later) to end of season.

Richard
 
Richard,

Thank you, at least we two senior citizens are not going completely crazy. We thought that posting this experience we might bring down upon us the "That's not possible" mob," and "Your science is all crazy" mob. But you have reassured us that there is at least a faint hope of understanding this decline.

Looking forward to your results.

Aloha
 
One thing I would do is test CYA immediately before backwashing and after refilling after backwashing so you know exactly how much CYA you are losing by backwashing.
 
Waterbear,

You were correct the about the CyA measurement. Before I drained down the pool I measured 25 CyA, with the drain down it should have read 19 ppm, I did not measure it after I drained it down but did add sufficient quantities to get it back up to 25 CyA, but upon re-measuring it came to only 21 ppm, so I added CyA again and now it shows about 28 ppm, good enough.

So next time I drain I will take your advice and measure both before after I have drained and re-filled. Still do not know why it lost so much CyA, but your method is superior.

Thank you and Aloha.
 
ChemGeek-

I have read many of your posts concerning the mystery of CYA degregation. After having my pool (in sunny Florida) for a year now, I emperically see a drop in CYA levels often. With minimal splashout and maybe an inch or two of rain each month (but no significant "pumping out" of excess water), I see about a 5-10 ppm drop in CYA levels each month. I keep my levels high due to SWG...usually 80-ish range. Perhaps there is a more noticeable drop for me due to the high absolute ppm that I am shooting for, but there clearly is a drop in CYA. I used to think that there was some problem with my SWG, perhaps excessive scaling. However, whenever I would go to clean the generator, there never is any noticeable scale on the plates. And I would also always check the CYA levels, and it is always lower when this happens.

It now seems I can predict when my CYA start to drop below 50, my steady-state FC levels start to drop and I have to crank up my SWG chlorine generation rate. What usually happens is my FC levels are stable for a month or two, then they start to drop some. It always seems to be that when this happens, it is the CYA levels drop below 50. I am now starting to use this effect as a gauge when I need to check my CYA levels.

Hope this helps. Thanks again for your input...always insightful and helpful.
 
Chem Geek and Florida Penguin,

Thank you both for your replies, very insightful. And at least I'm not going as crazy as my wife says I am. Chem Geek your response is posted above our bed for this evening at least.

Florida Penguin, you are right, it is possible I have a tiny, tiny leak. That could be one issue.

The CyA (target 30) definitely drops in both my friend's pool and mine. About 5 ppm per week in my pool (the tiny leak may account for the greater loss), and about 5 to 10 ppm per month in his. We have both been running our SWG with a target FC of 5, which could be too high. We recently lowered it to 4 and will, if all goes well, no algae, go for 3.

Amperage might play a part as well. I produce more chlorine (ozs per 24 hours) at lower amperage than he does. I think, but am not sure, that my cell the SC-60 (at power level 2) has a greater surface area than his Jandy 1400. This in combination with iron in the water, which is constantly added to by iron from volcanic dust (Ferric Oxide) blown into the pool by trade winds could also be a culprit.

Combine these factors with high UV levels in the summer, a too high FC level of 5 thereby making the SWG cell run too long, I am not too sure about temperature in our case 83 F. My best guess is that the high FC level with SWG cell running longer is the main culprit; the iron just helps speed it up.

My pool's surface area is proportionately much greater than his as his pool is much deeper. This too might explain why I lose CyA ppm faster than he does, as more iron is deposited as a percentage due to the greater surface area.

Will test over the next two weeks at FC 4 and report back, then drop it to FC 3 and see if that helps.

I know one cannot say for sure but as SWG seem to operate at lower FC levels I think the target of FC of 3.5 with a CyA of 30, according to the CyA chart by Chem Geek could go as low as 2.1 FC, or 60% of 3.5. I will see, then if the CYA stops dropping, and no algae presents itself, we may have an answer.

Chem Geek, it could mean that in a SWG pool the HOCl (as ppm Cl2) is higher than the PoolEquations spreadsheet states at a lower given FC, can that HOCl (as ppm Cl2) be tested for physically as opposed to by an equation?

Well I think I have covered all the bases, except for little CyA eating Hawaiian Menehunes playing in my pool late at night, we may need to add CyA to fish!

http://en.wikipedia.org/wiki/Menehune

If the issue can never be solved then I will buy CyA in bulk from the local Chemical Supply store, which supplies labs, hotels, manufacturing industries, etc. at a 60% discount to the pool store.


Aloha
 
Almost forgot here are my new numbers:

Measured pH 7.5
Total Alkalinity (ppm CaCO3) 70
Free Chlorine (ppm Cl2) 4.0
Cyanuric Acid (ppm CYA) 30
Calcium Hardness (ppm CaCO3) 500
Total Dissolved Solids (ppm) 3,620
Total Sulfate (ppm SO42-) 0
Total Borate (ppm Boron) 50.0
Total Ammonia (ppm Nitrogen) 0.0
U.S. Gallons 10,000
Temperature (oF) 83

Total Chloride (ppm NaCl) 3212
Carbonate Alkalinity (ppm CaCO3) 53.7
Langelier Saturation Index (LSI) -0.05
% HOCl (vs. Total Free Chlorine) 1.5%
OCl- (as ppm Cl2) 0.072
HOCl (as ppm Cl2) 0.058
Calcite Saturation Level (CSL) 0.66
Calcite Saturation Index (CSI) -0.18
 
smallpooldad said:
Chem Geek, it could mean that in a SWG pool the HOCl (as ppm Cl2) is higher than the PoolEquations spreadsheet states at a lower given FC, can that HOCl (as ppm Cl2) be tested for physically as opposed to by an equation?
Click to expand...
In the bulk pool water, the SWG won't have a different HOCl level, but in the SWG cell itself the FC level and the HOCl level near the chlorine generating plate is extremely high as the FC is very high relative to the CYA and it's very acidic. So it's possible that CYA is getting oxidized there but then I'd expect to see that in most SWG pools relative to non-SWG pools. Perhaps the combination of an SWG with high iron content water is causing this more rapid degradation of CYA.

Richard
 

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Chem Geek,

One again thank you for the reply.

Dropped the FC to 2.8, will see if that helps. If not it is most probably as you stated, iron. The only thing I can do then would be to drain, normally done once a year. I will then see if draining is more or less expensive than just adding CyA every week or so. I really do not want to drain every 3 months.

I will advise.

Aloha
 
I think I have posted enough on my CyA drops, and others, and many have tried to figure out why.

In other posts I stated that I thought it might be high iron content in the water, or possibly too high a FC (4-5) level for a CyA of 30-35.

So as promised I dropped my FC down to 2.8, and the daily loss is 0.5 with my pool covered most of the day (lowers the acid feed considerably - plus keeps the pool warmer) save 2 hours in the evening (my friends pool also losses 0.5 per day and his is uncovered - same specs as mine except his is 20k and mine 10k). The SWG PoolPilot SC-60 is now set at power level 2, pump runs 3.5 hours per day, and a production rate of 19% (no more ORP for me - I have come to agree with JasonLion it fluctuates too much and is pretty much useless except to impress lawyers, and their ilk).

I determined an FC of 2.8, by looking at the CyA chart for a CyA of 30 with potenial for mustard gas and it reads 4.6. As SWG FC seem to need to operate at only 60% of liquid chlorine it came to 4.6 x 0.60 = 2.76 FC. At FC 2.8 the pool is clear as a bell, at 80 F - 82 F.

Well, low and behold, the CyA dropping slowed considerably, and now only looses about 2 ppm, per week, and that could be just a reading error. If I lowered the FC to target to FC 2.2, CyA chart target 3.5 x SWG adjustment of 0.60 = FC 2.1, my guess is that there might be no CyA drop.

So, it seems that in some pools the CyA drop might be caused by having too high a FC for the given CyA content, and that a 60% of the charted FC is sufficient and/or the generator being on for too long.

On another note if I and my friend are only loosing 0.5 FC per day at 30 CyA; why then does everyone seem to think that 60=70 CyA is good! I'm beginning to think that this opinion is being more based on the all to human need to be different. And before everyone responds that it works, I am sure it does, but it works equally as well at 30 CyA (REAL LIFE EXPERIENCE for those who have stated these word far too often ), try it and you will see. There is really nothing wrong with being an average Joe, and you get to use less chemicals as well. Please note I have an automatic pH feeder at 7.4, my friend does not but keeps it at 7.5, checks daily, needs adjustment once every 3 days on average.

Aloha.
 
smallpooldad said:
On another note if I and my friend are only loosing 0.5 FC per day at 30 CyA; why then does everyone seem to think that 60=70 CyA is good! I'm beginning to think that this opinion is being more based on the all to human need to be different. And before everyone responds that it works, I am sure it does, but it works equally as well at 30 CyA
Click to expand...
Actually, it doesn't work as well at 30 ppm CYA. This thread will give you a primer as to why.

viewtopic.php?f=21&t=3663

And THIS has been backed up by quite a bit of real life experience in more than just one or two pools!

You half answered it yourself when you said your friend needs to add acid every 3 days (and I wonder how stable YOUR pH would be if you didn't have the acid feeder and pH probe!)

The short answer is that the higher CYA levels lead to greater pH stability and eliminate many of the problems associated with pH instability with a SWG.
 
And this post describes the experiment that Mark did (repeatedly) that shows how higher levels of CYA reduce the rate of breakdown of chlorine from sunlight even at the same FC/CYA ratio. This lower breakdown rate allows one to lower their SWG on-time and that reduces the rate of pH rise. In this post I speculate on why this might be so, perhaps due to CYA shielding lower depths of chlorine (in a non-linear way) possibly combined with imperfect circulation near the water surface.

Now if the pool isn't in strong sunlight then the above effect will be lessened. It also appears that the effect may be stronger in deeper pools though that is more anecdotal (i.e. not from Mark's experiment). Also, in pools where there is large bather load, such as many commercial or public pools, the rate of loss of chlorine from that bather load (i.e. ammonia/urea from sweat/urine) is very high and overwhelms the loss of chlorine from sunlight above around 20-30 ppm CYA so any higher CYA levels won't appear to have much incremental effect.

The easiest way to isolate the chlorine loss from sunlight from the other effects is to look at the chlorine loss overnight (or in a pool with an opaque pool cover) and preferably with no bather load.

So while a higher FC and CYA level may result in a faster CYA drop and a corresponding drop in FC from that effect, the lower loss in FC due to sunlight more than makes up for that effect. Like anything else, it's possible that there are exceptions for reasons we have yet to understand, but most SWG pools have benefited from the higher CYA levels with a net savings in chlorine and lowering of the SWG ontime. Most SWG users who were at 30-50 ppm CYA with around 2-3 ppm FC found that their FC loss was far higher than 0.5 ppm per day (if they turned off the SWG to measure such a loss) and had to have their SWG on-times fairly high. More typical FC loss per day in very sunny environments at 30 ppm CYA is at least 50% of the FC level (far higher than what you are seeing). When they raised their CYA to 70-80 ppm with 4 ppm FC, most found they could cut their SWG ontime significantly meaning the FC loss per day was lower in spite of the higher FC level.

Something doesn't make sense about your pool being covered and losing 0.5 ppm FC per day while your friend's pool is uncovered and loses the same 0.5 ppm FC per day. If your friend's pool is exposed to sunlight, it should be losing more chlorine (as a percentage of FC). It is quite understandable why your pool won't see a benefit from higher CYA levels because it's covered so doesn't lose much from sunlight. Is your friend's pool in shade? If so, then that would explain a lot since having it uncovered would mean more outgassing so more acid needed but being in shade would mean a similar low FC loss so not much benefit to higher CYA levels (or even being worse off at higher CYA levels as you describe).

Richard
 
Waterbear and Chem Geek,

Thank you both for your very clear replies, but as you see I have questions.

Chem Geek you are correct that my friend's pool goes into the shade starting at noon while ours is out of the shade nearly completely by 12:30 pm. We have a bubble pool opaque cover. Waterbear you are correct that at a pH of 7.4 the acid struggles, while at 7.5 it is pretty much fixed. He has a solar heater which runs with pump, we have a heat pump which I only use if we have no sun.

Your observations seem valid so we will experiment with a CyA of around 70 ppm, with a FC of 5.0, our chemical supplier is going to love you two! The first question I have is that the CSI index in the "PoolEquations" spreadsheet calculates differently from "The Pool Calculator" website for the same given parameters, which should I use?

"The Pool Calculator" gives a CSI -0.22, and -0.21 respectively, where "PoolEquations" gives -0.28 and -0.34

What would you expect the FC daily drop to be at a CyA of 70 ppm, and would you expect the CyA to drop; and if so by how much over a 30 day period?

My friend is left, I am to the right as I use borates.

Measured pH 7.5 7.50
Total Alkalinity (ppm CaCO3) 65 75.0
Free Chlorine (ppm Cl2) 5.0 5.00
Cyanuric Acid (ppm CYA) 75 75.0
Calcium Hardness (ppm CaCO3) 550 425.0
Total Dissolved Solids (ppm) 3,344 3,646.0
Total Sulfate (ppm SO42-) 0 0.0
Total Borate (ppm Boron) 0.0 50.00
Total Ammonia (ppm Nitrogen) 0.0 0.00
U.S. Gallons 10,000 10,000
Temperature (oF) 82 82

Total Chloride (ppm NaCl) 3200 3200.5
Carbonate Alkalinity (ppm CaCO3) 40.0 44.1
Langelier Saturation Index (LSI) -0.15 -0.22
% HOCl (vs. Total Free Chlorine) 0.5% 0.5%
OCl- (as ppm Cl2) 0.033 0.033
HOCl (as ppm Cl2) 0.027 0.027
Calcite Saturation Level (CSL) 0.53 0.46
Calcite Saturation Index (CSI) -0.28 -0.34

Aloha.
 
When I put in the numbers you listed into The Pool Calculator, I get a CSI for your friend's pool of -0.23 while for your pool I get -0.27 and not the -0.22 and -0.21 you listed. When I change the 50 ppm Borates number to 0 in The Pool Calculator, the CSI only changes to -0.26 so I suspect that the CSI formula isn't using the Borates adjustment. In my spreadsheet down at lines 427 to 479 is the derivation of a reasonably accurate CSI formula that's pretty close to the equations in the main body of the spreadsheet. I suspect that the Borates part of the CSI formula Jason is using isn't correct (or is missing). The term that reduces the "ppm CarbAlk" in the formula is "-4.63*(ppm Borates)/(1+10(9.11-pH)). Since you are using the spreadsheet anyway, you can use the CSI that is there though these differences are not a big deal.

As for the rate of CYA drop, it may be proportional to either the Cl2CY- species as Wojtowicz surmises or could be proportional to the HClCY- species which is the most dominant. If you go from a CYA of 30 to 70 ppm but keep the hypochlorous acid constant by roughly keeping the FC/CYA ratio constant, then (turning on the "Use Temp. Dependent Cl-CYA" at line 224) at 82F the Cl2CY- concentration goes up by a factor of 2.4 while the HClCY- concentration also goes up by a factor of 2.4. So my best guess would be that the rate of CYA degradation would be higher by roughly that factor of 2.4. I don't know how much that would be over a 30-day period since this seems to depend on other factors since Wojtowicz has numbers that are very different from my own pool experience and this is very different than your experience.

Richard
 
Bottom line on the CSI measurements, you are talking about approx a .05 difference in the readings and that is, essentially, insignificant when you consider that the accuracy of the other measurements that go into the calculation (such as your test results) do NOT have that much precision! For example, if you are titrating the CH and TA then you have, at best, a +/- 10 ppm precision with Taylor chemistry (and less with LaMotte....their $1000 waterlink express colorimeter and reagents is +10/-25 ppm on the CH test and +10/-20 on the TA test! The CYA test is an approximation at best with Taylor and other view tube tests (disappearing dot tests) and even with a meter such as the LaMotte it is +10/-25 ppm. So if the precision of what you are inputing is not that great why are you splitting hairs over about a .05 difference in the CSI?
It's insignificant since your error is already greater than that!
 
chem geek said:
I suspect that the Borates part of the CSI formula Jason is using isn't correct (or is missing). The term that reduces the "ppm CarbAlk" in the formula is "-4.63*(ppm Borates)/(1+10(9.11-pH)).
Click to expand...

Indeed, my Pool Calculator was missing the borate correction to the carbonate alkalinity. I just installed an update with the proper borate correction.
 

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