CYA vs UV Chlorine Loss Test Update

mas985

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Many years ago, I did a test of chlorine loss vs CYA level and published the results here:


There were a few issues with the test that I wanted to resolve with some additional testing but never got around to doing it...until now.

This time around, I wanted to use distilled water for dilution to avoid adding anything into the water that might affect the results. Also, for this round, I wanted to use shallow containers (wine tumblers) to determine if the depth of the solution had any impact to the benefits of higher CYA. Also, it wasn't practical to get that much distilled water for buckets so I kept the experiment small.

I started with my own pool water which was at FC level 7.2 ppm with a CYA of 80 ppm. All FC tests were performed using FAS/DPD. One glass had no dilution so had a FC/CYA of 7.2/80. The second glass was diluted 1:1 or FC/CYA of 3.6/40. The third glass was diluted 1:3 or FC/CYA of 1.8/20. Note that FC/CYA ratio is kept constant for all three because of the dilution method. Left in the sun for 2 hours, here are the results:

1:0 FC/CYA 7.2/80 --> FC/CYA 6.8/80 - 0.4 ppm FC Loss
1:1 FC/CYA 3.6/40 --> FC/CYA 2.6/40 - 1.0 ppm FC Loss
1:3 FC/CYA 1.8/20 --> FC/CYA 0.4/20 - 1.4 ppm FC Loss

So as in previous experiments, FC loss decreases with increasing CYA levels although with this test, the benefits seemed to be much greater. This is probably due to the depth of the water as it offers less FC protection due to lower scattering and absorption. The deeper the water, the more UV protection provided by the water so the benefits of CYA are diminished for deeper water.

This is a pretty easy test to perform so I would encourage anyone else to repeat these tests for corroboration.
 

UKjames

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this is really interesting as I’ve often wondered what is the optimal CYA level.

however I always thought that higher chlorine levels deteriorated faster than lower ones, so having a very high cya is counterproductive as the correspondingly high fc results in a greater loss. Have you tried increasing the cya and fc to see at what point the trend you are seeing no longer happens?
 

mas985

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I have not gone beyond 80 ppm but I don't think it is a good idea either. The problem with high CYA is it requires higher SLAM levels. Plus there is always some organic consumption of FC so there is likely diminishing returns with CYA higher than 80 ppm.
 

mgtfp

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Very interesting. That seems to be an even higher increase of UV-protection with increasing CYA-level than your old experiments indicated.
 

JamesW

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Does the glass block any of the UV light?

Does the dilution change the pH of the sample?

Higher pH should increase the fc loss since there would be more hypochlorite than with lower pH.
 

mgtfp

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I wouldn't expect a big pH change when diluting with distilled water, at least not with the 1:1 dilution. There could be a noticeable effect with the 1:3 dilution.
 

mas985

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Very interesting. That seems to be an even higher increase of UV-protection with increasing CYA-level than your old experiments indicated.
Yes, I believe that is due to the container. Both were plastic but the test in the link was a 5 gallon Lowes bucket while this latest test was a clear plastic wine tumbler so the later should have a bit more UV exposure than the former. Also the water depth probably played a factor as well.

Does the glass block any of the UV light?

Does the dilution change the pH of the sample?

Higher pH should increase the fc loss since there would be more hypochlorite than with lower pH.
The plastic probably does block some UV but I did the test intentionally from 12 noon to 2 pm. 1 pm is solar noon so the sun was very high in the sky and the angle of incidence was such that most of the sun's rays went through the open top of the container rather than the sides. But all three containers are exactly the same with the same exposure so any UV blocking should affect all three samples equally.

Distilled water right after production is neutral (7.0) but left out and it tends to absorb CO2 and the PH drops a bit. The sample I used had a PH of 6.9 while the pool water was at 7.2. So the diluted samples would have a slightly lower PH than the non diluted samples which means they might show a muted extinction effect. Without any PH change, the results might show even more of a difference.

The intent here is not to quantify the absolute extinction rates but to show that there is a difference. The very first test I did was in my spa and that used just pool water with different levels of CYA added. I can't seem to find that post but the results were similar. The second test with the buckets and tap water dilution as well as this test both showed a similar trend. The lower CYA levels lost FC faster on a ppm basis than the higher CYA levels even though the FC/CYA ratios are the same.
 
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mgtfp

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The very first test I did was in my spa and that used just pool water with different levels of CYA added. I can't seem to find that post but the results were similar.

I believe that's the post you were looking for - one of my most precious bookmarks:

I was going to wait until I repeated some the results that I have but given this topic is pretty heated at the moment, I thought I would give the results that I currently have. These are preliminary so don't flame on me. I need to repeat them to make sure I did not make any errors.

I have been doing an experiment in my SPA with different CYA levels. The spa is a bit easier to control the CYA levels as well as shorten the run time for the SWG to measure chlorine (24 min.).

Because I use my pool water as a starting point here are the starting numbers:
45 ppm CYA
2.5 ppm Cl
7.4-7.5 PH
110 ppm TA (yes I know it is a bit high but I am having trouble lowering it because my fill water is at 250)
225 ppm CH
3000 ppm salt (test strips say 3400).

The spa is 700 gallons
SWG run time for spa is 24 min.
At 1.45 lbs per day, the SWG should add 4 ppm by my calculations however it only adds about 2.5 ppm. I have not figured out why yet.

So here are steps that I am following.

Step 1 Run the spa in spillover to replace most of the water. SWG is off.
Step 2 Add enough CYA for 80 ppm or skip this step for 45 ppm.
Step 3 Test chlorine level. Pool is about 2.5 ppm so that is what the spa is.
Step 4 Run the spa in filter mode with SWG on for 24 min.
Step 5 Test chlorine level and CYA level.

I performed these tests in early evening after the sun has gone down.

So far I have done one test at 45 ppm CYA and one at 80 ppm CYA and both seem to be raising the chlorine level about the same amount, +2.5 ppm. I cannot tell the difference of < +-0.25 ppm in chlorine with the test kit so there might be a slight difference but that would probably be less than 10%.

However, what I did notice was a very large increase in chlorine retention over the next few days. I let the water sit in the spa after each test for a few days in direct sunlight.

At 45 ppm, the chlorine levels dropped by 50% per day or from 5 ppm to 2.5ppm and then down to a bit over 1 ppm the second day.

At 80 ppm, I hardly noticed a drop the first day (~4 ppm) and the second day the chlorine level was ~3.5 ppm . This is a retention of about 85% per day.

My plan was to repeat the tests next week to see if I get the same results.

I also ran a test with my pool cover on and off for a week each (I did the above tests during the week it was on). The pool is at a CYA of about 45 ppm and my pool cover is clear plastic. With the cover on at the end of the week, the chlorine was at a level of about 2.5 ppm. Without the cover, the chlorine level did not seem to drop much maybe 2.0 ppm. So the clear cover does not seem to help much for chlorine degradation.

Any suggestions on test modifiers?
 
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jonpcar

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Mark, I have stumbled across that old post of yours a few times and have always made a note to try and test it’s tenant on my pool. The problem, of course, is that it’s a one way street with CYA addition, haha. I made a mental note to run the test (increase CYA) on my pool just before I change my pool water due to calcium buildup, something I need to do every few years…but of course I forget. More important…your experiment is probably best carried out this time of year (sun position, chlorine use, water temp etc)…so the timing probably wouldn’t correspond to a pool water change.

I dose chlorine using a Stenner pump and this year chlorine prices have definitely had an impact. I had started some experiments using time distributed dosing (just-in-time…more like a SWG) to see its impact on my pool’s chlorine usage. But I wouldn’t expect results to have close to the potential impact (in reducing chlorine usage) that your test case seems to suggest.

Anyway, this thread/experiment has got me interested in trying this again…thanks for posting.
 
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