Effect of FC on CYA test?

[tomfrh]

My cya was 70. I slammed the pool due to algae outbreak. It passed and FC is now down to 18. CYA is now saying 40?

any idea what’s going on?

 

[aussieta]

high water temps and high fc can break down cya
algae can also break down cya
however i dont think it would total this amount
was the cya test performed correctly each time or maybe cool water and overcast one day

 

[Leebo]

While chlorine can break down CYA it only breaks it down a tiny amount over a long period of time. You’ve got something else going on.

What test kit are you using?

 

[AUSpool]

The question is a bit ambiguous to me. Do you mean ‘does high FC interfere with the CYA test?’ or ‘where has the CYA gone?’ I don’t know if there is a FC interference on the CYA test, the extended instructions don’t mention anything about it. How much algae did you get and how low did your FC get and for how long? I would bet your looking at a trifecta, algae and/or bacteria, water lose due to rain or backwashing and a bit of degradation due to high chlorine and sun. We have had a bit of rain on the coast, one of my pools was low on salt, CH and CYA.

I would wait for your FC to sneak under 10ppm, test again and dose accordingly.

 

[duraleigh]

Leebo is right. CYA test is not related to high FC.......there is no connection. The connection is testing error or a HUGE leak in your pool. I doubt the leak is there so testing error seems the most logical by far.

The 10 ppm significance of FC is for the pH test only......it again is not related to CYA test.

 

[kimkats]

Lets go back to the test kit....which one are you using? Did you get some new regents during the SLAM?

I have a good reason to ask these questions so will wait for the answer.

Kim

 

[tomfrh]

Clearchoice labs kit.

CYA was originally 50-60. measured 50-60 multiple time over several months. I added 1kg of stabilzier, so would have been 70-80. I didn’t test it.

shortly after I realised I needed to slam (low FC and algae). Brought FC to ~40. SLAM finished now, and CYA is reading ~40. Measured it twice now in different places on different days.

same reagents.



i was wondering if FC binds with the CyA and stops it precipitating, or something like that...

 

[mknauss]

i was wondering if FC binds with the CyA and stops it precipitating, or something like that...

No.

What is your water temperature? Higher water temperatures combined with high FC levels do degrade CYA more rapidly.

Here in the southwest desert of the USA, in summer, with water temperatures around 88F (31C), I lose about 20% of my CYA level each month.

 

[kimkats]

Did you do a lot of backwashing or such? Lots or rain that could cause you to drain some of your pool water?

 

[tomfrh]

40mm of rain. That’s all.

did some backwashing after slam, but only the excess from the rain.

- - - Updated - - -

Water temp has been 30C, recently fallen to 26 due to clouds.

the CYA has fallen inside of a week. Before slam 70+

now its 40.

FC is currently 15, and was much higher during the slam. Maybe the very high Fc during the slam degraded the CYA?

 

[duraleigh]

Maybe the very high Fc during the slam degraded the CYA?

Tom, you have asked the same question three times and each time you have been told NO. I have no idea what you want to hear but the CYA doesn't degrade from the presence of high FC

 

[tomfrh]

Duraleigh, several people above have said high FC cause CYA to degrade more rapidly, so I am merely continuing that conversation.


take it up with them if you beleive there is no association whatsoever.

 

[cfherrman]

Clorine oxidies cya, very very slowly. It's often given here less than 5 ppm a month.

The test kit margen on error is often +/-10 ppm, and the cya test is the hardest and you have to do it consistently each time with lighting being the same.

Water dilution from the skimmer being high to low can move it as well.

Don't worry about it, and if your saying your cya is 50-60 then your not doing the test right

 

[AUSpool]

Geday Tom,
the most important thing is the best management of your pool. I would test it again and adjust as required. As you’ve discovered there are lots of reasons why CYA can and will decrease over time and I omitted testing error and water leak. Given my pools have also had a decrease of CYA (due to rain heavy rain overflow, normal degradation and testing error) I was thinking water exchange the most plausible. But a water leak is a possibility and somthing well worth investigating. Has your salt or CH levels decreased over the same time by close to the same factor?

 

[tomfrh]

Nothing has changed that I can think of except for all the liquid chlorine I added. That’s why I’m stumped!

 

[Leebo]

I'm going to break this next post into two different sections......one truth and one theory. Please note the difference and please take a theory for what its worth.

Truth............
Your high levels of FC very likely did not break down the CYA. This has somewhat been discussed in this thread some time ago. I'd love to share a quick quote that simplifies the thread but truth be told, I'm also fighting the kids to go to school.

Theory...........
In the past my memory says the CCL test kit has had some issues with the repeatability of their CYA test. I will find the thread once I get the girls off to school that is somewhat like your thread and I'll post it again. My theory (and only a theory) is that this maybe a test error resulting from the CCL test kit.

 

[tomfrh]

Leebo,

ok, unlikely the CYA has been degraded by that much in such a short time.

CCL did have issues with the 50ppm standard solution. I encountered this myself. Is that what you meant or did they have reagent issues too?

in any case I’ve been using the same CCL reagents both before and after my fall in CYA numbers.

 

[AUSpool]

...

Truth............
Your high levels of FC very likely did not break down the CYA. This has somewhat been discussed in this thread some time ago. I'd love to share a quick quote that simplifies the thread but truth be told, I'm also fighting the kids to go to school.
...

As I read through that thread it suggests to me that high FC levels do oxidise CYA where the rate increases as FC, temp and pH increases. At normal FC/CYA ratios a loss of ~12ppm per month can be expected but can be as much as 3 times higher at shock levels, as much as 36ppm per month at 85F. This data was based on water temps at 85F and can be even greater as the temp increases. [edit] But it still does not explain a loss of 30ppm in a week. [end edit]

...

The primary step is the cleavage of the triazine ring and it is proposed by Wojtowicz that this primarily involves a fully chlorinated isocyanurate species Cl₂CY^-. The decomposition was first order with respect to average chlorine and increased with pH. The decomposition rate was a decrease in Free Chlorine at a rate of 0.0147 per hour or 1-EXP(-0.0147*24) = 30% per day while the calculated rate of loss of CYA at 4 ppm FC was 0.87 ppm per day [EDIT] (I get 4*0.30/2.47 = 0.49 ppm CYA per day) [END-EDIT], but that was at FC/CYA ratios (in ppm units) of 0.34 (close to shock levels) whereas the more typical ratio in our pools is around 0.1. His experiments at lower FC/CYA ratios of 0.029 with 4 ppm FC and 138 ppm CYA showed a lower CYA decomposition rate of 0.24 ppm/day which is a factor of 3.6 lower. At 4 ppm FC and 11.76 ppm CYA (a ppm ratio of 0.34), the concentration of Cl₂CY^- (at temp 85F) is a factor of 9.9 higher while HClCY^- is a factor of 6.9 higher than the concentrations at 4 ppm FC and 138 ppm CYA which might mean that HClCY^- is the rate-critical species that degrades (this is speculation on my part). In another paper "Effect of Cyanuric Acid on Swimming Pool Maintenance" (in the same JSPSI collection), Wojtowicz describes a chlorine loss rate due to oxidation of CYA at 12.5% per day at 85F which is roughly consistent with a 30%/3.6 = 8.3% rate especially since his 12.5% number came from FC levels starting higher (5.4 ppm for indoor pools, 7.6 to 9.2 ppm for outdoor pools). So, assuming a CYA loss rate of around 0.4 ppm/day in our pools this comes to 12 ppm per month which is clearly enough to be noticeable as the months pass during a swim season. If one shocks the pool, then the rate of loss could be about 2-3 times faster.

Wojtowicz also shows a strong temperature dependence on the chlorine oxidation of cynauric acid where every 10F increase in temperature results in roughly doubling the rate of degradation. So his data was with pools at 85F so pools at 90F could have degradation rates about 1.4 times higher.

The thing is that some of what Wojtowicz has seen does not seem to be consistent with some of what we have seen in our own pools, especially with regard to chlorine loss rates. Wojtowicz implies that there is little breakdown from sunlight of the chlorinated isocyanrates -- that only hypochlorous acid and especially hypochlorite ion are affected. Yet the experiments mas985 (Mark) made showed that higher levels of CYA did protect chlorine better in a non-linear way in sunlight and he did not see losses overnight (that is, without sunlight) which Wojtowicz saw in indoor pools. My own pool is at 86-90F and is exposed to sunlight 1-2 hours most days (it has an opaque electric safety cover on most of the time) and about a 1 ppm FC per day loss which includes use of the pool (1 person bather load most days). The rate of chlorine loss from oxidation of CYA is, in ppm units, about 2.5 times higher so even if I assume 1 ppm FC loss per day all oxidizing CYA, that would be 0.4 ppm CYA loss per day or 12 ppm per month. I should be able to measure that as I started with 30 ppm CYA when I opened and added more CYA around April/May. [EDIT] I just measured my pool's CYA level and it's a little above 25 ppm so even attributing a 5 ppm CYA drop over 3 months, that's pretty low and could be explained by the error tolerance of the test or some by splash-out (I have an oversized cartridge filter that is only cleaned once a year so the only dilution is from splash-out). It's certainly not near 12 ppm per month of loss. [END-EDIT]

So I can see that it is possible for CYA to degrade slowly over time in pools that are at higher temperature or at higher FC/CYA ratios such as extensive periods of shocking....

 

[Leebo]

Leebo,

ok, unlikely the CYA has been degraded by that much in such a short time.

CCL did have issues with the 50ppm standard solution. I encountered this myself. Is that what you meant or did they have reagent issues too?

in any case I’ve been using the same CCL reagents both before and after my fall in CYA numbers.

As I mentioned, I’m only saying a theory so please take it with a grain of salt. This thread below is one that I can find that reminds me of your situation
CYA Testing

Additionally,
CYA Testing - Not doing it right

 

[AUSpool]

I found the CCL CYA test a bit hard to use. I was lucky and reverted to the Taylor test tube. The CCL test tube is a bit clunky and the shake tube would be better as a dropper bottle. The CCL test is a turbidity disappearing conical flask rather than a disappearing ‘dot’ test. The disappearing dot test is a bit like the secchi depth test and designed around a circular indicator. I think CCL could improve on their test by changing the conical flask to a dot. I think it adds an extra element for error in what is already a difficult test.