More about recommended CYA levels ...

Aug 20, 2011
17
#1
I have done some more serious reading about CYA and its effects and was surprised by the findings.

CYA basically trades chlorine power with chlorine longevity. Trying to better understand the tradeoffs I have been looking for numerical test results.
I have found the following, as can be seen in the attached graphs:

1. 95% of the chlorine longevity benefits is achieved at CYA levels as low as 20 ppm
2. CYA level of 70 ppm is a saturation point for chlorine power, the power stays almost the same regardless of the amount of FC.

If these graphs are correct then I see no reason why to use more then 20 ppm CYA since you can get most of the benefit without scarifying too much power.
 

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MattM

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Jul 14, 2011
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San Diego, CA
#2
I'm not an expert, but:
- I don't see anything in these charts stating the conditions under which the test is performed (noon sunlight on warm day would be best)
- The data charted seems to counter to my own experiences where I've started a new 24K gallon mostly covered outdoor pool with a CYA of 20 and slowly moved CYA level upwards until I could find an optimimum point where reasonable FC could be maintained and runtime of an ic60 SWG kept to <50% with a daily pump cycle of 8am-7pm. Southern california summer sunlight when cover is open for swimming plus the natural bursts in bather load just overwhelmed the IC60 SWG when CYA <30. And, to simplify operations and handle pool parties, I noted that CYA >40 substantially reduced incidents requiring shock and that CYA levels between 50-60 helped to avoid having to occasionally add bleach.

I suspect the graphs above were not under true conditions stressful to FC. They may work in some locations and light seasons...but certainly not southern california summer...and not with pools regularly heated to 85-90 water temps.

It's true that ORP based systems have trouble with CYA levels >30, although they somewhat work at higher levels, which creates an incentive for pool equipment manufacturers to encourage customers to use lower cya levels. I suspect it's possible that the charts above were meant to be marketing to support this approach.

I'm still struggling to find the ideal CYA level for my pool, but it certainly isn't 20 -- I may end up setting the lower limit to 40 and the upper to 50. I have to take into account that higher cya levels reduce the runtime requirements of the SWG while lower cya levels increase ORP accuracy. If you're looking to deploy an ORP system, I'd be very careful about using the graphs above to justify the assumption that CYA levels < 30 will be sufficient.

If I recall correctly, other intellichem orp users here on tfp have recommended a cya level of 40.
 
Aug 20, 2011
17
#3
You can read the full article at:

http://www.ppoa.org/pdfs/PrP_Cyanurics% ... 20Bomb.pdf

It is one of the best sources I have found that doesn't go into chemistry and yet well explains the subject.


I'm located in Israel with similar sun and temperature conditions as in southern CA. I'm using CYA level of 40 ppm which worked fine for me last year with the SWG. I didn't try higher or lower numbers yet. I guess next I'm going to try lowering it to 20-30 ppm and measure the effect.

I'll also try to look for some more hard data on tests performed in different weather conditions.
 

JasonLion

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May 7, 2007
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#4
That article contains an amazing mixture of really good and really misleading information. A number of the conclusions drawn in that article are not actually supported by the data they present. For example, both of the points mentioned in the opening post of this topic are simply false. Higher CYA levels continue to improve total chlorine consumption as CYA increases, right up to and well past 100, even though you have to increase the FC level to compensate for the reduced activity level of the chlorine. Likewise, it is always possible, even easy, to raise FC high enough so that chlorine remains effective, regardless of how high the CYA levels goes. There are some very significant reasons to not use CYA levels over 80, but not for any of the reasons stated in that paper.

That paper has been covered in more detail before, for example here and here. There is some great background information on the actual chemistry here.
 

chem geek

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Mar 28, 2007
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#5
Yeah, from our experiences here on the forum and some experiments Mark and others have made, those charts and info from the PPOA are misleading in some cases and wrong in others. Yes it is true that small amounts of CYA dramatically reduce chlorine loss compared to no CYA, but the implication that the gain from higher CYA levels is marginal isn't true and one can easily compensate for this by proportionately raising the FC level. Remember that without CYA, the FC drops by half in about 1 hour in an average 4.5 foot depth pool in direct noontime sun. Having 30 ppm CYA slows this down, but it still means you lose 70-80% of the FC over one day -- half is lost in perhaps the equivalent of 6-8 hours of direct noontime sun. At 50 ppm CYA, the loss is down to around 45% for the day. At 80 ppm it's down to around 25% and at 100 ppm down to around 15%. These numbers mean that even if you were to increase the FC proportional to the CYA level, you still end up losing less absolute FC at higher CYA levels in spite of the higher FC levels. This is why having 4 ppm FC with 80 ppm CYA for SWCG pools lets one have a lower on-time compared to 2 ppm FC with 40 ppm CYA (see this post for Mark's experiments).

As for ORP, see this post with graphs showing that the effect on ORP is not at all surprising since ORP is roughly proportional to active chlorine (hypochlorous acid) concentration. In fact, this is why we have the chlorine/CYA chart in the first place -- because the chlorine bound to CYA doesn't kill/prevent algae growth (at least not enough to matter).

I think what is going on is that there are multiple factors not being taken into account. First is that the chlorine bound to CYA still breaks down in sunlight (the CYA core remains intact, however), but not nearly as quickly as unbound chlorine. Second is that CYA itself (as well as chlorine bound to CYA) absorbs some UV directly shielding lower depths and this is a non-linear effect.