Fundamental Question Related to CYA and Chlorine

[eqbob]

I have a fundamental question related to understanding the relationship between CYA and Chlorine. Prior to asking it, I would like to state:
a) Yes I've read pool school. Yes it tells me what to do, not why.
b) No I'm not trying to upset anyone
c) I am NOT saying I'm not following or using TFP methodology, nor am I trying to diss the forum, the members, the philosophy, the expensive experience, etc.

I'm simply saying I don't understand WHY and HOW this relationship works, and would like the background knowledge.

With all that....

Why does the recommendation exist for higher levels CYA?

As I've read, I think I understand that higher CYA binds more chlorine to it, which therefore keeps it around longer, but also reduces the amount that is available to fight "bad things in the water". I recall several detailed posts by ChemGeek which had figures of something like only 24% of the chlorine was actually available to be used, the rest was bound.

So I guess I don't understand the trade-off. If you have to add more chlorine, or run your salt cell longer or harder to create more chlorine, so that the higher level of CYA can bind to it, how is that more efficient or better? If I have to push my salt cell harder at a higher CYA level, why is that better than a lower CYA level and not pushing the salt cell as much or adding in more bottles of bleach at $3 a pop?

Again, I am NOT trying to inflame members or question the TFP methodology, I am simply looking to understand the background of it more than I do, for this particular relationship.

Since CYA is known to dissipate very slowly does it release the bound Chlorine over time and bind to new chlorine? How does that process work?

If the test is only accurate to +/- 10 ppm is there really a difference between 60 and 70, especially, especially since the reading scale is logarithmic?

 

[singingpond]

Not sure if you already read this one:

http://www.troublefreepool.com/threads/558-Pool-Water-Chemistry


edited to add:
are you mainly asking about the higher CYA recommendation when running a SWG?

 

[mrcarcrazy]

At least part of the reason is to protect the FC from the sun. More CYA protects better than less...this means the SWCG can be run less. Which allows it to last longer.

another as indicated above is
"Salt Water chlorine Generation (SWG) pools seem to require a higher level of CYA, about 70-80 ppm, to operate efficiently. The theory is that the CYA is slow to "store" the chlorine as it is being generated so without enough CYA there is a build-up of chlorine that degrades the performance of the salt cell. I would prefer that the SWG manufacturers offer a larger lower-power (per length) cell that would work efficiently at lower CYA concentrations." - ChemGeek

" Several users of SWG pools have found that raising the CYA to higher levels, especially approaching 70-80 that some manufacturers recommend, has a dramatic increase in FC levels at the same SWG output. Though one theory is that the CYA makes the SWG cell more efficient by combining with the generated chlorine in the cell "hiding" it from the plates in terms of equilibrium (thus making the generation proceed more quickly), an alternative explanation proposed by some is that the higher CYA levels simply protect the chlorine from destruction from sunlight at a rate faster than the baseline theory outlined at the start of this post." - ChemGeek

Both quotes are from link above

ON the question as to whether there's a difference between 60 and 70 I'd say " of course there is"....but to quantify that difference would require someone like ChemGeek who is well versed on that. (I don't have a SWG...so I don't really care about all of that. I just go with the recommended levels since they are industry standard and not argued against by anyone).

 

[prs]

i understand that cell run time is actually reduced at higher CYA levels because less FC is lost to the sun. It does seem counterintuitive as you would think that the higher pool FC concentration required by higher CYA would result in more losses, however my personal experience does seems to confirm that higher CYA levels allow reduced cell run time

I must admit I don't understand why this same logic doesn't apply to non SWG pools.

 

[jblizzle]

The recommended CYA levels are a balance of the amount of FC lost, the risk of high shock FC levels if a problem develops, and the longevity of the SWG system.

You could certainly raise the CYA level for a non-SWG pool and have lower FC losses to the sun, but if you have to SLAM, the required FC levels are much higher.

With a SWG, they are slow to add FC and thus need the added protection from the sun and by raising the CYA, they run less and the cells should last longer. Also, with a SWG, there is less likelihood of human error in maintaining adequate FC and thus the risk of needing to SLAM should be lower.

 

[eqbob]

The recommended CYA levels are a balance of the amount of FC lost, the risk of high shock FC levels if a problem develops, and the longevity of the SWG system.

You could certainly raise the CYA level for a non-SWG pool and have lower FC losses to the sun, but if you have to SLAM, the required FC levels are much higher.

With a SWG, they are slow to add FC and thus need the added protection from the sun and by raising the CYA, they run less and the cells should last longer. Also, with a SWG, there is less likelihood of human error in maintaining adequate FC and thus the risk of needing to SLAM should be lower.

Thank you.. So perhaps what I don't understand is the nuance of the salt cell operation. It's plugged in, it's on, water flows through it, is it not always generating? Or is it only generating periodically?

If it's always generating, then I'm (hopelessly?) confused as to why a higher CYA would prolong the life of the cell.
If it is a periodic generation, then I could certainly understand that it could.
It was my understanding that it is constantly generating, but I could certainly be wrong.

 

[eqbob]

Not sure if you already read this one:

http://www.troublefreepool.com/threads/558-Pool-Water-Chemistry


edited to add:
are you mainly asking about the higher CYA recommendation when running a SWG?

Found yes.
Read as much as I could comprehend.
Understood some aspects, got lost in the details and nuances. My Dad was a PhD in organic chemistry, not me <G>
As for your edit to ask?: Basically, yes. I understand and completely get that it works at that level and the combined experience is that it works well. NOT questioning that. I'm just trying to understand the why and the mechanism of how it works, at a higher level than those postings discuss/I can understand.

 

[jblizzle]

Thank you.. So perhaps what I don't understand is the nuance of the salt cell operation. It's plugged in, it's on, water flows through it, is it not always generating? Or is it only generating periodically?

If it's always generating, then I'm (hopelessly?) confused as to why a higher CYA would prolong the life of the cell.
If it is a periodic generation, then I could certainly understand that it could.
It was my understanding that it is constantly generating, but I could certainly be wrong.

Most SWGs have a % setting which is the amount of time (over some period, 1-2 hours) that the cell is actually energized and generating. So, with higher CYA and lower losses to the sun, the % can be set lower.

 

[eqbob]

Most SWGs have a % setting which is the amount of time (over some period, 1-2 hours) that the cell is actually energized and generating. So, with higher CYA and lower losses to the sun, the % can be set lower.

Ok. So that all starts to make some sense now. Wasn't thinking about the %age and the parameters set on the cell itself. I need to go look at my control panel and refresh what my parameters are set to.

Thanks.

 

[prs]

eqbob, when the pool pump is running SWGs turn on or off according to their % setting, so running your pump 12 hrs per day with a 50% SWG setting gives you the same cell run time as 6 hrs of pump with a 100% SWG setting. If you FC is high you can either lower your %SWG or reduce pump run time.

IMHO lowest cost operation is achieved by setting the SWG to 100% and reducing the pump run time to the minimum required to maintain FC at the level determined by your CYA. Of course if that minimum pump run time doesn't give you enough filtration to keep the pool water clear, then you will have to increase pump run time and lower %SWG accordingly.

 

[eqbob]

eqbob, when the pool pump is running SWGs turn on or off according to their % setting, so running your pump 12 hrs per day with a 50% SWG setting gives you the same cell run time as 6 hrs of pump with a 100% SWG setting. If you FC is high you can either lower your %SWG or reduce pump run time.

IMHO lowest cost operation is achieved by setting the SWG to 100% and reducing the pump run time to the minimum required to maintain FC at the level determined by your CYA. Of course if that minimum pump run time doesn't give you enough filtration to keep the pool water clear, then you will have to increase pump run time and lower %SWG accordingly.

Thank you Peter. Good explanation and obviously I wasn't thinking along those lines as you and others have pointed out. Appreciate the insight.

 

[chem geek]

Why does the recommendation exist for higher levels CYA?
:
I recall several detailed posts by ChemGeek which had figures of something like only 24% of the chlorine was actually available to be used, the rest was bound.

Actually, with 4 ppm FC at 80 ppm CYA, over 98% of the chlorine is bound to CYA. Fortunately, it takes a low level of active chlorine to kill pathogens and prevent algae growth.

Because CYA acts like a hypochlorous acid (HOCl) buffer, the amount of HOCl is proportional to the FC/CYA ratio. So the amount of unbound chlorine is the same at 2 ppm FC with 40 ppm CYA as it is at 4 ppm FC with 80 ppm CYA. If only the unbound chlorine were broken down by the UV in sunlight, then one would expect no difference in chlorine loss between these two scenarios. However, in practice we do find that the 4 ppm FC with 80 ppm CYA loses chlorine more slowly to sunlight and therefore is more economical since less chlorine is used/consumed per day. We are not sure why this is, but speculate that there is some UV shielding effect of CYA (or perhaps chlorine bound to CYA) that protects lower depths from the UV in sunlight.

At 4 ppm FC with 80 ppm CYA one should be able to lower the % on-time for the SWG cell since less chlorine needs to be generated compared to 2 ppm FC with 40 ppm CYA. With a lower on-time, the SWG cell will last longer. When the SWG is "on", it outputs the same amount -- there is no setting for it to output different amounts, only on/off time.

 

[prs]

eqbob, I've found it takes a week or two for the FC test results to settle out after a change in %SWG or pump run time, so take it easy on your changes.

Also I've found my FC test result spikes after a CYA addition, I don't know why!! If this happens to you. just set your %SWG to 0% until FC test is back down to normal.

 

[chem geek]

The FC test spike after a CYA addition makes sense (if you are measuring the FC after some period of time say during the day when the sun is out) since the higher CYA level protects the chlorine from breakdown from UV in sunlight so a lower SWG on-time is needed to maintain the chlorine level. You should find that at a higher CYA level you need a lower % on-time for the SWG, assuming there's the same amount of sunlight. If you don't lower the on-time after increasing the CYA level, then the FC level will climb higher since it is getting used up from sunlight more slowly.

 

[prs]

chem geek, I think you have the answer! I've only been collecting data since April but from then to mid-July my CYA dropped from 70 to 50 ppm (constant +/-100°F days without a cloud in the sky here in Bakersfield). I added enough CYA in July to bring it back to 70 ppm and have been reducing cell run time ever since, I'm down from 2.8 to 1.6 hours per day. I didn't expect such a small change in CYA could have that big an impact on cell time.

From now on I'm going to adjust CYA every time it gets down to 60ppm.

 

[eqbob]

The FC test spike after a CYA addition makes sense (if you are measuring the FC after some period of time say during the day when the sun is out) since the higher CYA level protects the chlorine from breakdown from UV in sunlight so a lower SWG on-time is needed to maintain the chlorine level. You should find that at a higher CYA level you need a lower % on-time for the SWG, assuming there's the same amount of sunlight. If you don't lower the on-time after increasing the CYA level, then the FC level will climb higher since it is getting used up from sunlight more slowly.

ok. This and your other post on FC/CYA makes sense. What is the role of hypochlorous acid (HOCl) in all this? Although I've seen it in your other posts--I haven't really figured out what it is.

 

[chem geek]

We know that in direct noontime sun that hypochlorous acid has a half-life of around 130 minutes while hypochlorite ion has a half-life of around 20 minutes. At pH 7.5 with a roughly 50/50 mix of these two chemicals the half-life is around 35 minutes. When CYA is in the water, around 97+% of the chlorine is bound to it (specifics depend on the FC and CYA levels) and this bound chlorine may not break down in sunlight or does so more slowly. Since the FC/CYA ratio determines the amount of unbound chlorine, in theory the higher CYA level with proportionately higher FC level should not have lower loss, but we see that it does so the higher CYA level must be protecting chlorine from breakdown via an additional mechanism. We speculate that there is some sort of shielding of UV from lower depths.

So the bottom line is that even with the same FC/CYA ratio, so same active chlorine level to prevent algae growth, a higher CYA level (so higher FC level to go along with it) actually loses less absolute FC per day. That is what you were seeing and the effect is quite noticeable going from 50 to 70 or even 60 to 80 ppm.