CYA for Salt Chlorinator Pools II

[chem geek]

Sean,

I wasn't clear with the amount of chlorine levels in the generator. I was talking about the normal operation that generates the incremental chlorine for the pool and referring to what some people were claiming was a natural superchlorination in the cell during this process. I was also referring to the entire cross-section of area that the water flows through and not just an area near the plates (which has incredibly high concentration of chlorine).

I believe what you are talking about is a superchlorination mode which runs the cell at a much higher power level. If you ran at this level during the duration of one pool turnover, then the pool itself would get to that 10 ppm level. Is this what you are talking about? I apologize for my ignorance about the mode you have for your cells.

Richard

[Poolsean]

No apologies, but thanks for "slappin me in the face" and making me aware that I sounded like a sales guy. That's the last thing I want to sound like here as it irritates Ben if I do.

The normal operation, when you inspect anyone's cell, is for the water to pass through and around the electrodes, or blades. Let's face it, the chlorine is only produced between the blades so the space around them does not get the same concentration of chlorine. However, it is quite high between the blades AND still quite high surrounding the blades too. Oddly, if you test the water coming out of the cell, you're prone to only get 1-5 ppm higher than what's tested in the body of the pool. You may see a similar result if you take a water sample from the closest return line to the pool equipment.
Now, the design of the cell is going to be rather different from manufacturer to manufacturer. If you inspect the various designs, you'll see different dimensions of blades, spacing between the blades, solid vs mesh, % ruthenium oxide coatings on the blades, and power (milliamps/sq cm) to the blades. So when you asked for more details, it's not easy...besides, most of this is proprietary information.

What I am taking about is not just under superchlorination mode, this is under normal operation. Besides, most Superchlorination mode does not increase the power to the cell, it simply extends the time the cell is energized. I believe there is only one manufacturer that claims to increase the output to 125%.
Most manufacturers cycle their cells on and off, during the day. Some run the cell continually, for a % of the pump run cycle (I'm not sure how this is determined without having to enter your physical pump run time). Some constantly energize the cell and fluctuate the power to the cell between a low output, up to a high output. Perhaps this last one is the model you're describing? Those are typical of Australian systems that also require higher salt levels to operate (4000 - 6000 ppm).

Hope this helps

[Tredge]

I've been fascinated by this discussion and I'm continually trying to understand.

Are you saying that because the water "between the blades" is 80+ppm chlorine while the water exiting the cell is only 1-2ppm higher than the pool itself that the high CYA acts as a transport of that high chlorine? Sort of a way to get it into the pool?

That logic makes sense to me.....what Doesnt make sense is when Ben's best guess chart comes into play.

The chlorine in the pool, regardless of where it came from, has the same chemical properties as any other chlorine.

In my experience, when I ran my SWG at 80ppm CYA, my chlorine maintained at a lower power setting....as suspected. However, Algae on the walls and in the pool ran out of control and I fought it all season.
Ever since I lowered my CYA to 35-40ppm, I've not had to shock my pool once and haven't had a hint of "slimy walls" or the onset of algae.

There could be other factors at play since my tests were over the course of 2 seasons...

[tphaggerty]

Yet my results are the exact opposite of Tredge. Running at about 30ppm CYA, I just couldn't keep my 30K IG pool "stable" (Aquarite). Every time I had any type of high bather load or lots of rain or a New York pollen "storm", the pool would get cloudy.

Now I am running at about 60 ppm CYA (I wonder if anyone can read the test more accurately than a 10ppm swing) and I have been able to scale back the SWG from 60 to 50 to now running at 40%. Water is clear and I am at about 4ppm FC. The pool is much more stable, it just stays clear and nice. It is possible that my results come from finally clearing out any residual "gunk" from opening, but I'm inclined to think it is due to running at or close to the SWG instructions.

Plus, my PH seems rock solid at 7.4/7.6.

[chem geek]

Sean,

Yes, you've got my point. The fact that some small area of the cell and therefore some small percentage of volume of water going through the cell is exposed to 80 ppm of chlorine is interesting, but doesn't mean that all or even most of the water is exposed to that level except through many, many turnovers of the pool water.

An analogy with what happens when you add liquid chlorine (or bleach) to the pool might be helpful. Someone else [EDIT](let's give credit to waste where credit is due)[END-EDIT] mentioned this in this or another thread, but I'll describe the situation a little more deeply. When you add 6% bleach to your pool, you are adding chlorine in a concentration of 60,000 ppm. As the chlorine diffuses into the pool, at some point you will have the chlorine diffused into 1% of the total pool volume at which point the averge chlorine concentration in this volume of the pool will be 100 ppm assuming you added enough chlorine to raise the entire pool's chlorine level by 1 ppm. At a later point in time when the chlorine has mixed with 10% of the total pool volume, this volume has on average 10 ppm, etc. Now of course I'm oversimplifying since the mixing is not uniform (though I used the term "average" which covers up many sins) and there is not an absolute line of demarcation between pool water that has mixed with the chlorine vs. pool water that has not. But the principle is very much the same as what happens in the cell. Though some small amount of water is exposed to very high chlorine levels, it's still a small amount of total pool volume.

As an aside, the above "analysis" (and I use that term loosely since I was not at all precise in the above discussion) implies that it may be better to add liquid chlorine (or bleach) to the pool in a way that mixes rather quickly with a large volume of the pool water, but not with all of the pool water at once (which is impossible to do anyway). So adding the chlorine over a jet or into the skimmer may be better then putting it into one place in "calmer" water. I'm not sure how distributing it manually around the pool's entire edge would do, but personally I don't like to take that risk of splashing the 60,000 ppm chlorine onto my clothes!

Anyway, the bottom line is that I think that marketing a salt cell as somehow super-zapping your pool's water by using the high chlorine measurement example is telling the truth, but not the whole truth (I'm not accusing you -- you have been very forthright and honest in your discussions -- I'm talking about sales folks less knowledgeable than yourself). Unless there is good science to explain otherwise, what the salt cell does is not much different from what happens when you add liquid chlorine to your pool except that the cell is doing so continually over a longer period of time. It is possible that this is more optimal in producing the 10% volume / 10 ppm mix through the pool water. It is also true that the chlorine that is initially produced by the salt cell is unbound by CYA and therefore the true disinfecting chlorine concentration may be much higher, though the exposure time before this gets combined with CYA is rather short. Though the same thing occurs with liquid chlorine put into the pool, again the rapid nature of dumping chlorine in the pool is probably less optimal than the continous process of the salt cell.

Richard

[chem geek]

Are you saying that because the water "between the blades" is 80+ppm chlorine while the water exiting the cell is only 1-2ppm higher than the pool itself that the high CYA acts as a transport of that high chlorine? Sort of a way to get it into the pool?
:
:
In my experience, when I ran my SWG at 80ppm CYA, my chlorine maintained at a lower power setting....as suspected. However, Algae on the walls and in the pool ran out of control and I fought it all season.
Ever since I lowered my CYA to 35-40ppm, I've not had to shock my pool once and haven't had a hint of "slimy walls" or the onset of algae.

I don't believe the purpose of the high CYA is to "transport" the chlorine into the pool, but you are close to what I do believe is the purpose of the high CYA in making the salt cell more efficient. I'm going to try to explain this non-technically, but if I fail I apologize in advance.

The efficiency of the salt cell in generating chlorine is dependent on how much of this generated chlorine already exists near the generating plates. If there is already a lot of chlorine close to the plates that generate chlorine (due to its building up and not getting swept away fast enough), then the efficiency of the process drops and less chlorine is generated. The purpose of the CYA seems to be to combine with the chlorine that is generated and effectively "hide" this chlorine from the salt cell so that as far as the salt cell is concerned, the concentration of chlorine near its plates is not 80 ppm but something much less. The key to making this work is that the chemical reaction that combines the chlorine with the CYA to "hide" the chlorine needs to be faster than the production of the chlorine from the salt cell and it turns out that the critical point where this takes place is around the 60 ppm or so of CYA that is the minimum recommended by the manufacterer of the salt cell.

Your experience with algae is consistent with what I was saying in my earlier posts that Ben's chart is important to follow because the concentration of the disinfecting form of chlorine needs to be high enough throughout the pool, not just in pool water that passes through a salt cell. On the other hand, many people have reported no algae problems at higher CYA and Sean's experience is that those with lower CYA have had algae problems while those with high CYA have not, but that might have been due to the low-CYA cases having such low efficiency of salt cell chlorine generation that they couldn't produce enough FC to kill algae even at the lower level of CYA. At this point, I would say the jury is still out and my guess is that the continual dosing of chlorine from the salt cell is somewhat more efficient and does probably zap more bugs, oxidize more organics, and kill more *free-floating* algae than the manual dosing of liquid chlorine. That said, your experience shows that this greater efficiency of continual dosing from a salt cell is not enough and that you still need to have "Ben's chart" chlorine levels throughout your pool.

I do believe that it would be in the best interests of pool owners if the manufacturers of salt cells would at least consider the possibility that designing salt cells to operate at lower levels of CYA would be beneficial to "the rest of the pool" including pool surfaces where algae can develop (and bacteria can hang out in biofilms). All the salt cell makers have to do is simply increase the area of their plates over a larger volume and drop the power so that the local chlorine generation rates are lower since that will require a lower concentration of CYA to be able to "keep up" and "hide" the chlorine that is produced. And yes, this means that the salt cell may have to be bigger or, most likely, longer than it is today and that means more bulk and more piping to deal with this, but if it means a better situation for pool owners, then that's the right thing to do.

I just want to thank all of you that post on this and other threads. I have never experienced such a wealth and diversity of information and assitance in an environment that has remained civil. It's refreshing!

Richard

[tphaggerty]

Richard & Poolsean,

This is so great for those of us on the edge of understanding it all. Its like reading Scientific America - I don't understand most of it but it sure stretches the mind.

The ONE thing that hasn't been addressed in your analysis, Richard, is the fact (using the term loosely) that almost everyone using a SWG with fairly well balanced water reports CC at 0 all the time. This is not true (I believe) of those that use bleach to maintain normal FC, at some point they will get a buildup of CC and have to shock (which is another interesting thread you are involved in!). This would seem to support the idea that the SWG is "super-shocking" in the cell and it must be some larger percentage of the water that passes through the cell than you are calculating.

In thinking about this though, you did say that it might take a couple of days to "shock" all of the water in the pool, perhaps this is happening and is "good enough" to keep the CC reading at or close to zero. I am wondering why or whether this information is available in Austrailia where they have been using these systems for a lot longer than here (at least for non-commercial pools).

[chem geek]

Richard & Poolsean,
The ONE thing that hasn't been addressed in your analysis, Richard, is the fact (using the term loosely) that almost everyone using a SWG with fairly well balanced water reports CC at 0 all the time. This is not true (I believe) of those that use bleach to maintain normal FC, at some point they will get a buildup of CC and have to shock (which is another interesting thread you are involved in!). This would seem to support the idea that the SWG is "super-shocking" in the cell and it must be some larger percentage of the water that passes through the cell than you are calculating.

In thinking about this though, you did say that it might take a couple of days to "shock" all of the water in the pool, perhaps this is happening and is "good enough" to keep the CC reading at or close to zero. I am wondering why or whether this information is available in Austrailia where they have been using these systems for a lot longer than here (at least for non-commercial pools).

A properly maintained outdoor pool using manually dosed chlorine will also report a CC of 0 all the time. If the chlorine level gets too low, then [EDIT]when you later add some more chlorine[END-EDIT] you can build up chloramines, but if it is constantly maintained, then this is much less likely [EDIT]since breakpoint occurs "on the fly" when sufficient chlorine is present[END-EDIT]. Also, outdoor pools exposed to sunlight may have UV breakdown the chloramines [EDIT]and possibly chlorinated organics[END-EDIT]. If you think about it, if you were to manually drip liquid chlorine (remember this is 60,000 ppm chlorine concentration) into your pool near an output jet and did so more or less continuously whenever the pump was running, you are essesntially doing exactly the same thing that the SWG system is doing [EDIT]including exposing some volume of water to very high chlorine levels[END-EDIT]. This implies that for a manually dosed pool it *may* be better to add chlorine in small doses more frequently rather than wait to do it in larger doses every few days [EDIT](and maybe this slower dosing helps keep the CC at 0 by oxidizing CC albeit a small amount of pool water volume at a time).[END-EDIT]

[EDIT]From what I have read, chlorinated organics (aka Disinfection ByProducts or DBPs) are more likely to form at higher chlorine concentrations (which CYA keeps down low) and in particular they form even more when there are chloramines also present so an incomplete breakpoint is the worst situation to be in (i.e. to not have enough chlorine to meet bather loads). The "bad" chlorinated organics, known as THMs, form more at higher pH while at lower pH you get more non-volatile chlorinted organics that are not considered harmful to health, but would still show up in CC if they persisted.[END-EDIT]

To really test your theory, an analysis of indoor pools with and without SWG would be the place to start. Indoor pools have a much harder time avoiding CC since there is no sunlight to break them down nor is there good air circulation to sweep away the products of breakpoint chlorination (though I've never really been convinced that this is an issue since nitrogen and oxygen gasses are already pretty dominant in air so I doubt they get that much higher in concentration above an indoor pool's surface -- perhaps it's other gasses, such as carbon dioxide, that need to be swept away???).

Richard

[chem geek]

Evan (waterbear) also makes good points. If we really want to get to the bottom of this, we would need ALL of the relevant pool water chemistry information that all of you in these forums know is what is asked of you when you want people to help you with your water problems. Of particular relevance for this SWG algae discussion would be minimum FC level, CYA, and pH. The TDS is already roughly known and not a big factor nor is the CH or alkalinity (except that very low alkalinity can lead to large pH swings you might not be aware of) -- I'm not talking about water balance to protect plaster, but rather those components that affect disinfecting chlorine concentration.

Anyway, I don't want to bash SWG systems in any way. Even though I don't have one, I think they are great and the most important service they provide is that they automatically maintain FC levels. And then there's that silky feel of salt water...oooooooo, you've got me thinking about getting an SWG myself!

[Sentient]

The ONE thing that hasn't been addressed in your analysis, Richard, is the fact (using the term loosely) that almost everyone using a SWG with fairly well balanced water reports CC at 0 all the time. This is not true (I believe) of those that use bleach to maintain normal FC, at some point they will get a buildup of CC and have to shock (which is another interesting thread you are involved in!). This would seem to support the idea that the SWG is "super-shocking" in the cell and it must be some larger percentage of the water that passes through the cell than you are calculating.

I hope this doesn't duplicate something said farther below, where I haven't read yet.
[Edit]Sure enough, it was beat to death. Sorry to waste the bandwidth[/Edit]

I have a SWG and have seen CC chlorine levels above zero. It has been after a swim party or other event that put a good load on the pool. My solution was simply to add some bleach, and let things circulate over night.

This does not deny that some form of superchlorination may be occurring in the cell, but it also shows that water not having gone through the cell recently is still at risk.

[Edit]I think I will try and up my CYA based on what I read here. I was keeping it lower because I didn't understand how it would help. Now that I can grasp a reason why it is a good thing I will follow through.

In reading all of this I had a couple of thoughts.

First, my aquarite turbo cell has what looks like plastic mesh around the plates. This should spit out lots of vortices to both slow the water down and mix it a lot near the plates, suddenly that makes sense. High turbulence should move the chlorine away from the plates more quickly. Clever mechanical tricks are always a good thing.

Second, someone with a two speed pump should be able to test whether slower flow is more or less efficient somehow. This would not be an easy test since fairly accurate estimates of flow rate would be needed, but worth considering.
[/Edit]

Sentient