CYA for Salt Chlorinator Pools II

[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