After thinking about this a bit more, I wondered what the actual chlorine ppm would be leaving the cell given the production rate of the cell. To find out, I started with the maximum production level of my Aqualogic cell which is 1.45 lbs/day of chlorine (assumes 100% setting running 24 hours/day). Thus the production rate in GPM would be;

1.45 lbs/day * .125 gallons/lbs * 1 day / 24 hrs * 1 hr / 60 min = 1.26e-4 GPM

For my pool, the water flows at about 90 GPM (estimated from pump head curve, psi and suction in. mg.) so the maximum ppm leaving the cell would be 1.26e-4/90 or 1.4 ppm continuously. This is of course is added to the existing ppm but still it is not exactly shock level.

So if it is true that the cell does indeed have a very high level of chlorine in the cell, then the cell must either produce chlorine that is very unstable and reverts back to salt shortly after production and/or, very little of the water going through the cell is actually between the plates of the cell (ie. reduce volume of water, increased CL ppm).

Looking at the interior of my cell, I noticed that the plates only take up about half of the area of the cell which means about half or a bit more of the water never touches the cells but goes around them. Still this would only double or triple the ppm level in the cell.

So the only plausible explanation for a very high chlorine levels in the cell is that the chlorine gas is very unstable, which I think someone has already pointed out. It would seem that most of the chlorine reverts back to salt before or shortly after exiting the cell.

Therefore, the only explanation I can gather for the need of high CYA, is that during this temporary phase of high chlorine, the high CYA may prevent some of this unstable chlorine from reverting back to salt. So the CYA may have two jobs, one for UV and another for stabilizing the chlorine in the cell.

Also, from what I understand, chlorine has two jobs. First is for sanitation which kills all of the nasty bugs in pool water. From what I have read, this does not require a very high residual to accomplish so even 60-80 ppm CYA can usually accomplish this fairly quickly or quick enough.

The second job of chlorine is oxidation which removes all of the other stuff such as sweat, suntan oil and algae which is a big one. Higher levels of chlorine are required to accomplish this. However, it is not necessary to remove this stuff in seconds or minutes but is sufficient to remove in hours or even days.

So it would seem that the high CL in the cell does much of the oxidation, which can be accomplished over hours or day, and the CL residual is responsible for killing the bugs, which can be accomplished with a low CL and a fairly high CYA.

The only anomaly to this whole theory is that my residual chlorine did not increase with an increase in CYA which I would have expected. I am willing to bump it up to 70 just to make sure there is not something else going on.

Sorry for the long thought experiment but I would like to fully understand the SWG process and as Waste has pointed out, so far, we have not seen a comprehensive explanation. This is my first attempt at one but I am sure it has some flaws.