chem geek, I believe that when Poolsean talks about lowering the GPM, he is assuming that the pump run time will be adjusted to maintain one turnover per day. That means the SWG will be run longer and so be able to produce more chlorine.
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It seems that we mostly, or perhaps entirely, agree on the facts, but are getting stuck on wording. Several of the words we are using don't have definite fixed interpretations. We started with "not designed", which in this context is a kind of vague notion dealing with how suitable something is for a specific purpose, which is open to various interpretations both in the "suitability" and in the "purpose".
I'm going to try and be more specific. I am specifically taking about shocking the pool. Further, when we talk about shocking the pool we want a procedure that will work more than 98+% of the time, not often or sometimes, but nearly always. TFP recommends shocking when the FC level goes to zero. FC at zero does not automatically mean that shocking is required, but it is well correlated with shocking being required.
I have a significantly oversized SWG, an AutoPilot SC-60 on a 19,000 gallon pool. Running 24 hours a day at 100% it can produce about 12 ppm of chlorine. That is significantly oversized. Even though we recommend oversizing the SWG, mine is at the high end of what you tend to find in the world and dramatically larger than what is minimally required.
Even with my setup, the SWG may never produce enough chlorine to recover from a full blown green swamp. A full blown green swamp can often sustain it's self in the face of 15 or even 20 ppm of chlorine every 24 hours indefinitely. Assuming a CYA level of 60, the shocking procedure TFP recommends could easily require the addition of 80 ppm of chlorine in the first day, possibly more. Even my dramatically oversized SWG setup could not come close to that.
A typical ammonia situation has similar requirements, though the timing is not as tight. Ammonia problems can often required more than 100 ppm of chlorine, something that would take my significantly oversized SWG almost two weeks to produce running 24/7 at 100%.
Obviously these situations are not the routine day to day situation most people are in. Even a much smaller SWG run for many fewer hours can produce enough chlorine to keep up with routine demands. That is after all why people buy SWGs. Likewise, nearly all SWGs can produce extra chlorine above day to day requirements, and that extra chlorine is often enough to deal with relatively mild but still unusual events. A typical SWG set on boost/super chlorinate can handle the very beginnings of an algae bloom, or recovering from a pool party, or producing some extra chlorine for whatever reason the user wants extra chlorine.
The specific case that started this discussion was a pool with an FC level of zero. We don't know why FC was zero. Perhaps the SWG was broken. Perhaps there is a full algae bloom going on that wasn't mentioned. Perhaps they just opened the pool and there is a significant amount of ammonia in the water. Certainly there is some chance that simply going to boost/super chlorinate will take care of whatever is happening. At the same time there is a significant chance that it won't make any visible difference. To reliably recover from this situation, more chlorine needs to be added than plausible residential SWGs will be able to produce.
All of that appears to me to be straightforward and solidly based on facts. Things get much fuzzier when we explore the shadings of meaning of my original statement in full: "A SWG is not designed to raise the FC level up from zero." Certainly, there are cases where a SWG can raise the FC level up from zero. Likewise, there are cases where a SWG can not do so.
In the TFP sense of recommending procedures that are 98+% reliable, a SWG can not raise FC up from zero 98+% of the time given the distribution of situations that actually occur when FC is zero. Nor, I believe, would anyone claim that it was ever intended to. To my mind the entire discussion really comes down to that, and the disagreement is about applying that 98+% reliable rule and how fair/true/representative of the capabilities of a SWG it is to do so.