Re: CYA Testing
So in the commercial pools with daily FC testing, I presume that is local staff and they also add some chlorine as well? Then your twice a week visit is for things like brushing the pool and verifying that things are done properly? It is surprising to have the lower end of the FC range with the higher CYA level and not have any algae in any of those pools. With only once a week visits to residential pools, do your customers have to add more chlorine? If so, are they all using Trichlor for continuous feeding or do they add other sources regularly?
The SWG pools at 3 ppm FC with 80 ppm CYA is on the edge, but given the magic water you apparently have it's perfectly fine. Well now I can see why you don't see any sort of chlorine/CYA relationship because you can run your commercial pools as low as 1 or 2 ppm FC with 60-80 ppm CYA and not see algae developing. Perhaps just lucky with low phosphates or nitrates or something else inhibiting algae growth. It's certainly not the experience of many residential users nor Ben Powell's experience with his commercial pools nor others I've spoken with (at the NEHA conference I recently attended, most commercial/public pools had much higher FC levels almost all above 3 ppm and some above 5 ppm and this was done both for algae prevention and to have a sufficient chlorine buffer for peak bather loads).
[EDIT] If there is high bather load in the commercial pools, then perhaps they have CC, specifically monochloramine, since that can be effective at killing or inhibiting algae. That could explain why these pools are able to tolerate a lower FC level than the low bather load residential pools. It's at least a possibility and a little more scientific than magic water! [END-EDIT]
Richard