SWG/CYA question

[JCarsten]

I've been re-reading Pool School now that we finally have the pool finished and filled. The chemistry is making much more sense now that I have moved out of the "conceptual" and into the "hands on" learning phase.

One thing still puzzles me. I have a SWG and see where the water balance section and some of the other guides on this site recommend a higher CYA level for SWG pools. Or more accurately, the chart recommends a more narrow CYA level for a salt pool.

I'm still trying to wrap my head around these 2 concepts:
1. Why is the range recommended in the Chlorine/CYA chart for SWG pools more narrow than for non-SWG?
2. Why is there a different recommended chlorine level by CYA in one vs. another? For instance, in a non-SWG pool, at 60/70/80 CYA respectively, the chart puts the recommended FC level at 7/8/9. However, for the same CYA level in a salt pool, the recommended FC level is 4/5/6.

I understand "what" I need to be targeting. I just don't get the "why". Can someone help me clarify?

 

[JasonLion]

1) Because the SWG lasts longer at higher CYA levels, but you don't want to go over 80 for other reasons. This optimizes the SWG's lifetime without adding undue difficulties for the pool owner.

2) Because it works. Seriously, we don't really know why, but a SWG works with a lower FC level than you would otherwise need. There are a dozen theories, but no one is sure exactly which one, if any, is correct.