I think I've read enough to understand the basics of FC vs CC vs TC, and how your CYA levels dictate your target FC. What I'm having trouble understanding is if/how CYA "gives up" it's hold on FC when it's needed.
It seems from what I've read, that CYA basically holds some of your FC hostage so that the sun wont break it down, but that this also makes that FC not truely "available" for sanitizing purposes. It also seems from what I've read that without CYA, the sun would rapidly cut your FC down by about half every hour or so? If this were all "absolutely" true, then wouldn't it be impossible (with a CYA level of let's say 40), for your FC reading to drop to zero? Does CYA protect your FC from sun, but then "give it up" when it's needed for sanitizing?
Thanks for any help you can give explaining this.