Why does stabilized CL test as fc instead of cc?

[Themadczar]

Follow up question: why then have I read on these forums that pools with higher cya levels need to keep higher than normal minimum fc levels to remain sanitized? Doesn't the stabilizer continue to release the CL as it's needed until there is no more CL in the pool? If it does so when reacting to dye, why not with contaminants?
While I am not a chemist, I can hold my own in the math/science departments. The details of the chemistry above were beyond me but I still find all of this very fascinating!

[chem geek]

Follow up question: why then have I read on these forums that pools with higher cya levels need to keep higher than normal minimum fc levels to remain sanitized? Doesn't the stabilizer continue to release the CL as it's needed until there is no more CL in the pool? If it does so when reacting to dye, why not with contaminants?
While I am not a chemist, I can hold my own in the math/science departments. The details of the chemistry above were beyond me but I still find all of this very fascinating!

Another good question. It's because there is a difference between having a chlorine reserve or capacity (i.e. what is measured as FC) vs. the amount of chlorine that is active. Reaction rates that determine the rate of killing pathogens, preventing algae growth, and oxidizing bather waste are dependent on the concentration of active chlorine. The amount in reserve is not relevant to that reaction rate.

An analogy might be helpful. Think of fighting a war against an enemy and that the front-line soldiers are the ones doing the killing. As those front-line soldiers get killed, sacrificing their lives to kill the enemy, they are replaced by more soldiers from the reserve/reservoir of additional troops. The rate of killing has nothing to do with how many troops you have in reserve nor with how quickly front-line troops are replaced from the reserve. The number in reserve just tell you how long you can keep fighting, but not whether you are killing the enemy faster than they can reproduce.

The reaction of chlorine with dye is very, very fast. The reaction of chlorine to kill pathogens is slower, though still in seconds to a minute or so for bacteria (for a 99.9% kill), but algae are harder to kill due to thicker cell walls and slower permeability to water (and active chlorine). So with algae, you very much notice differences in the active chlorine level whereas with the dye the reaction is so fast that even a low active chlorine level still shows up as color in far less than a second.

Because of chemical equilibrium, the amount of active chlorine is proportional to the FC/CYA ratio (technical derivation in this post). 3 ppm FC with 30 ppm CYA has the same active chlorine level as 6 ppm FC with 60 ppm CYA and is the same as 9 ppm FC with 90 ppm CYA. The FC levels at each CYA level were determined by Ben (PoolDoc) years ago to be what was needed to inhibit algae growth regardless of algae nutrient (phosphate and nitrate) level and under ideal growth conditions of sunlight and temperature. That is, these are roughly worst-case so that nearly all pools can be managed from that chart.