How much FC can a given CYA protect

[Rangeball]

Using the pool calculator, for a CYA of 20ppm, it recommends a range of 2-5ppm of FC. Does that imply that 20ppm CYA can protect 5ppm FC from UV degradation?

Say those levels are in a pool in full sun in a perfect world, no organics or swimming activity, would you expect to still have 5ppm FC when the sun goes down?

 

[jblizzle]

No. The higher the CYA, the higher % of FC it will protect ... this is kind of overly simplified.

For example (hypothetical numbers):
If you had 20ppm with full sun and started with 5ppm ... it would probably be down to around 2ppm by the end of the day (loss of 60%)
If you had 40ppm, started with 5ppm ... you might have 2.5ppm left at the end of the day (loss of 50%)
If you had 60ppm, started with 5ppm ... you might have 3ppm left at the end of the day (loss of 40%)


So while higher CYA levels require high minimum FC levels, you usually end up actually losing fewer ppm ... and thus are using less chlorine each day even though your FC levels are higher.

 

[BoDarville]

Using the pool calculator, for a CYA of 20ppm, it recommends a range of 2-5ppm of FC. Does that imply that 20ppm CYA can protect 5ppm FC from UV degradation?

Say those levels are in a pool in full sun in a perfect world, no organics or swimming activity, would you expect to still have 5ppm FC when the sun goes down?

No, this is not correct.

Think of CYA like sunscreen, it offers some protection from sunlight but you will still burn if you stay in the sun too long. CYA will protect FC from sunlight consumption to a greater degree than no CYA, but you will still lose some FC to sunlight. The CYA does this by holding some of the FC in reserve. However, this lowers the effective strength of the FC. This is why higher FC levels are needed at higher CYA levels (see ABC's of Pool Water Chemistry and Chlorine CYA Chart)

The FC targets for a given CYA are those that will provide adequate sanitation for the pool.