# How much FC can a given CYA protect

#### Rangeball

##### Well-known member
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

TFP Expert
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.

TFP Expert