Free Chlorine & CYA Levels

[poolneophyte]

I have an Jandy Aquapure Purelink AP1400 SWG and have a question relating to the target FC levels. The Chlorine/CYA chart in Pool School shows the target FC levels of 4, 5 & 6 for CYA levels of 60, 70 & 80, respectively. The manual for the Purelink SWG recommends the CYA range to be 50-75 PPM and the FC range to be 1-3 PPM. There is a warning note which says that above 3 PPM of FC may cause corrosion of pool metals. The manual also states that the Aquapure is warranted to operate properly only if these conditions are met.

Assuming I keep my CYA at 60 PPM and the Pool School recommended FC level of 4 PPM, it appears that I am voiding the warranty on the SWG. Also, when I need to shock the pool using the Pool School recommended levels, could this damage the SWG? I would appreciate any thoughts or comments on this.

 

[JasonLion]

FC levels above 3 can cause corrosion only if you don't have any CYA at all, and even then it isn't likely to do anything. They are saying that because people with indoor pools without any CYA tend to set the percentage too high and get very high FC levels (20+) which can cause problems without any CYA.

In an outdoor pool, with CYA around 70, the situation is completely different. It takes FC around 80 to match the active chlorine level of FC at 3 and no CYA. In practice, it actually takes FC levels quite a few times that high before the first sign of corrosion shows up. Somewhere around FC at 300+ with CYA at 70 might maybe start to cause a problem.

With CYA at 60, you could get by with FC at 3. But you will have fewer problems if you run with CYA between 70 and 80 and FC around 5.

Warranty clauses like that one are annoying, because you will never be able to prove that your FC level never went over 3, even if you were super careful and documented everything. So they have an excuse to get out of the warranty any time they want to.

 

[chem geek]

Actually, around 90 ppm FC with 70 ppm CYA would be equivalent to 20 ppm FC with no CYA. The FC/CYA ratio rule breaks down when the ratio gets high. When the FC is higher than the CYA level, the equivalent FC is closer to FC-CYA. Nevertheless, the point is still valid that with CYA in the water it is very difficult to get the chlorine level high enough to cause corrosion.

 

[poolneophyte]

Thanks to both of you for your help. I will target CYA of 70 and FC of 5 this summer.