Quick question - Just about everything added to the pool contains some salt in it (incl bleach, soda ash, baking soda). I understand that it might not be much salt, but it is some salt.
Why doesn't the salt level just continue to rise - slowly but continuously? Why don't you eventually end up with a pool that has too much salt?
I understand that with a sand filter there is some water loss with backwashing which may explain why the level doesn't go up, but what about with a cartridge filter?
Is the process so slow that the pool needs to be drained for some other reason (such as liner replacement), well before it becomes a problem?