I've read over and over again that there is often a lag time when adding CYA to a pool. It is usually attributed to the slow rate at which it dissolves, especially when it is poured into the skimmer. I've never been a big fan of this idea. It just doesnt' seem to make much sense to me. In order for the granules to fit through the skimmer basket, and end up in the filter, they must first get smaller. They get smaller as they are dissolved. They then have to fit through the pump basket, where they are further dissolved. Yes it is entirely possible that a portion is captured in the filter, but with the rate of flow through that filter, its hard for me to believe that the CYA remains in the filter more than a few hours at most.
Today I had an experience with a pool that makes me suspect that the lag has nothing to do with CYA being trapped in the filter until it dissolves. I added 4lb of CYA to a pool yesterday afternoon. This morning I returned to the pool, and tested the CYA to find that it hadn't increased yet. Without thinking, I backwashed the filter. After doing so, I remembered that CYA is supposed to take time to dissolve, and you are supposed to wait a few days before backwashing. So if there was CYA in the filter, it should be gone now.
Since this pool is pretty much on my way home, I stopped by again to check on it. I was going to add more CYA since I had backwashed away everything I added the day before. I decided to test the water one more time. This time the CYA had come up by about 10 ppm. So I decided not to add more, and see just how far it comes up. I'm going to try to check on it tomorrow again.
It seems to me that even if some of the CYA is trapped in the filter, a significant portion of it has to dissolve before it will even pass through the skimmer basket. Why doesn't that portion show up right away?