At the startup it was about 9 psi and has now risen to about 20.
I'm on my way to church, and can only take a minute, but you don't want to let that happen again. That sort of pressure differential can PERMANENTLY damage your cartridges by collapsing the pleats so they remain in contact with each other.

I am glad to have made the decision to build our first pool even though it took till I retired to do it.
If you've got grandkids, a pool is a great thing for them!