Diaphragm expansion tanks are smaller than the old-school, plain-steel compression tanks they’ve replaced over time. This is a good thing because boiler rooms are stingier on space these days. Oh, and diaphragm tanks seem so carefree. Well, at least on the surface.
These tanks separate the air from the system water with a rubber membrane and pre-charge the air side of the membrane to 12 psi at the factory because that’s what you need in most American houses.