The air in a diaphragm-type expansion tank is separated from the water by a flexible rubber membrane (see Figure 10-44). These tanks are smaller than the closed steel tanks and come from the manufacturer precharged with compressed air. When the tank arrives at the site, the diaphragm is fully expanded against its inside surfaces. When the tank is installed and connected to the system piping, water enters the other side of the tank chamber and presses down on the diaphragm.
As a rule, diaphragm tank manufacturers will precharge their tanks to 12 psi, which is sufficient to match the water-fill pressure requirements of the typical house or small commercial building.