Diaphragm Pressure Tanks are more compact and have a rubber diaphragm that is fixed in place separating the lower section of water from the upper section of compressed air. As water is pumped into the tank ,the diaphragm pushes up into the compressed air section until the preset water pressure level is met and the pump shuts down. This will help maintain a steady water pressure downstream in your house. As users daw water from the system, the diaphragm then pushes back down (keeping steady system water pressure) until the over all water pressure falls to the point where the pumps comes back on to rebuild pressure once again. Setting the compressed air level correctly is critical to optimal pressure tank performance.