Air actuators are mainly available in two forms - diaphragm and air cylinders type. The diaphragm actuators are used with control valves, while air cylinders are found on on-off service. Diaphragm actuators with considerable diameter are used to provide sufficient force to overcome unbalanced flow forces across the valve seat as well as internal friction from stem packing. It operates from 3 to 15 psig as well as 6 to 30 psig of air. Some air cylinders utilize 125-psig service air or higher pressure where there is a desire to minimize cylinder diameter. These provide modulating valve stem movement and can operate in any position, although a diaphragm or cylinder arrangement above the valve body is preferred in order to avoid moisture accumulation within the diaphragm or cylinder.