The diaphragm pump is a volumetric hydraulic pump that processes the working fluid in a control volume that varies periodically over time, with typically non-steady flow. In PHOENIX diaphragm pumps, the fluid travels from the lower suction manifold to the pump body and then to the delivery manifold through the alternating movement of two opposing diaphragms. In English, it is called Air Operated Double Diaphragm AODD because the movement is imparted to the diaphragms pneumatically through a shaft controlled by a distributor, into which compressed air is introduced into a chamber adjacent to the pumping chamber. Thanks to this mechanism, the diaphragm pump can also be called a pneumatic diaphragm pump, or more precisely, a double diaphragm pneumatic pump.

In the double diaphragm pump, the head of the working fluid is high, despite its relatively compact size: if already primed, it can suck liquid from 9 meters and pump up to 9.8 meters high on primed suction. These heights can be limited by the viscosity of the fluid itself, the higher the viscosity, the lower the head.

The supply pressure is instead constrained by the frictions between the components of the distributor and the air passage channels available. The fluid-air ratio is about 1:1. Double diaphragm pneumatic pumps can be made for small and medium flows, and medium and high heads.

Diaphragm pumps typically operate with supply pressures in the range of 1.5 bar to 8 bar.