A diaphragm pump (also Air Diaphragm Pump referred to as a Membrane pump) is a positive displacement pump that uses a mixture of the reciprocating actions of a rubber, thermoplastic or teflon diaphragm and suitable valves on either aspect of the 
diaphragm
A double diaphragm is a positive displacement pump which utilises two flexible diaphragms that reciprocate back and forth, creating a short-term chamber, which both allures and expels fluid through the pump. … The two diaphragms that are connected by a shaft through the centre section where in fact the air valve is located.
Though it depends upon the pump and application, a positive displacement pump, is generally a self-priming pump. … For instance, air managed diaphragm pumps self-primary by creating a pressure differential in the diaphragm chamber. This draws in air flow and pulls the liquid into the suction port.