A diaphragm pump (also called a Membrane pump) is a positive displacement pump that runs on the combination of the reciprocating actions of a rubber, thermoplastic or teflon diaphragm and Air Diaphragm Pump 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 draws in and expels fluid through the pump. … Both diaphragms that are connected by a shaft through the center section where the air valve is located.
Though this will depend upon the pump and application, a positive displacement pump, is normally a self-priming pump. … For example, air managed diaphragm pumps self-primary by creating a pressure differential in the diaphragm chamber. This allures air flow and pulls the liquid into the suction port.