A diaphragm pump (also referred to as a Membrane pump) is a positive displacement pump that uses a mixture of the reciprocating action of a rubber, thermoplastic or teflon diaphragm and suitable valves on either part of the diaphragm
A dual diaphragm is a positive displacement pump which utilises two flexible diaphragms that reciprocate back and forth, creating a temporary Air Diaphragm Pump 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 situated.
Though this will depend upon the pump and application, a positive displacement pump, is normally a self-priming pump. … For instance, air operated diaphragm pumps self-primary by creating a pressure differential in the diaphragm chamber. This allures air flow 
and pulls the liquid in to the suction port.