A diaphragm pump (also referred to as a Membrane pump) is a positive Air Diaphragm Pump displacement pump that uses a combination of the reciprocating actions 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 short-term chamber, which both draws in and expels liquid through the pump. … The two diaphragms that are linked by a shaft through the center section where the air 
valve is situated.
Though it depends upon the pump and application, a positive displacement pump, is generally a self-priming pump. … For instance, air operated diaphragm pumps self-prime by creating a pressure differential in the diaphragm chamber. This draws in air and pulls the fluid in to the suction port.