A Air Diaphragm Pump diaphragm pump (also known as a Membrane pump) is a positive displacement pump that uses a combination of the reciprocating action of a rubber, thermoplastic or teflon diaphragm and suitable valves on either side 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 allures and expels liquid through the pump. … The two diaphragms that are connected by a shaft through the centre section where 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-prime by creating a pressure differential in the diaphragm chamber. This allures air and pulls the fluid into the suction port.