A diaphragm pump (also called a Membrane pump) is a positive Air Diaphragm Pump displacement pump that uses a mixture of the reciprocating actions of a rubber, thermoplastic or teflon diaphragm and suitable valves on either part of the diaphragm
A double diaphragm is a positive displacement pump which utilises two flexible diaphragms that reciprocate back and forth, creating a temporary chamber, which both draws in and expels fluid through 
the pump. … Both 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 generally a self-priming pump. … For instance, air operated diaphragm pumps self-primary by creating a pressure differential in the diaphragm chamber. This allures air and pulls the fluid in to the suction port.