A diaphragm pump (also known as a Membrane pump) is a positive displacement pump that uses a mixture of the reciprocating actions of a rubber, Air Diaphragm Pump thermoplastic or teflon diaphragm and suitable valves on either side 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 allures and expels fluid through the pump. … The two diaphragms that are linked by a shaft through the centre section where the air valve is located.
Though it depends 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 draws in atmosphere and pulls the liquid in to the suction port.