A diaphragm pump (also referred to as a Membrane pump) is a positive 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 double diaphragm is a positive displacement pump which utilises two flexible diaphragms that reciprocate backwards and forwards, creating a temporary chamber, which both draws in and expels liquid Air Diaphragm Pump through the pump. ... Both diaphragms that are connected by a shaft through the center section where the air valve is located.
Though this will depend upon the pump and application, a positive displacement pump, is normally a self-priming pump. ... For example, air managed 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.