Double Diaphragm Pumps are classified as positive displacement pumps because the diaphragms acts as a limited displacement piston.
The pump is design and constructed such that two opposed diaphragms are driven by the reciprocating motion by mechanical linkage, compressed air, or fluid from a pulsating, external source. Double diaphragm pumps are design to delivery twice the flow of single diaphragm type pumps.
The pump and diaphragms construction eliminates any contact between the liquid being pumped and the source of energy. This eliminates the possibility of leakage, which is important when handling toxic or very expensive liquids. Disadvantages include limited head and capacity range, and the necessity of check valves in the suction and discharge nozzles.