Are the most common industrial PD pumps. They are sometimes referred to as a membrane pump. A typical configuration consists of a single diaphragm, a chamber, and suction and discharge check valves to prevent backflow. The piston can either be directly coupled to the diaphragm or can force a hydraulic oil to drive the diaphragm. Variations include bellows-type diaphragms, hydraulically actuated double diaphragms, and air-operated, reciprocating double-diaphragms.