Positive displacement flowmeters, also known as PD meters, measure volumes of fluid flowing through by counting repeatedly the filling and discharging of known fixed volumes. A typical positive displacement flowmeter comprises a chamber that obstructs the flow. Inside the chamber, a rotating/reciprocating mechanical unit is placed to create fixed-volume discrete parcels from the passing fluid. In the round gear meter (also called a spur gear meter) the chamber is like two intersecting circles within the body (6). Running in the chamber are two gears (5) sensed by a pickup (1). The fluid travels around the outside turning the gears and creating a positive displacement of fluid for each pulse output.
Image courtesy of TÜV NEL