Paddlewheel and Positive Displacement (PD) Flow Transmitters: Paddlewheel devices have a paddle wheel mounted perpendicular to the flow path, and are used determine the flow, in an enclosed pipe, of gas, air, and liquids. PD devices measure the volume or flow rate of a moving fluid or gas by dividing the fluid or gas into fixed, metered volumes or parcels. These devices have a rotating or reciprocating mechanism that allows passage of these parcels. The number of parcels that pass through the chamber determines volume. The rate of revolution or reciprocation determines the flow rate.