A positive displacement pump, or a rotary vane pump, uses moveable vanes mounted on a rotor in side a chamber to displace air, gas, water, solvents, or hydraulic fluid. The vanes in a rotary vane pump are mounted in slots and slide in and out of the slots. Pushrods, springs or hydraulic pressure pushes the vanes against the housing of the chamber. The center of the chamber is offset relative the center of the rotor; this creates a smaller volume in the chamber as the fluid enters and a larger chamber as the fluid leaves the chamber. The smaller volume becoming a large volume creates pressure (or vacuum).
Positive displacement pumps are ideal for gases and low viscosity liquids; they can develop relatively high pressures and they are very good for creating a vacuum. The extension of the vanes by springs or pushrods compensates for wear. The rotary vane pump can be primed dry and can run dry for short periods. Positive displacement pumps are not as good for high viscosity fluids or for creating very high pressures.
Some manufacturers of positive displacement pumps, aka rotary vane pumps, for the laboratory are Gast, Welch, Edwards, Labconco, and Brandtech.