Pumps can be broadly categorized according to three techniques: Positive displacement pumps use a mechanism to repeatedly expand a cavity, allow gases to flow in from the chamber, seal off the cavity, and exhaust it to the atmosphere.   Momentum transfer pumps, also called molecular pumps, use high speed jets of dense fluid or high speed rotating blades to knock gas molecules out of the chamber.   Entrapment pumps capture gases in a solid or adsorbed state. This includes cryopumps, getters, and ion pumps.