Piston pumps are used for some large scale vacuum Rotary Piston Vacuum Pump applications. These pumps possess a sliding 
sleeve by which the gas enters, then drawn right into a cylindrical chamber and exhausted by a rocking piston plunger. These pumps are very hardy and have pumping speeds that range between 15 cfm to 2,000 cfm and the pumps can reach a pressure of just one 1 x 10-4 Torr. Rotary piston pumps could be ready for corrosive and oxygen service.
