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