Pump Performance Measures
The pumping speed is also known as the volume flow rate.
It is measured at the pump’s inlet in liters per second , cubic feet per minute, or cubic meters per hour.
The throughput refers to the pumping speed multiplied by the gas pressure at the inlet and is measured in torr-liters/second and similar units.
The throughput is related to any vacuum leaks and refers to the volume leak rate multiplied by the pressure at the vacuum end of the leak.
The leak throughput can be compared to the pump throughput. The base pressure refers to the drop in the pumps mass flow to the same level as the mass flow in the chamber and represents a constant pressure.
The evaporation into a vacuum is called outgassing. The most common source of this is the water absorbed by materials in the chamber. At some point, the vacuum will approach the pump’s ultimate pressure or the best vacuum that this type of pump can achieve under ideal situations.
Adding more pumps in parallel or bigger pumps of the same type can still improve the pump-down speed but they will not reduce the base pressure below the ultimate. This is where better pumping technologies must be used to go beyond this barrier.