Ultimate Pressure - Welch 8890 Owner's Manual

2.3 Effects of Continued Pressure Reduction
The quantity of gas in the vessel (6) is reduced with each evacuation cycle. The gas remaining in the vessel
expands to fill the vessel and consequently with each cycle the pressure in the vessel is reduced. This is a
manifestation of Boyle's law which states that for a constant temperature, the volume of a body of gas is inversely
proportional to its pressure, i.e. if the volume is enlarged the pressure must be reduced.
As the amount of gas in the vessel is steadily diminished, its pressure is correspondingly reduced. The action of
the pump must therefore compress a successively smaller quantity of gas with each cycle to something greater
than atmospheric pressure in order to expel it from the pump.
At the beginning of an evacuation sequence, the compression ration is very small. In the first cycle of operation
the pump draws in a volume of gas at atmospheric pressure and expels it at approximately atmospheric pressure.
In contrast, at ultimate pressure, a pump draws in gas at (for example) 100 millitorr and must compress it to more
than 760,000 millitorr (atmospheric pressure) in order to expel it from the pump. Since the exhaust valve is
generally spring loaded to provide a good seal, the pressure required to open it is somewhat greater than
atmospheric pressure. Therefore, at an ultimate pressure of 100 millitorr (1 x 10 Torr) the compression ration
performed by the pump is approximately 10,000 to 1.

2.4 Ultimate Pressure

As described previously, a quantity of gas is removed from the system with each cycle of the pump. Therefore,
the pressure of the gas remaining in the system is reduced with each pump cycle. Since the pump can remove
only a small portion of the gas with each pump cycle, it is obvious that this method of evacuation can never
completely remove all the gas in the vessel. In addition, all the components of the system contain minute sources
of gas leakage which are impossible to seal completely against atmospheric pressure. Outgassing of materials
within the system provide additional sources of gas.
As a result, after prolonged pumping, a state of equilibrium is reached in which the gas introduced from all the
leakage sources is balanced by the ability of the pump to remove gas from the system. This state of equilibrium is
referred to as the ultimate pressure, or blankoff pressure, of the pump and its system. No matter how much
additional pumping time is provided, no further reduction in system pressure will be accomplished once ultimate
pressure is attained.
2.5 The Gerotor Vacuum Pump
The Model 8890 Vacuum Pump uses patented technology, (U.S Patent No. 4,519,755) developed by Welch
Vacuum, employing two gerotor stages to pump gases. Each gerotor stage consists of an inner rotor and an outer
roto Figure 2-2 is a cutaway view of a single gerotor pump showing the inner and outer rotors. There is a one-
tooth differential between the two rotors; the inner rotor has one less tooth than the outer rotor. The inside
diameter of the outer rotor is somewhat larger than the outside diameter of the inner rotor.
Figure 2.2
Gerotor Pump Chamber,
Schematic Diagram
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