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Pfeiffer <strong>Vacuum</strong><br />
Page 64<br />
Formula 2-15<br />
Ultimate pressure<br />
<strong>Vacuum</strong> <strong>Technology</strong><br />
Dividing the backing pressure by the compression ratio yields the ultimate pressure.<br />
p e =<br />
p v<br />
K 0<br />
Whether ultimate pressure will be achieved will hinge upon the size and cleanliness of both<br />
the equipment and the pump, as well as upon the bake-out conditions. After extreme bakeout<br />
(to 300 °C) only H , CO and CO will be found in the residual gas. These are gases that<br />
2 2<br />
are dissolved in the metal of the recipient and continuously escape.<br />
The gas ballast in the backing pump that is being used should be activated occasionally to<br />
prevent hydrogen from accumulating in the backing-vacuum area. In many cases, the actual<br />
ultimate pressure will be a factor of the desorption conditions on the high vacuum side of<br />
the turbopump and its pumping speed, and not the compression ratios of the pumps.<br />
2.8.2 Application notes<br />
Generating clean vacuum<br />
Turbopumps are suitable for generating clean vacuums in the range of 10 -3 to 10 -10 mbar.<br />
Thanks to their high compression ratio, they reliably keep oil from the inlet area of rotary vane<br />
pumps away from the recipient. Models with stainless steel housings and CF flanges can<br />
be baked out. This makes these pumps ideally suited for research and development applications<br />
where contamination of the residual gas is undesirable. Turbopumps can be used for<br />
evacuating large vessels, with rotary vane pumps being employed as backing pumps. In the<br />
case of turbo drag pumps, even two-stage diaphragm pumps will suffice as backing pumps;<br />
however due to their lower pumping speed, it will take them a great deal of time to pump<br />
down larger vessels. The gas throughput of the combination will also be highly restricted by<br />
the diaphragm pump.<br />
<strong>How</strong>ever this combination is an extremely cost-effective solution for a dry pumping station.<br />
It is often used in connection with differentially pumped mass spectrometers. Pumping<br />
stations consisting of a backing pump and a turbopump do not require valves. Both pumps<br />
are switched on at the same time. As soon as the backing pump has reached the necessary<br />
backing-vacuum, the turbopump quickly accelerates to its nominal speed and quickly evacuates<br />
the vessel to a pressure of p < 10 - 4 mbar with its high pumping speed. Brief power<br />
failures can be bridged by the high rotational speed of the rotor. In the case of longer power<br />
failures, both the pump and the recipient can be vented automatically if the RPMs decline<br />
below a minimum speed.<br />
The effects that play a role in evacuating vessels are described in Chapter 7. Dimensioning<br />
issues as well as calculation of pump-down times are also presented in that chapter.<br />
Evacuating load lock chambers<br />
Evacuating load-lock chambers definitely requires clean handling when transferring the workpieces<br />
to be treated in a vacuum process. If these items are channeled in from atmospheric<br />
pressure, the chamber should first be pre-evacuated via a bypass line. The running turbopump<br />
is then connected between the backing pump and the chamber via valves.<br />
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