1.3.4 Bake-out
The following prerequisites must be satisfied in order to achieve lower pressures (< 10-8 mbar):
- The base pressure of the vacuum pump should be a factor of 10 lower than the required ultimate pressure
- Stainless steel vacuum recipients and components must be used
- Metallic seals (CF flange connections) are required
- PPump and equipment must be baked out
- Leaks must be avoided and eliminated prior to activating the heater
(use helium leak detectors!) - Clean work is a must, i.e. all parts must be thoroughly cleaned and must be installed with grease-free gloves
Bake-out significantly increases desorption and diffusion rates, and this produces significantly shorter pumping times. Bake-out temperatures of up to 300 °C are used. The instructions of the pump manufacturers relating to maximum bake-out temperatures and maximum permissible radiation levels in the pump flange must be observed.
Following installation the equipment is switched on, and after reaching a pressure of p < 10-5 mbar the heater is then switched on. During the heating process, all gauge heads must be operated and degassed at intervals of 10 hours. In the case of stainless steel vessels and the use of metallic seals, bake-out temperatures of 120 °C and heating times of approximately 48 hours are sufficient for advancing into the pressure range of 10-10 mbar. Bake-out should be continued until 100 times the expected ultimate pressure is attained. The heaters for the pump and vacuum chamber are then switched off. After cool-down, the desired ultimate pressure will probably be achieved. In connection with pressures p < 5 · 10-10 mbar and large interior surface areas, it will be advantageous to use a titanium sublimation pump that pumps the hydrogen escaping from the metals at a high volume flow rate.
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Pfeiffer Vacuum GmbH
Berliner Strasse 43
35614 Asslar
Germany
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Pfeiffer Vacuum GmbH
Berliner Strasse 43
35614 Asslar
Germany
T +49 6441 802-0
F +49 6441 802-1202