The starting pressure for a diffusion pump is typically atmospheric pressure, which is about 760 Torr (101 kPa). This is the initial pressure that the pump begins to operate against to reduce the pressure in a chamber to achieve high vacuum levels.
Explanation:
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Initial Pressure and Pumpdown Sequence: In a typical setup, the process of achieving a high vacuum begins at atmospheric pressure. A positive displacement pump is initially used to reduce the pressure from atmospheric levels (760 Torr, 101 kPa) down to about 25 Torr (3 kPa). This step is crucial as it removes the bulk of the gas from the chamber.
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Use of Diffusion Pumps: After the initial reduction in pressure, more specialized pumps like diffusion pumps are employed. Diffusion pumps are designed to achieve very low pressures, typically in the range of 10-2 to 10-6 Torr. They are particularly effective after other pumps have reduced the pressure to a manageable level where diffusion pumps can take over and further refine the vacuum conditions.
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Operational Requirements: Diffusion pumps do not operate independently; they require a backing pump (often a mechanical pump) to maintain a lower pressure at the inlet of the diffusion pump. This setup ensures that the diffusion pump can effectively reduce the pressure to the desired high vacuum levels without being overwhelmed by the initial high pressures.
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System Configuration: Depending on the specific requirements of the vacuum system, different configurations of pumps can be used. For instance, a two-stage system might start at atmospheric pressure and use a combination of pumps to reach a pressure of 4.5 x 10-2 Torr, whereas a three-stage system with a diffusion pump might aim for even lower pressures, such as 8.0 x 10-6 Torr.
In summary, the starting pressure for a diffusion pump in a vacuum system is atmospheric pressure, from which the system is progressively evacuated to achieve the desired high vacuum conditions. The sequence and type of pumps used depend on the specific vacuum requirements and the nature of the materials being handled.
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