Back streaming

At low pressure, two main sources of gas are present <2w gas desorbing from surfaces and <2l gas leaking into the container from outside (imperfect tightness of gaskets, permeation, back-streaming from pumps, and so on). If Qt represents the sum of all these inputs, the change of pressure p with the time t is described by ... 

Another drawback of oil-sealed pumps is the back-streaming of oil vapour into the roughing line, which may occur at low pressure. Contamination by back-streaming oil can be drastically reduced by using proper traps like molecular sieve traps with zeolite (see Section 1.6.4). 

A system of cascaded booster pumps has the characteristics of a medium-size rotary vane pump but does not present the drawback of the back-streaming. For this reason, it finds application in cryogenics, for example, for the circulation of the He mixture in dilution refrigerators. 

Back Streaming. A diffusion of vaporized oil entering the vacuum chamber. 

Saturation vapor pressure can be caused by either back streaming or back migration. Although diffusion pump oils have low vapor pressures (10 5-10 8 torr), in use they break down into multiple vapor pressures. The resulting higher... 

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Mechanical Pumps. Perhaps the most common form of vacuum pump is a mechanical pump that operates with some sort of rotary action, with moving parts immersed in oil to seal them against back-streaming of exhaust as well as to provide lubrication. These pumps are used as forepumps for diffusion pumps. Other common laboratory applications are the evacuation of desiccators and transfer lines and distillation under reduced pressure. These pumps have ultimate pressures ranging from 10 to 0.05 Torr, and pumping speeds from 0.16 to 150 L s or more, depending on type and intended application. 

An oil diffusion pump can be used without a trap if it is sufficiently well provided with baffles to stop back-streaming molecules, for the oils used usually have room-temperature vapor pressures less than 10 Torr when pure (see Table 1). However, pump oils generally undergo some decomposition in service, and some decomposition products may find their way into the system. Liquid-nitrogen cold traps are usually added when back-streaming and oil creep along the walls of the system must be prevented. 

Essential diffraction apparatus consists of electron gun (98-101), crystal manipulator, and beam detection components. These are housed in a bakeable chamber, in which vacuums of 1 X 10 Torr or better are easily developed. The usual material is stainless steel, rather than glass as in earlier years. Sputter-ion pumps are more commonly used than diffusion pumps, because their pumping speeds are greater and back-streaming problems associated with hydrocarbon (or mercury) vapors are reduced, and of course they are much more convenient. Gases are introduced through bakeable leak valves, either directly from flasks or from a header where they can be mixed to a desired composition. 

Another problem is that back streaming of the lighter components of the pump oil can contaminate both the ion source and analyzer e.g., contamination in El sources results in the appearance of characteristic hydrocarbon fragments, while contamination of analyzers leads to loss of resolution and sensitivity. 

Vacuum is also required in the counting laboratory, and these fines should be supplied with cold traps to prevent pump oil from back-streaming and depositing on sensitive counting apparatus (e.g., surface-barrier detectors). 

The components to be removed were installed in the supervised area of the plant. Thus they actually had to be non-contaminated. It was known, however, that certain inner surfaces of pipes had been slightly contaminated due to back streaming gas during plant operation. 

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