High vacuum pumps diffusion pump

Vacuum pump, diffusion pump (DP) A compression-type vacuum pump that operates by the collision of heavy vapor molecules with the gas molecules to be pumped, giving the gas molecules a preferential velocity toward the high pressure stages of the pump. Also called a Diff pump or Vapor jet pump. 

Figure 13.3 shows a typical pumping station. It includes a mechanical pump, a high-vacuum oil diffusion pump, a manifold, and the associated gauges and valves. Operation of most commercially purchased vacuum systems is now at least partially automated, but older or specialized systems are operated by manual valves. There are... 

Fig. 3.20 Schematic diagram of a high vacuum pump system with optionai operation of a Roots pump or a diffusion pump...

The exhaust from a rotary pump, especially if it is being run in the ballast mode, i.e. pumping a fair quantity of air or other gas, is an aerosol of oil in the gases from the line. A variety of filters is now available commercially for cleaning exhaust gas, but a good additional safety precaution is a wide tube fitted to the outlet so that the gas stream can be vented to the nearest fume-cupboard or window. The pumping efficiency of a rotary pump drops off rapidly below ca. 10 Torr even under optimum conditions, and such pumps are therefore usually installed as a back-up to a more efficient high vacuum pump, such as a diffusion or a turbomolecular pump. 

Diffusion oils for high-vacuum pumps (ultimate vacuum up to 13.3 pPa) are the most valuable and high-quality materials based on oli-gomethylphenylsiloxanes. Their vacuum properties cannot be rivalled by any other classes of chemical compounds. First and foremost, one should take into account that high-vacuum liquids for diffusion pumps should be resistant to oxidation at high temperatures. 

The foreline is the section of the vacuum system between the high-vacuum pump (i.e., diffusion pump) and the fore pump (i.e., mechanical pump). 

Efficient high-vacuum pumps generally do not operate well near atmospheric pressure. Thus the vacuum system must have a mechanical vacuum pump to evacuate the system to a pressure where the high-vacuum pumps are effective. Mechanical pumps require routine maintenance, such as ballasting and replacing the pump oil. The diffusion pump is the least expensive and most reliable high-vacuum pump. Turbomolecular pumps and cryopumps are also used on mass analyzers. The high-vacuum pumps also require... 

In current work the TA Instruments thermal analyst 220 was interfaced [46] to the Hewlett-Packard 5972 series mass selective detector (Figure 15) equipped with a hyperbolic quadrupole mass filter and vapor diffusion high-vacuum pump used in conjunction with a LaserJet 4 Plus printer. The TG analyzer s effluent tube was modified to terminate in a straight 1/4 in. OD glass tube. A 1/4 to 1/6 in. tube reducing union... 

From the operational point of view, reliable vacuum systems are a prerequisite for mass spectral measurements. In most cases, manufacturers apply differential stage pumping to achieve the required pressure range(s). Rotary pumps are used to provide an initial vacuum of approximately 10 to 10 Torr. High-vacuum pumps such as diffusion pumps (10 to 10 Torr), turbomolecular pumps (10 to 10 Torr), and cryopumps (10 to 10 Torr) are used to reduce pressure ftorther. Adequate knowledge in vacuum technology is essential in instrument design however, this is also beyond the scope of this chapter. 

Perfluoroalkyl ether greases thickened with polytetrafluoroethylene (MIL-G-38220 and MIL-G-27617) are used from —40 to 200°C in missiles, aircraft, and appHcations where fuel, oil, and Hquid oxygen resistance is needed (55). Polyphenyl ether greases find special use from 10 to 315°C in high vacuum diffusion pumps and for radiation resistance. 

If the pump is a filter pump off a high-pressure water supply, its performance will be limited by the temperature of the water because the vapour pressure of water at 10°, 15°, 20° and 25° is 9.2, 12.8, 17.5 and 23.8 mm Hg respectively. The pressure can be measured with an ordinary manometer. For vacuums in the range lO" mm Hg to 10 mm Hg, rotary mechanical pumps (oil pumps) are used and the pressure can be measured with a Vacustat McLeod type gauge. If still higher vacuums are required, for example for high vacuum sublimations, a mercury diffusion pump is suitable. Such a pump can provide a vacuum up to 10" mm Hg. For better efficiencies, the pump can be backed up by a mechanical pump. In all cases, the mercury pump is connected to the distillation apparatus through several traps to remove mercury vapours. These traps may operate by chemical action, for example the use of sodium hydroxide pellets to react with acids, or by condensation, in which case empty tubes cooled in solid carbon dioxide-ethanol or liquid nitrogen (contained in wide-mouthed Dewar flasks) are used. 

The ejector is widely used as a vacuum pump, where it is staged when required to achieve deeper vacuum levels. If the motive fluid pressure is sufficiently high, the ejector can compress gas to a slightly positive pressure. Ejectors are used both as subsonic and supersonic devices. The design must incorporate the appropriate nozzle and diffuser compatible with the gas velocity. The ejector is one of the ( to liquid carryover in the suction gas. 

Two vacuum systems are used to provide both the high vacuum needed for the mass spectrometer and the differential pumping required for the interface region. Rotary pumps are used for the interface region. The high vacuum is obtained using diffusion pumps, cryogenic pumps, or turbo pumps. 

In a diffusion pump, the dense oil vapour produced by the boiler (see Fig. 1.12) is ejected into the vacuum at high (or even supersonic) speed through the nozzles. 

The vacuum chambers were pumped down by means of an oil diffusion pump backed by a rotary vane vacuum pump. The base pressure achieved was 1 x 10 5 Torr (1.33 x 10 Pa). High-purity argon gas was bled into the chamber, the high-vacuum valve throttled, and the chamber pressure maintained as close as possible to 2 x 10 2 Torr (2.66Pa). For some of the experiments, the dc self-bias on the magnetron electrode was also measured. 

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