Editing Diffusion pump (section)

==Oil diffusion pumps==

An oil diffusion pump is used to achieve higher vacuum (lower pressure) than is possible by use of [[Positive displacement vacuum pump|positive displacement]] pumps alone. Although its use has been mainly associated within the high-vacuum range, down to
{{convert|1e-9|mbar|Pa|sigfig=1|abbr=on|lk=on}}, diffusion pumps today can produce pressures approaching
{{convert|1e-10|mbar|Pa|sigfig=1|abbr=on}} when properly used with modern fluids and accessories. The features that make the diffusion pump attractive for high and ultra-high vacuum use are its high pumping speed for all gases and low cost per unit pumping speed when compared with other types of pump used in the same vacuum range. Diffusion pumps cannot discharge directly into the atmosphere, so a mechanical forepump is typically used to maintain an outlet pressure around
{{convert|0.1|mbar|Pa|sigfig=1|abbr=on}}.

[[Image:Calutron diffusion pumps.jpg|left|thumb|250 px|Diffusion pumps used on the [[Calutron]] [[mass spectrometer]]s during the [[Manhattan Project]], visible as black cylinders in the upper half of the image]]

[[Image:Diffusion pump schematic.svg|thumb|right|250 px|Diagram of an oil diffusion pump]]

The oil diffusion pump is operated with an oil of low [[vapor pressure]]. The high speed jet is generated by boiling the fluid and directing the vapor through a jet assembly. Note that the oil is gaseous when entering the nozzles. Within the nozzles, the flow changes from [[laminar flow|laminar]] to [[supersonic]] and [[free molecular flow|molecular]]. Often, several jets are used in series to enhance the pumping action. The outside of the diffusion pump is cooled using either air flow, water lines or a water-filled jacket. As the vapor jet hits the outer cooled shell of the diffusion pump, the working fluid condenses and is recovered and directed back to the boiler. The pumped gases continue flowing to the base of the pump at increased pressure, flowing out through the diffusion pump outlet, where they are compressed to ambient pressure by the secondary mechanical forepump and exhausted.

Unlike [[turbomolecular pump]]s and [[cryopump]]s, diffusion pumps have no moving parts and as a result are quite durable and reliable. They can function over pressure ranges of
{{convert|1e-10|to|1e-2|mbar|Pa|sigfig=1|abbr=on}}. They are driven only by [[convection]] and thus have a very low energy efficiency.

One major disadvantage of diffusion pumps is the tendency to backstream oil into the vacuum chamber. This oil can contaminate surfaces inside the chamber or upon contact with hot filaments or electrical discharges may result in carbonaceous or siliceous deposits. Due to backstreaming, oil diffusion pumps are not suitable for use with highly sensitive analytical equipment or other applications which require an extremely clean vacuum environment, but mercury diffusion pumps may be in the case of ultra high vacuum chambers used for metal deposition. Often [[cold trap]]s and [[Baffle (heat transfer)|baffles]] are used to minimize backstreaming, although this results in some loss of pumping speed.

The oil of a diffusion pump cannot be exposed to the atmosphere when hot. If this occurs, the oil will oxidise and has to be replaced. If a fire occurs, the smoke and residue may contaminate other parts of the system.

===Oil types===
{{See also|Polyphenyl ether#Ultra-high-vacuum fluids}}

The least expensive diffusion pump oils are based on [[hydrocarbon]]s which have been purified by double-distillation. Compared with the other fluids, they have higher vapor pressure, so are usually limited to a pressure of
{{convert|1e-6|Torr|Pa|abbr=on|lk=in}}. They are also the most likely to burn or explode if exposed to oxidizers.

The most common [[silicone oil]]s used in diffusion pumps are [[trisiloxane]]s, which contain the chemical group Si-O-Si-O-Si, to which various [[phenyl group]]s or [[methyl group]]s are attached. These are available as the so-called 702 and 703 blends, which were formerly manufactured by [[Dow Corning]]. These can be further separated into 704 and 705 oils, which are made up of the isomers of  tetraphenyl tetramethyl trisiloxane and pentaphenyl trimethyl trisiloxane respectively.<ref>{{cite book | title=A User's Guide to Vacuum Technology | chapter=Pump Fluids | publisher=John Wiley & Sons, Inc. | location=Hoboken, NJ, USA | date=2004-12-07 | isbn=978-0-471-46716-8 | doi=10.1002/0471467162.ch13 | pages=229–246}}</ref>

For pumping reactive species, usually a [[polyphenyl ether]] based oil is used. These oils are the most chemical and heat resistant type of diffusion pump oil.