Talk:Diffusion pump

I seem to recall that cold traps enhance diffusion pump ability, not detract from it. Baffles probably do interfer though. -rmhermen

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To be honest, my experience indicates that pumping ability is improved when the cold trap is in use. However, the book I used as a reference for writing this entry, says that cold traps interfere with pumping ability. My guess is that the additional inlet tube length required for the cold trap reduces conductance and thus pumping speed, so pumping speed may be higher without any cold trap. However, if a cold trap is already installed, using it reduces flow of oil into the chamber, resulting in an improvement relative to not using the already installed cold trap. Admittedly, I'm not an expert, so I'd welcome input from anyone with more knowledge on the subject.

--Matt Stoker

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I think the trouble is that diff pumps are really bad at pumping water, which is mostly what you're pumping when you pump from atmosphere. Thus, employing the cold trap seems to improve pumping speed, since it pumps water very well.

--Brian Perkins

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I have a few years of experience with a variety of pumps and traps, and I've found that cold traps can sometimes improve and sometimes hinder pumping efficiency. Here's why:

1) Cold traps place baffles between the pump and chamber, and this reduces conductance to the pump. Use of undersize cold traps can also create a throat that is smaller than the pump, also reducing efficiency. Cold traps are usually not long enough for the length to have an impact. The problem is that in a high vacuum environment, gas molecules have to diffuse into the pump; they're not pushed into it by a pressure gradient. So the molecules have to accidentally find the hole or path leading to the pump in the course of their random walk around the chamber. Smaller holes and larger baffles hinder pumping.

2) Cold traps cryopump the chamber. You wouldn't know that from reading the wikipedia article on cryopumps, but it's true. Vapor molecules, especially water and other outgassing products, will condense out of your chamber and onto the cold trap. In a general-purpose chamber that has not been baked out, your cold trap will typically reduce your pressure by a factor of 100, until it saturates after about 8 hours of operation. In a tighter chamber with bakeout, the cold trap willl not make as much of an improvement, (because the vacuum is already improved to begin with,) but it won't saturate as fast.

In summary, a cold trap acts as a poor man's cryopump in cheap systems, but act as a hairball in ultra-clean systems. Check out the vacuum page for some more discussion.

--Yannick 04:22, 10 Jun 2005 (UTC)

Use of Cold Traps in the process of Vacuum Metallizing

Has anybody had any gain in the use of these pumps in Vacuum Metallizing?? —The preceding unsigned comment was added by 196.209.22.59 (talk) 17:59, 11 December 2006 (UTC).

In my experience typical systems made for metallizing under vacuum use a diff pump. D6stringer 22:38, 9 July 2007 (UTC)

Contradiction

This article currently says that diffusion pumps cannot discharge directly to atmosphere, and also says that a steam ejector is a diffusion pump. These cannot both be right as steam ejectors can and do operate directly to atmosphere.

I would guess a diffusion pump is to an ejector as a turbomolecular pump is to a fan-superficially similar but designed to work at very low pressures where gases behave as free molecules rather than as a continuum.--QuantumEngineer 17:04, 30 June 2007 (UTC)

These are two different application of diffusion pumps. An injector can exhaust to atmosphere, but they can produce a very poor vaccum. If you use them as high-vacuum pumps, they need forepumping. It should be mentioned that mercury diffusion pumps were used for decades. Generally they are somewhat more efficient than oil diffusion pumps, but due to environmental problems they are very rarely used nowadays. (Valdez from Hungary 9 Oct 2007)