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Re: Creating vacuum

Posted By: SVP Neon Equipment
Date: Sunday, 29 July 2007, at 9:40 p.m.

In Response To: Creating vacuum (Jan)


> Will adding a foreline trap (molecular
> sieve) to a refrigeration service vacuum
> pump (rated at 25 mikron) drop the pressure
> enough for neon service?

> Jan

In a word, no. The rated ultimate vacuum of a pump is never realized in the real world. As for Dennis' vacuum gauge reading, I would question the accuracy and calibration of the gauge used at the very least. Getting 4 microns from a refrigeration pump simply by adding a foreline trap is, well, hard to believe IMO.

Regarding the reference links cited, the problem with this sort of data is that it does not pertain to processing luminous tubing, e.g., heating part of the vacuum chamber (the neon unit, not the manifold) to a high temperature and trying to evacuate it to a high vacuum very quickly (less than 1 minute). Then, exposing the chamber to ATM and starting over again. This process goes completely against "normal" high vacuum practice. Therefore, normal high vacuum practice often does not apply to processing luminous tubing using the internal bombarding method.

For example, in the "Introduction" of the first link it states, "Baking can speed things up a lot but it is a real complication and may not be compatible with some set ups." But that is exactly what we are doing when we bombard a unit. We are baking it out, but only the unit and not the manifold system, and by using rapid internal heating as opposed to slow external heating. It goes on to say, "even if we can get rid of the water, the oil vapor is available in essentially an unlimited quantity and will continue to keep us from getting a low ultimate pressure." This is only true to a point where neon processing systems are concerned.

Do oil sealed mechanical vacuum pumps backstream oil vapor? The answer is yes. But the question becomes how much and how far - the same questions that concern diffusion pumps when they are down-played by turbo pump aficionados.

In HV and UHV set ups the mechanical pump (or diffusion pump) is usually only a few inches away from the vacuum chamber. In this case backstreaming is a real concern because it does not have to go very far to reach the chamber. In a neon processing system the mechanical pump is usually several feet away from the unit being processed. In this case oil vapor backstreaming is rarely, if ever, a problem.

If glass tubing is used for the connections between pumps (mechanical & diffusion) and manifold it can visually be seen how far and roughly how much any backstreaming is occurring simply by looking at the cleanliness of the glass. A "fog" inside the glass extending a few inches away from the pump indicates minor backstreaming while large droplets of oil on the inside of the glass extending several inches away from the pump indicates more problematic backstreaming.

A foreline vapor trap will reduce or prevent backstreaming from the mechanical pump, but will also reduce pumping speed and throughput (regardless of which type is used). In the case of a neon system where pumping speed is paramount for fast evacuation after bombarding is completed and before the unit cools off, a foreline trap may not be desirable as it may actually limit the ultimate vacuum obtained within the desired time frame.

When everything is said and done, in my opinion the most effective and still the least expensive method of consistently obtaining a fast, high vacuum in a system that is repeatedly cycled to ATM is the simple single stage glass diffusion pump.

Mark


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