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Re: NEON- Need pumping advice...

The 'fore' and 'aft' systems are both attached in parallel to the 
manifold.  The 'fore' is  a mechanical pump. Th 'aft' system has a 
diff pump betwen the manifold and the mechanical pump.

The roughing or fore pump does just as it's name suggests. The 
initial pumping down gets a heck of a lot of 'stuff' - particularly 
water vapor out of the tube. This and the fact that you can test for 
leaks without exposing the aft pump to the open air or the initial 
'crud' in the tube is the main obvious advantage.  The underlying 
advantage is that your aft system ends up with a faster recovery 
time.  

The very general technique is to rough pump and test for leaks, then 
reintroduce air, and proceed with the aft system.  Others go directly from 
fore to aft not reintroducing air - depends on your experience, the 
system, and the project.

The aft pump should not be confused with a backing pump which is 
attached behind a high speed molecular pump to provide a low pressure 
atmosphere for the molecular pump's exhaust.

Overheard and 'kibbutzed' a discussion at AC between Rusty (EGL's 
neon demonstrator) and Dave A. sort of debating whether a vacuum 
system works by exerting a force that continually 'drags' molecules out of 
the tube and manifold, or if the system (both diffusion and mechanical) is 
merely a one-way valve which prevents molecules from returning to the 
tube.

It's an interesting chicken/egg sort of thing.  The mechanical pump 
can be seen as a valve because the oil seal between the rotor and 
housing prevents trapped gases from backflowing.  At the same time 
the 'gulp' of gas by the rotor changes the pressure differential and 
therefore you can say that the pump is causing a current to flow. 

Longtime neon-l members (neon-el-ders?) will remember an old battle over 
processing methods where we went into much vacuum theory in depth - 
particularly good explanations of  laminar and molecular flow.  
The discussions are very interesting if you can filter out the 
content from the flames. That would be in the first archive (1995).

Kenny



> 
>          In my search for a new employer I am sure to come across
>          manifolds
> using a 'roughing pump', D-pump and a final vacuum pump.  As I am
> only familiar with a single pump, can anyone give me some insight on
> how to pump with one of these systems?  
> 
>            I've got a pretty good idea how to make poor equipment
>            work, but
> am lost when it comes to a good system.  Thanks in advance.
> 
> Best Wishes, 
> 
> Sean
> 
Kenny Greenberg --  Neon - Scenic and Environmental Art  
                    Internet Site Consultant and Author   
KRYPTON NEON 34-43 Vernon Blvd Long Island City, NY 11106
         Phone: 718-728-4450  Fax: 718-728-7206  
http://www.neonshop.com  -  The Internet's Neon Shop
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