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Re: "Diffusion" pumps

Here my .02 on dif pumps...

Like Tom said, if everything is really optimal, and your mechanical pump 
is in great shape with clean oil, you can meet the minimum requirements 
for producing good tubes.  But there is very little margin here and a lot 
of variables just waiting to work against you.

There has been little bona fide research on the use of secondary vacuum
pumps (like diffusion pumps, aspirators, turbomolecular pumps, cryo pumps,
etc, etc) verses the "quality" of neon produced.  But my observation is to
this date unchallenged:  shops that use secondary pumps and maintain them
properly have few problems with all the other stuff people complain about. 
Snaking months after a tube is installed, blackening at the bends, dim
output after just a few months, staining of all sorts near the trodes &
elsewhere.  On the other side of the coin, I have seen people without 
secondary pumps using top-grade equipment with all sorts of elaborate 
"backstreaming filters" and such.  They seem to offer little, as these 
shops still have tubes comming back with the above problems after a few 
months in service.  The trend right now seems to be blaming tubing 
maufacturers for not "coating" properly. 

Consider too that diffusion pumps have a long, proven history in both the
neon industry and other hi-vac industries like television tubes, vac
tubes, and semiconductor fab. 

I think a 10 l/s glass unit is a good first dif pump.  You get to learn by
seeing how it works and how the operation if affected by the heater and
the vacuum.  Metal units are no doubt more efficient, and can be
manufactured to close tollerances.  There are 100 liter/second metal dif
pumps that use only 50ml of fluid.  They also have built in temp. 
regulation so in some sense they are more plug-n-playable. 

I have a glass 10 liter/sec unit.  It has served my needs extremely well,
and has required little in the way of maintenance. It does need attention
while operating however, and there is certainly a proper proceedure to
follow when turning it on and shutting it down.  You can ruin that 
expensive fluid if you're careless.

I guess I haven't really answered the question as to whether they are 
necessary.  I'd like to hear others' views here.

       -John Anderson