[Prev][Next][Index][Thread]

"Diffusion" pumps

Don,

 Let me start off by explaining what we are talking about.  A diffusion pump
is similar to the pump most often used for neon work called an aspirator. The
main difference is that a diffusion pump has very tiny jets that can produce
gas speeds close to the speed of sound and an aspirator pump usually has a
larger venturi type system.  The glass ones sold by neon suppliers are usually
aspirator pumps. Metal ones are usually true diffusion pumps although it is
possible to make each out of the other material. Confused yet?  Anyway, it is
a matter of semantics and practicality. It really comes down to what type of
fluid you are using in the pump, and a balance between the pump and the system
it is attached to at both ends.  You can also adjust the speed of the pumping
of the aspirator via a control of the heating element to achieve this balance
as well.

 Without an aspirator,  you can only achieve so much vacuum.  With it you can
get a more pure tube before filling with the gas of choice.  While you can get
away with making a tube without one that will light and may give good service
it is what I call a matter of "headroom".  This is a term borrowed from my
audio experience.  Let's say you have a system without an aspirator. If it
pumps as good as it can ( this means that the fluid is new and clean, nothing
leaks in the manifold, and it is very clean ) then you can make good tubes. 
Because you can pump only to the level acceptable to make good tubes under
perfect conditions, if you have even a small problem with contamination you
will now make tubes that are not acceptable.  You can age them forever and let
the electrodes absorb the remainder of the impurities but the tube will be
borderline.  But if you have an aspirator and you can pump beyond what is
necessary, and you have a problem with something, then it will still be within
acceptable tolerance.  Just the way I like to do things.  In rigging we call
it a safety factor.  A steel cable that is rated to hold 2000 lbs. is only
good for vibrational loads ( for instance.. speaker cabinets ) of 200 lbs. 
That's a 10-1 safety factor and is what we have to work with for
rigging...legally.  You could take the cheapo approach and say that the book
says it will hold 2000lbs. and it might just do that.  I'm from Atlantic City.
I understand gambling. Do you want to gamble on your tubes and your reputation
or do you want something that is just adequate?  Would you stand under that
speaker in a rock concert?  That is what it comes down to.  IMHO and $.02
worth.


  E-mail from: Tom Biebel, 19-Nov-1995
 

!^NavFont01F0006MGHHOI2773