The gas would be air until some surfaces oxidize during the pumping/baking
process. The nice thing about carbon coated elements (particularly the
filament) is that reaction oxygen produces either CO or CO2 that would be
pumped out. I guess the same would be true for nitrogen (NO and NO2) if the
elements were hot enough. But aside from that, a million molecules is not
much to worry about.
If every one of the million were ionized by the removal of one electron
then the available charge would only be 1.6E-13 Coulombs.
And most of the molecules are not where they will be ionized. The ones
that will be ionized will be between the elements. They would be ionized
by the massive flow of high energy electrons from the cathode. But
there just are not that many atoms to amount to much.
In fact that is how they measure such small vacuums. They use an
ionization gauge tube. It is very much like a triode but the collector (no
longer an anode) is negatively biased. The electron current flows between
the directly heated cathode and grid and most of the electrons go into the
space between the grid and collector but return to the grid since the
collector is negative. The molecules of gas in that region become ionized
and their electrons go to the grid and the ions flow to the collector
producing nanoamps of current which is measured by a sensitive meter with a
scale calibrated for nitrogen. The collector is usually just a wire rather
than a cylinder.
73
Bill wa4lav
At 08:55 AM 2/8/2006 -0500, Tony King - W4ZT wrote:
>Ian White GM3SEK wrote:
> > GGLL wrote:
> >>>> Does it still has nothing?. So it must be equal regardless inside
> >>>> volume.
> >>>>
> >>>
> >>> Nothing?
> >>>
> >>> Tubes are typically pumped down to about 1E-8mmHg, which means that the
> >>> "vacuum" space of an amateur-sized transmitting tube will contain well
> >>> over a million free gas molecules.
> >> Well, I ignored that and simplified things also; then, after
> >> "vacuum
> >> process", and to "kill" those over a million molecules tube's
> >> manufacturer use the getter method?.
> >>
> >
> > I don't have any specific information (so this message is also being
> > blind-copied to a couple of people who might know better) but my
> > impression is that the getter in transmitting tubes is mostly intended
> > to *maintain* the quality of the vacuum, not to improve it.
> >
> > That wouldn't be true of small glass tubes that use a barium getter -
> > the silvery-looking film that has been evaporated onto the surface of
> > the glass. In those tubes the getter film is not created until *after*
> > the tube has been pumped and sealed. In that case there would be a big
> > improvement in the vacuum when the film is thrown onto the glass (it's a
> > standard cleanup technique in high-vacuum lab work). But that is all
> > part of the manufacturing process; afterwards, the getter would tend to
> > *maintain* the vacuum in its post-manufactured state.
> >
> > Transmitting tubes are different because the getter is already in place
> > when the tubes are being pumped down, flash-heated way above the
> > operating temperature to remove as much gas as possible, pumped some
> > more and finally sealed. In that case, the getter surface would be
> > pretty close to a chemical equilibrium with the residual gas pressure
> > *before* the tube is sealed, so afterwards it would tend to *maintain*
> > that condition.
> >
> > We're also aware from a few weeks ago that the getters used in
> > transmitting tubes need to be hot in order to work at optimum
> > effectiveness, so the vacuum can deteriorate when the tube is left cold
> > for long periods.
> >
> > However, the basic principle is that electron tubes are quite happy to
> > share the space with quite large numbers of gas molecules. That is
> > because the electron current is usually much larger than the small
> > reverse ion current... but if it isn't, things can go badly wrong.
> >
> >
>
>So the question that remains for me is: What kind of gas molecules
>remain? Surely they are not an easily ionized gas like helium, neon or
>argon. Surely not oxygen (big no no); nitrogen? I would think that the
>atmosphere used in the manufacturing process just prior to evacuation
>would be kept as free of the big four as possible so that any remaining
>molecules wouldn't be one of them. Does the "getter" material only
>collect oxygen molecules or does it collect others as well? Does anyone
>have more information on this?
>
>Thanks and 73, Tony W4ZT
>
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