On Tue, 5 Apr 2011 09:53:16 +1000, "Phil Allison"
wrote:
"Jeff Liebermann"
When we talk about a "gassy" vacuum tube, it's not a leaky glass
envelope or seal.
** The term covers that eventuality too.
Ok. My bad grammar. I meant that if the tube is deemed "gassy", it's
probably not an air leak in the envelope or seal. It's more likely
gaseous diffusion of helium or neon through the envelope.
Any tube that has even a tiny air leak will glow a pinkish purple inside
like a *******.
Yep. I've seen the pink glow. The only way I could tell the
difference between the nitrogen glow of an air leak:
http://en.wikipedia.org/wiki/Ionized_air_glow
and that of a helium diffusion through the glass leak:
http://en.wikipedia.org/wiki/Noble_gas#Discharge_color
was that the nitrogen glow would not last very long as the filament
would burn out due to oxidation.
The getter does best with reactive gases and does
nothing for eliminating noble gases, which will not react with metals
(or most anything else). It's those noble gases that remain (helium,
neon, argon, krypton, xenon, and radon) that you're seeing glow.
** Maybe - but how would any significant amount of Argon get inside the
tube UNLESS there was air leaking in ?
Remember, Argon makes up less than 1% of air and does not react with metal
parts.
Oops. Y'er correct. Only helium and neon will diffuse through glass.
Argon and larger gas molecules won't diffuse through glass.
The vacuum inside a tube is less than one millionth of atmospheric pressure.
Yeah, but the pressure differential is 14.7 lbs/sq-in. Over the
surface of the 6L6, that's about (assuming a cylinder):
Surface Area = 2 Pi r^2 + 2 pi r h
= (2 * 3.14 * 0.7^2) + (2 * 3.14 * 0.7 * 3.0)
= 16 sq-in
Surface pressure = 16 * 14.7 lbs/sq-in = 235 lbs.
(Yes, I'm guessing at the dimension for a 6L6 as I don't have one
handy). That's quite a bit of pressure pushing the helium and neon
atoms through the glass. Still, for helium, the diffusion rate is
slow (helium through pyrex at STP):
http://teaching.matdl.org/teachingarchives/browser/trunk/matml/transport/problems/hepyrex-solution.pdf?format=raw
8.0 x 10^-8 m^3/hr
Nitrogen mixed with a little CO2 makes a nice, white glow under low
pressure - its called a "Moore Tube"
http://en.wikipedia.org/wiki/Daniel_McFarlan_Moore
His untimely death was a bit shocking.
I guess his murderer didn't do a proper patent search before starting
work on his invention. We're more civilized these daze. Instead of
murder, we have litigation. It's much like murder in slow motion.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558