Hi again,
I made some tests with my vacuum system today: I opened the ball valve
and let the pump run for two hours straight. After about ten minutes,
the pressure arrived at 9E-3mbar and stayed there for the rest of the time.
I heated my flanges with a hot air fan for about five minutes, but the
pressure did not change at all. So I guess this means that there is not
so much residual material left on the inside?
During these two hours, I closed the ball valve and left the pump
running. The pressure increased to 0.1mbar in about 3s and to about
1mbar in 30s. After opening the ball valve, the pressure went back to
9E-3mbar in 5s. I repeated this experiment several times, always with
roughly the same results.
After the two hours I shut off the pump and closed the ball valve. The
pressure rose from 9E-3 mbar to 1mbar in 10s (really fast) and to 4mbar
in about 2 minutes. After 10 minutes I had 100mbar or more.
So there appears to be a leak, this cannot be mainly caused by
outgassing, can it?
Jens
Hello Dalibor,
my valve comes from Leybold, see here:
http://www.ld-didactic.de/index.php?id=ld-artikel&a=378776
With 240 EUR it was rather expensive, but it is the best valve I have
found. Also, I got my whole vacuum equipment for free from a friend of
mine here in Germany, so this is all I paid.
By the way, this is the ball valve:
http://www.ld-didactic.de/index.php?id=ld-artikel&a=378777&L=0
Jens
Hello Jens,
You are definitely right with dosing the argon, there must be around
0.5% of argon and with pressures about 30torr we are talking about
really really small amount of argon.. Where did You get the valve
from and how much does this cost? I am considering building one
myself, but I am afraid of wasting time and money on it and
eventually buying a real one :-D
Thanks!
Dalibor
Dne 30.10.2012 23:35, jb-electronics napsal(a):
Hello all,
a friend of mine will supply me with a suitable adapter for 9mm
glass tubing. Really amazing! I actually found some at ebay for $20
or so as well, the company Swagelok makes them (among others).
Thanks for drawing my attention to the ball valve. I checked, and as
it turns out it has been made for high vacuum applications, so I am
confused why it should be the source of the leak. Maybe there is no
real leak and it is just outgassing:
I need to get some vacuum grease and recheck the system after some
longer evacuation time, so thanks again for the hint, Dalibor and
Ron! Once I have re-greased the flanges I will leave the system
running for some time and then check the pressure.
Regarding the needle valve: I wanted a reliable valve that allows me
to adjust the neon partial pressure very accurately, because later
on I will also use this valve to let in argon (very low partial
pressure) and then fill up the rest with neon to create a penning
mixture. For this procedure I need an accurate valve. Granted, it
might have been possible to save a little money on this one, but I
am not the craftsman to build something like that by myself, so I
took the shortcut of purchasing a slightly overengineered valve. But
then again, I only have to buy it once.
There are many ways to a well-working manifold. I chose the way of
using KF16 flanges for all parts, while others might find it more
convenient to build components on their own. But I really want to
focus on tube-making and not on building a ton of equipment which
just takes too much time for me.
But then again, it is up to everybody's personal habits and abilities.
Jens
Hello!
You most likely have a serious leak in the ball valve. You might
get temporary seal in it by disassembling it and packing the seal
areas solid with vacuum grease. This would be temporary and subject
to begin leaking again at any time but could give you a few days of
leak free operation without changing out the valve to a better one.
You would do better to get a hardware store water valve with a
standard packing that you could remove and replace with a teflon
plug packing. I have done this and made valves that are tight to 10
-6 torr! Be sure to get a "plug" valve and not a gate valve. (a
plug valve rams a rubber plug onto a seal surface. A gate valve
slides a metal gate into a groove.)
You should not have any problems at all with the rubber hose in
those short sections so long as you use grease on the connections.
Permeability of the rubber is minimal at room temperature and would
take hours to affect the vacuum at the pressures you are using. How
long will the pressure stay constant when you turn off the power
and close the ball valve? It should remain below 100 microns for
several hours if you don't have a leak. Important: Note that to
outgas the system to do this test it will take about 4 hours of
continuous pumping regardless of leaks! (surprise?!) Yes, it takes
that long if you cannot heat the system a bit with a torch or heat
lamp! Most new users in vacuum are very surprised at how long it
actually takes to outgas a system that has been at atmospheric for
any length of time! You may not have a leak, just normal outgassing!
Just out of curiosity, why do you need such a close control of the
neon into the tube? For nixies it surely isn't necessary. What are
you going to make?????
glasslinger
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