Hi Dalibor,
what do You think by the "insufficient greasing on my needle valve"?
The rubber o-ring used to connect the valve to the system was not
greased properly?
yes, there was some fibre or something hair-like that caused a small leak.
I think that the valve stem is not fully pressed to the valve seat
when set to zero, when You screw it further below zero, it gets full
contact with valve seat this making thight seal.
Yes, that is exactly what happened. I just assumed that the scale
position "0" implied that it was fully in its seat. It is connected
properly (the needle towards the vacuum, it is a rectangular configuration).
By the way, how long did You pump the system before closing and making
the test?
About 10 minutes. I did a second test today and pumped it for 20 minutes
and the result is much much better (half the leakage rate, Q = 2E-6 mbar
l/s).
I guess that this is as good as it is going to get with my two-stage
pump. On the long run I will purchase an oil diffusion pump, but not
now. As Dalibor explained later, this pressure range is fine for testing
and making Neon experiments.
What pressures are You going to achieve for pumping the nixies before
filling by gas?
7E-3 mbar is the best I have achieved so far.
What valve do You use for isolating the system from vacuum pump (is it
the ball valve we discussed recently)?
The ball valve is leaking, I proved that with another curve. No, I am
using a bellow valve that Ron got for me. He is great. I don't know what
I would do without his help.
Now I need to connect the pressure reducer next.
Jens
Dalibor
2013/1/26 jb-electronics <webmas...@jb-electronics.de
<mailto:webmas...@jb-electronics.de>>
Hi folks,
I am still working on Nixie tubes, but I have to take it a little
slow at the moment because I switched my university and I am
currently sort of living in two cities ;-)
However, today I had some time so I took a look at my vacuum
system. My problem was that even after I closed the valve to my
pump at a pressure of 1.5E-2 mbar, the pressure would rise up to
20mbar in 20 minutes. So there was a leak.
I then checked every part of my system (swagelok adapter, needle
valve) by replacing them with a blind flange and recording the
pressure after 10s, 20s, 30s, 40s, 50s, 60s, 120s, 300s, 600s,
1200s and plotted them. I realised that there was insufficient
greasing on my needle valve, and the most interesting thing is
that it is not airtight if the scale is adjusted to just zero. You
have to (gently) adjust it below the scale zero point. Then I get
a leakage rate of just 1.2 µbar/s which is very low.
My system has a volume of at most (!) 65ml, so the leakage rate
now is Q = 5.4E-6 mbar l/s which is pretty good I think. See my
results here:
http://www.jb-electronics.de/tmp/leakage.png
http://www.jb-electronics.de/tmp/leakage_extensive.png
Note that the Q value in these diagrams needs to be multiplied
with 65ml.
The next step will be to find a good connection to the pressure
reducer for my gas bottle. Right now I am using a PVC hose
(terrible), and I have huge leakage rates.
But as always: One thing at a time ;-)
Jens
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