Vacuum Check

11 02 2012

All photos.

I have Swiss TV journalist Yves Gerber in the lab today. I will try the electron gun again while he is here.

Previously in the comments, Olivier suggested the vacuum pressure is too high. Indeed, at 1.66 millitorr the pressure was higher than I wanted.

My first goal today is to check the empty vacuum chamber with blank flanges. A best case scenario.

With the initial pump down I only got down to ~3 millitorr…  about the same as last time. I used the stethoscope to listen for a leak but did not hear one.

I tried tightening the flanges one last time, and suddenly the pressure started dropping again.  I forget how much torque these conflat flanges need to fully seal.

Now I am seeing pressure in the range of 0.098 millitorr and dropping. Much better!

So now let’s install the electron gun, and see what we get.

UPDATE:

With the electron gun components in the chamber I am able to get down to 0.27 millitorr… not bad!

But when I turn on the electron gun… still no beam.

Exhausting!

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11 responses

13 02 2012
Glenn

Good stuff. Thanks for the update!

Might be useful to loosen up one of those nuts and use the stethoscope to listen for the related leak when you know roughly where it should be, just to practice.

13 02 2012
Christopher Hotchkiss

I’m glad you’re able to post updates on this again, however your photo link doesn’t seemto be working.

14 02 2012
Chris

Keep on trying though man, we are all pulling for you! And yeah, thanks for the long awaited update lol, can’t tell ya how many times I’ve stopped by and gone “Awww man…” *sadface* lol

15 02 2012
theorbtwo

Possibly you should get a torque wrench, so you can reliably tighten the bolts to a known torque? That does give you another variable to find the correct value for, as were, but it will at least move it from the “unknown unknown” to the “known unknown” category.

22 02 2012
Remy Dyer

Conflats use a copper gasket – technically you’re supposed to use both a required torque as well as replace the gaskets every time the flange is reinstalled.

(this is very expensive)

The sealing method basically depends on the copper deforming as the edge cuts into it, creating a seal that’s basically perfect. This is also used in some very high pressure seals.

I suggest use of a torque wrench, and some experiments to determine how much torque you need to get good vacuum on the first, second … n times that gasket is used.

You’ll probably find you need to do it up a little tighter each time, as the copper deforms and exposes more surface area for the edge to contact.

At least this way you’ll be able to get the maximum reuses out of your copper gaskets.

I’d also be tempted to try with Al-foil or Al-sheet gaskets as well – it’s sometimes used in place of copper (which has been in use for this kind of deforming seal for a very long time).

Might be worthwhile making up a set of dies to cut your own gaskets – it’ll allow you to make your own gaskets and not pay the “research tax” these lab parts otherwise seem to attract.

Best of luck

8 03 2012
Mark Suppes

The problem I’m facing is one of my conflats has a ding! So it takes much more torque to seal than the same conflat would by specification.

22 02 2012
Remy Dyer

Oh, another thought about why the e-gun isn’t working. When you broke it out of it’s envelope, you would have removed the high tension positive “attraction ring”, which is formed by the metal coating about the tube.

This metal coating acts as an electrode to attract the electrons towards it’s centre, and as it’s wrapped around the tube, they fly through the middle.

The vacuum chamber won’t work in this manner – as it forms a faraday cage when seen from the inside. If the cathode is at large negative volts with respect to the vacuum chamber, the electrons will just take the shortest path directly to it, not forming a beam at all.

Point is, to throw the beam across the chamber, you’ll need a ring shape in front of the gun, at a high positive voltage with respect to both the gun and the vacuum chamber, in order to attract the electrons towards it.

You don’t necessarily need much voltage between the focussing ring and the cathode – it’s really intended just to change the width of the beam, acting as an electrostatic lens – hence it will need to be negative.

You also don’t necessarily need much negative voltage on the gun itself – probably enough to overcome the work function of the cathode surface.

When you run a CRT externally, you can get away with wiring the gun to high negative, and the ht connection around the screen to ground, and it’ll work, because the screen is still the closest “positive” to the gun. But in the vacuum chamber this won’t work until the gun is pointed at the nearest wall of the chamber.

As always, best of luck, mate.

23 02 2012
Remy Dyer

Another couple of thoughts about sealing,

These guys reckon Glyptal is good, so long as you bake it.
http://www.coultersmithing.com/OldStuff/piranigage.html

Also, that metal gasket really just needs to be soft metal without any pores. Could try moulding your own out of solder?

I’m not certain that solder would be all that good – may have too much dissolved gasses in it, but it’s certainly soft, cheap, and reusable as well.

Best of luck.

7 03 2012
Steven Hosemans AKA Dustin.

To compound your problems, in one of the photo’s it looks like the filament is running in air. The filament is coated in carbonate compounds that increase the electron emission. These are poisoned if the filament is run in air and electron emission can drop 10-30 times once poisoned.
Why not try a magnetron filament?
Steve.

8 03 2012
Mark Suppes

I never meant to run it in air. But I may have accidentally. Something to rule out.

15 04 2012
Remy Dyer

Found a link on home-made crt’s from couliersmithing’s forums:
http://www.sparkbangbuzz.com/crt/crt6.htm

Perhaps you’d be better off making your own E-gun?

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