24" steel vacuum chamber progress

Hi Jim , Well I am guessing this is not a high Vac system . If you were to be able to do a pump down with a tester that has gages on you can introduce helium along the weld and when the needle goes up then that is where the leak is ,another idea is to pressurize it and use a water soap mix and it will produce bubbles as well .I have built many many of high Vac chambers when I was working at Cornell University with the Chemistry Deopt .We built them all out of either SS Steel or Al .we were dealing with the minus figures and it was all heliarc welding of which I have lots of experince with ( 20 years ) till my retirment 7 years ago .we delt with the figures of 10 to the minus 4 - 8 range .all there stuff was done in high Vac systems . Project sounds interesting . One thing about using the oil or the likes of is the contaminion of the area and if it is to be welded it will not be clean and will create trouble ,so try and keep the trouble area cvlean so you can fix it .

Good luck , Bernie

HI Bernie,

Well not yet :)

Honestly, this post was a little premature, I was just rather excited that the chamber door closed AND was able to seal against my "custom fabricated" (aka Home-Made) O-Rings. (We really mis-treated the door, and had to do a lot of heat shrinking to get it back into some semblance of "flat")

While I tried sharing my excitement with SHMBO and the neighbor, neither really "Got it" :)

I probably should have held back on posting till I had some semblance of a real vacuum, or at least fixed the three gross leaks I found last night.

I did something similar when I built my 12" chamber a few years ago, but instead of Hydrogen (I didn't have any handy) I used Automotive Carburetor Spray cleaner (or spray brake cleaner, which ever was closest at the time). While this worked, I imagine hydrogen would work even better, especially on the smaller leaks. The 12" chamber still has some leaks as it won't hold the 2x10-5 torr for very long once isolated from the pumps.

Helium should work almost as well as hydrogen, yes? If so I can get that fairly easily...

Unfortunately my o-ring implementation won't work with positive pressure inside the chamber. This is attributed to my inability to machine proper o-ring groves, and instead make do with the metal working tools available (OA torch, Welder, Grinder)

How's that saying go? "When you only tool is a hammer, everything starts to look like a nail!" :)

This chamber will only be my second, the first being fabricated with only a grinder and an AO torch. While the first one works well enough (typically 2x10-5 torr range) I'm finding its just not big enough.

Take Care, James Lerch

(My telescope construction, Testing, and Coating site)

OK, here is the poor-man's leak checker:

1) if you can get down into the thermocouple range - below 500 microns or so - then simply spritz a bit of acohol or acetone on the suspected leak area. Depending on teh size of the leak, one of two things will happen: for large leaks, the solvent will be sucked into the chamber and the thermocouple gage will show a pressure increase. If the leak is smaller, the solvent will freeze as it enters through the leak, and plug it up. Then the pressure will spike downwards. 2) More sensitive technique: Put a piece of glass tubing in the rough pump line, in series with your rubber hose. Beg borrow or steal a high voltage sparker coil, like a ford coil or a tesla coil. Wind a turn or two of wire on the outside of the glass tube and attach to the high voltage terminal.

Turn on the spark coil and observe the color of the gas discharge in the glass tube - it will be fairly blue, the lower the pressure, the fainter the discharge. Then apply solvent to the leaks, and the dischage will turn pink when solvent is being vaporized into the chamber through the leak.

Jim

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This is a fancy leak checker, which uses a mass spec tube to analyze for helium gas. This cannot be done with simple equipment like he has, with only rough pressure gages.

Also the chamber will have to be leak tight down into the tens of microns before one of these could be used to good advantage.

I respectfully suggest the glass tube in the foreline, and some acetone.

Jim

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Not really. This requires a real mass-spec leak checker to work.

Jim

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Thanks,

On my web-site, item 18 in the left side (towards the bottom) are the details on my existing vacuum cart, plumbing and my first 12" chamber made from a Propane tank.

Well, I read John Strong's section on vacuum technology in his book "Procedures in Experimental Physics" which pretty much pointed me in the right direction. From there its been mostly Internet research and many hours of sleepless nights.

Not actually sputtering, but thermal evaporation using tungsten filaments. Basically the same method I currently use on my 12" chamber, just scaled up 3x

Thanks, so far the only thing I've found to really screw things up is "Neosporin Ointment" used for treating wounds. That stuff is just plain nasty inside a vacuum chamber! :0

Thanks,

Take Care, James Lerch

(My telescope construction, Testing, and Coating site)

Press on: nothing in the world can take the place of perseverance. Talent will not; nothing is more common than unsuccessful men with talent. Genius will not; unrewarded genius is almost a proverb. Education will not; the world is full of educated derelicts. Persistence and determination alone are omnipotent. Calvin Coolidge

Hey James,

Wow!! It's not a cylinder. Why does that surprise me? Won't you get quite a bit of "motion" with a box type?

Take care.

Happy New Year.

Brian Laws>Greetings All,

Hi Brian,

I was a little concerned about the rectangular shape of the chamber. In an effort to have it hopefully keep its shape, and not inadvertently collapse and crush a $4k optic, the chamber is fabricated from 1/2" thick sheet steel. The opening of the chamber is also re-enforced with 5/8" square bar stock around the outside perimeter as additional bracing and a place for the door o-ring to seal against.

Last night, even with the leaks, I made it down to ~18torr of pressure. Atmospheric pressure is ~760torr. Consequently last night I made it to within 97% of my working pressure and could not measure any deflection across the back wall or door. I'm sure it did bend a little, but not enough to be measured with a straight edge.

On the flip side, just to give an idea of the forces involved, the door occupies 625 square inches of surface area.

625 * 14.7psi = 9188 pounds of pressure on the door.

Since the back wall is the same size as the door, I believe we can double this to a staggering force of 18,375 pounds trying its best to make the door meet the back wall, with our precious optics in the middle :-0

So far so good, but I will admit I stood mostly behind a concrete block wall as I watched the pressure drop :)

Take Care, James Lerch

(My telescope construction, Testing, and Coating site)

Press on: nothing in the world can take the place of perseverance. Talent will not; nothing is more common than unsuccessful men with talent. Genius will not; unrewarded genius is almost a proverb. Education will not; the world is full of educated derelicts. Persistence and determination alone are omnipotent. Calvin Coolidge

Looks good so far. I highly recommend you don't use any kind of paint or oil/dye for leak testing. It will be very tough to get it all out again later, and will forever contaminate the chamber and maybe your optics when you start sputtering. Helium is used for leak checking, not hydrogen. It permeates leaks very well and thermocouple and ionization gauges respond about ten times less to it than air so you can easily see it on the gauge when you put a small flow on a leak. Plus it's clean; no puddles to wipe up :-). Methanol is my favorite solvent for leak checking, as someone else described (just allow several seconds after each spritz to see if the gauge moves, before going on to the next spot). Don't use acetone since it soaks into orings and then takes forever to dry back out - I once spent three days in grad school watching an ion gauge drop from 10-7 to 10-9 torr as I baked the acetone out of an oring I soaked while leak checking. What kind of gauging do you have to work with? At 18 torr I'm guessing a mechanical vacuum gauge, but hopefully you also have a thermcouple gauge for the next step after you get the bigger leaks taken care of. I think you could clamp the door on and sit the chamber on a flat plate and pressurize it up to a few psi without your orings blowing out, so you can use the soap and water method, as well. Oh, if you have a refrigeration leak checker you should be able to put it in the exhaust of your vacuum pump (maybe put some cotton balls in a tube before it to keep oil out), and then use dust-off (mostly R134A these days) or helium (the leak detectors respond to pretty much anything that is "not air") or even CO2 if you have a MIG and CO2 bottle. Anyway, good luck with it.

-- Regards, Carl Ijames snipped-for-privacy@verizon.net

No, not so. The forces are in opposite directions. (If they werent, the chaber would go flying across the room.) This is no different than if you had a 9000 Lb weight on top, and it was sitting on the floor. The floor must exert 9000 Lb force to keep the chamber from falling through the floor. But, that doesn't double the load on the side walls.

I don't think you have much to worry about (with the possible exception of welds failing). We have chambers of similar size here that are made with thinner material. 1/4" would have been adequate, I would think, for the side walls. The 1/2" front and back look much larger, and they may need that thickness to keep deflection down to where the seals won't get lifted. Otherwise, you could use external braces to keep the front and back from bending inward.

Jon