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GluD

Thu Jan 13 2011, 10:59PM

Registered Member #1221 Joined: Wed Jan 09 2008, 06:17PM
Location: Odense, Denmark
Posts: 196

Hello

I am trying to make a vacuum system for a plasma display.
There was a brief discusion about it in the chatroom the other day, based on some new tests I think I do in fact have a leakage.

The pressure seems to stop falling at around 150mbar, which is a fairly high pressure in my opinion. When I the close the system and stop the pump, the pressure seems stable but if left alone for a long time (serval hours) it slowly rises to atmospheric pressure.

Should I just stick it in a bucket of water and pressurize the system and see if it bubbles?

Some infomation:
It is mostly made from 3/8" brass fittings sealed with PTFE tape, the joint between the brass plug and the cobber tube is soldered with my soldering iron and the solder I also use for electronics, PCB's and such. Was that a bad move?
The copper tube is 6mm on the outside and 5mm on the inside.
The glass is a 2000ml roundbottom laboratory flask from duran schott.
The vacuum pump is a two stage rotary vane type.
The valve is a gas approved ball valve and the manometer is some strange thingy that seems to measure from atmospheric presure to "minus 1 bar". I think its pretty silly to measure pressure in negative units but it was the only one I could find at the time. Now I've located some digital ones but I'd like to use what I got.

Please ask if you wish more details.

I would appriciate any comments and ideas about my construction if you would be so kind.

Some pictures:

Ash Small

Thu Jan 13 2011, 11:12PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Can you get hold of any helium (balloon gas)? If not, can you get any acetone?

GluD

Thu Jan 13 2011, 11:16PM

Registered Member #1221 Joined: Wed Jan 09 2008, 06:17PM
Location: Odense, Denmark
Posts: 196

I have some acetone, and I'm fairly sure I could get some ballon gas.

Turkey9

Thu Jan 13 2011, 11:48PM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

I wouldn't be surprised if your threads are leaking.
Threads, even with PTFE tape, are almost impossible to seal. I've had systems with threads like yours leak no matter how tight I thought the joint was. If your joints don't need to be unassembled, I would say mix up some JB Weld and epoxy them together.

Ash Small

Thu Jan 13 2011, 11:58PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Balloon gas is the best thing to use to find leaks. you don't need much. While it is not pure helium, the helium in it will 'seep' through the leak much quicker than any other gas, resulting in an almost instant rise in pressure.

PTFE is a thread lubricant, not a sealant. It works by allowing you to tighten up the joint tighter than you could without it, however, for vacuum use, you are better off with nothing on the threads.

GluD

Fri Jan 14 2011, 12:01AM

Registered Member #1221 Joined: Wed Jan 09 2008, 06:17PM
Location: Odense, Denmark
Posts: 196

I'm not familiar with JB Weld, but I have some loctite epoxy I could try. What about outgassing, would that be an issue with epoxy?

edit

Hi Ash

uh oh I thought PTFE was suposed to be a sealant. I get the point with the helium, but what about the acetone?

Ash Small

Fri Jan 14 2011, 12:18AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Acetone is supposed to do pretty much the same thing, but not as well. as it evaporates it is supposed to find it's way through the leak, resulting in a pressure rise, but I've not tried it myself.

I'd try dry threads, but epoxy could be used as a last resort. JB Weld is supposed to be the best for vacuum use among the fusor crowd, but it will outgas if it gets hot. A small amount on a thread shouldn't be a problem, though.

GluD

Fri Jan 14 2011, 04:55PM

Registered Member #1221 Joined: Wed Jan 09 2008, 06:17PM
Location: Odense, Denmark
Posts: 196

Hello again

I just tried without the PTFE tape, it pumped down to about the same pressure and then the pump stalled again, but when I closed the system the pressure raised instantly.
I'll epoxy it tonight and then try again tomorrow. The fittings shouldnt get hot so I dont suppose outgassing will be an issue then.

Proud Mary

Fri Jan 14 2011, 05:18PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

There are high vacuum epoxy adhesives like Torr Seal which is good to 10E-9 Torr.

Plazamatron once told me he'd found that some much cheaper epoxies would work well, but I've forgotten what he said.

klugesmith

Sat Jan 15 2011, 11:15PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Couple hints before you glue all your plumbing joints:

1) For plasma glow work, you need a way to measure absolute pressures of a few torr.
Thermocouple gauges are popular, but a closed-end manometer filled with vacuum
oil can do. A fancy home-made one for neon sign makers is shown here:

2) How low a vacuum can you get with the pump connected to gauge and nothing else?
If that's plenty low, and you don't even get close with your larger apparatus, then it's reasonable to look for leaks.

3) I've had good luck with brass pipe thread joints made up with teflon tape or pipe joint paste. The latter is easier to apply. They -are- sealants. And regular tapered pipe threads are NOT designed to seal dry; the thread crests are truncated more than the roots so there is always a helical leakage path.

4) to detect leaks in threaded joints etc., apply some vacuum oil to the outside of the joint with a small brush (e.g. an acid brush) while your pump and gauge are running. If the threads are leaking, the oil (vastly more viscous than air) will temporarily plug the leak and your pressure will go right down.

[edit] and you can use Loctite-type anerobic thread sealant liquids, which are less tenacious than epoxy if you want to take the joint apart later.

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