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Vacuum Pumps - I need an education

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GrantX

Sun Aug 04 2013, 04:45AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

A while ago I was given a vacuum pump. At the time I didn't really have a use for it, but I've now become more interested in gas discharge tubes and would like to set up a basic vacuum system.

The pump I have is almost new, and apparently unused. The name plate says it's a 1.8CFM 2 stage rotary vane pump with a 1/4 hp motor and an ultimate vacuum of 3x10^-1 or 25 microns. I have no idea if this is 'good' or 'bad' but I was told the pump was <$100 so I guess it's at the budget end of things. I'm certain this is the exact model: HERE.

I'm just curious as what additional parts I will need for an entry-level system. I've seen some primitive setups on youtube that consisted of a single hose connecting the pump to the chamber (no valves/gauges). And I've also seen extremely complicated systems with a ton of copper tubing, valves, gauges and extra chambers.

What's the most affordable option that will make best use of this pump? Are there any inherent safety considerations I need to make? I think the health of the pump, and personal safety are my key priorities here, over and above depth of vacuum. I'm seriously terrified of something imploding or violently re-pressurizing, so any advice on chamber construction is also welcome!

Daedronus

Sun Aug 04 2013, 08:25AM

Registered Member #2329 Joined: Tue Sept 01 2009, 08:25AM
Location:
Posts: 370

Plastics and rubber are bad in vacuum.

Silicon sealants are extremely bad, they never quite fully cure and the outgass substance is corrosive for the pump.

For copper tubing brazing do not use lead containing solder, or flux, flux will outgass.
2 parts epoxies are okysh, sometimes, the slow cure ones are supposed to be better.

If you must use rubber make sure it's viton, o-rings are quite cheap on ebay.
There is a seller from Europe called bohemia_seal that specializes in o-rings, and has almost any size imaginable.
For o-ring seals make sure the pressure helps make the seal, and it's not pushing against it.

If you will have a plasma discharge, make sure it won't touch any kind of rubber or epoxy.

If you have screws in the vacuum, make sure there is no air trapped in the back, drill hole trough them, or cut a groove along the side.

If you have glass parts make sure you have a blast shield of some sorts in front of them.
6mm glass plates should be good to around 50-60mm dia view ports.

Last but not least, keep everything as clean as possible, no greasy fingermarks :)
Might not be a big problem at 25microns but once the plasma touches them they will outguess like crazy.

Ash Small

Sun Aug 04 2013, 11:26AM

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

The 25 micron figure will be with no chamber attached, just a guage. You won't achieve this figure on a 'proper' vacuum system.

If you need more vacuum than the roughing pump will provide, the cheapest option is a small diffusion pump as well. (diff pumps and turbo-molecular pumps won't work without a roughing pump)

The maximum pressure on any vacuum system is ~15 PSI.

Electro-polished stainless steel is generally the best material for a system, but generally, any stainless will do. Steel is not as good, but is used sometimes.

Avoid copper inside if you are generating plasma/ions. Copper 'sputters' very easily, and will contaminate/ cover everything in a layer of fine pink copper .

Aluminium is very dificult to sputter, but does outgas.

For high vacuum, don't use viton, use copper seals (or silver plated copper)

Proud Mary

Sun Aug 04 2013, 06:32PM

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

GrantX

Mon Aug 05 2013, 12:21PM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

Thanks for the replies, they've been very helpful so far. The Bell Jar site was very enlightening, I haven't sifted through all the material yet, just their pages on the basics and a few forum posts.

I'm thinking of following this diagram from the 'basics' page on belljar.net:
HsgLf5U

Just the 2-stage pump -> isolation valve -> chamber, gauge and vent valve.

However, I read a forum post stating that it's not healthy to turn the pump off while the chamber is evacuated: apparently oil can backflow from the pump and damage is possible. So I guess that means the isolation valve is not optional? Would I also need a vent valve on the pump side as well as the chamber side?

Speaking of valves, I found some affordable brass gas fittings and ball valves intended for use with household gas appliances. However gas mains pressure here is only 2.75 kPa (0.4 PSI?!). Would these valves even be safe at ~15 PSI? If these aren't suitable (I guess not), are there any decent online sources? Apart from Ebay, I'm not sure where else to shop.

I've been trying to decide what materials to use for hose/piping to connect it all together. I have some 15mm copper gas tubing, but currently do not have the means to bend and shape it without crushing/damage. I've also found two types flexible hosing which would be much easier to work with, but I have doubts. The first is some "vacuum PVC" which was rather stiff, but very thick-walled, and the other was "silicon vacuum hosing" sold at an automotive store and designed for use in an engine bay, however it was so soft and squishy I think it would just collapse as soon as the pressure dropped (also would the silicon tubing have corrosive outgassing?).

As for the chamber itself, I've been looking at some glass jars on ebay, but haven't found anything suitable or affordable yet. I'm trying to avoid plastics where possible, but one of those large-diameter lexan or perspex tubes would be nice and easy to work with.

Thanks for the help so far, I'm making some progress!

Daedronus

Mon Aug 05 2013, 12:47PM

Registered Member #2329 Joined: Tue Sept 01 2009, 08:25AM
Location:
Posts: 370

What is your target pressure ? This determines what materials are acceptable.

Ash Small

Mon Aug 05 2013, 01:28PM

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

This is a bit on the pricey side, but there is vacuum equipment on Ebay Australia.

This is a bit pricey too:

This is a bit better, pricewise:

How about this:

You just have to keep watching until something you like comes up at the right price.

Hazmatt_(The Underdog)

Tue Aug 06 2013, 01:03AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Just a FYI, last I checked, Diff. pump oil was about $1 per cc, so try to get it on ebay if possible.

GrantX

Tue Aug 06 2013, 04:51AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

While my pump claims to be capable of 25 microns, I don't think I currently need a deeper vacuum than ~100-200 microns. Don't think I'm going to invest in an oil diffusion pump any time soon.

Ash, those are some serious industrial components, that gate valve is enormous!
Is the single stage jigtool pump and the small speedivac pump far superior to my 2-stage Chinese pump? The speedivac isn't too expensive if I absolutely require an upgrade from the Chinese pump.

Would the brass ball valves, designed for use with gas installations and appliances at 2.75 kPa, be too leaky to achieve 100-200 microns with my 1.8 CFM pump?

If this "beginners" set up works well, then I may consider an upgrade to something more professional for constructing plasma globes or attempting some DIY vacuum tubes, but that's all in the distant future. If this first system can make some pretty glow discharges then I'll consider it a success.

klugesmith

Tue Aug 06 2013, 05:59AM

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

I think you'll be fine with the pump you have, and hardware store ball valves.
Spend your money on getting a base plate and bell jar. If the latter was made for canning food,
think about implosion protection -- e.g. plywood box with acrylic window on one side.

Oh, also (even for just pretty glow discharges) you will soon want a vacuum gauge that reads absolute pressure. Thermocouple or Pirani gauges are readily available for not too much money, and can read down to on the order of 10 microns.

When pumping down your chamber, if the indicated pressure levels off at something higher than you expected then start looking for leaks. Vacuum oil, applied with a brush to the outside of a leaky joint, will temporarily plug the leak and the pressure will drop. By the way, do you know if your vacuum pump is due for an oil change?

Check out the vacuum discussions and FAQs on the fusor.net forum.

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