indigofuzzy
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Vacuum (2-7 torr range) Can it be achieved cheaply?
I was wondering if it's possible to achieve a 2-7 torr vacuum without spending several hundred us dollars. I'd like to experiment with low pressure
gas discharges (and make things like plasma spheres, "neon" signs, etc.)
Does anyone know of a way to achieve this level of vacuum without spending a fortune? Or am I simply asking the impossible?
(edit) changed title. sorry about the mislabel....
[Edited on 3.26.2007 by indigofuzzy]
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Sauron
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The short answer is no.
You need the following:
A good two stage oil sealed mechanical pump
A 13X Zeolite custom made trap
Valves, fittings, tubing etc
Optional cryo trap
Several vacuum gauges of various types and ranges
This is a bare bones system as simple as they get
If you go instead with auxiliary pump(s) like diffusion pumps rrather than the zeolite trap, price goes up.
I estimate MINIMUM cost at a few THOUSAND dollars, so as I said the answer to your question as stated, is no.
[Edited on 26-3-2007 by Sauron]
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indigofuzzy
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Hmmm... this makes me severely doubt the validity of those "Do-it-yourself plasma tornado jars". Of course, Argon and helium are said to sustain arcs
even at 1atm....
So, Maybe a better question to ask is: to make a plasma globe or "neon" sign, do I really need at 2-7torr vacuum? If not, what would be needed to
obtain the required pressure range?
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Sauron
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Do you mean 2 x 10 to the -7 torr?
Neon signs do not use such a hard vacuum, because no current would flow at that, no discharge would occur.
My advice is: Google for a website called The Bell Jar and read all the free information and articles they have. Follow the links to related sites and
commercial suppliers. You will learn all you need to know. One of the suppliers listed there specializes in equipping the sign making trade.
1- X -7 is the range for X-ray tubes and particle accelerators not plasma tubes and gas discharge tubes.
You want medium vacuum that you can achieve easily with a good 2 stage pump even without a zeolite trap. You still need gauges and hardware though.
Steve HGansen sells the first five years of the Bell Jar newsletter for about $70, and is preparing the second five year compilation now. Buy them and
subscibe. But furst read all the freebies.
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indigofuzzy
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I apologize for the ambiguity. I meant within the range of 2 torr up to 7 torr, (1/300ish to 1/100ish of an atmosphere)
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UnintentionalChaos
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Well, this may be quite impractical, but would probably be much cheaper. Lithium metal will react with every major component of air at room
temperature, namely nitrogen (although I am not sure how quickly) and oxygen, as well as removing any moisture. It should not, however, react with
noble gases like argon or helium. To this end, you build a neon sign with a little bulb at one end for containing the lithium. The lithium and gas you
want in the tube should be added in the proper amount and the whole deal sealed up, ampoule style. For a neon sign with 1m^3 internal volume, which is
enormous and will never happen, the most lithium it could possibly require to consume all the gas is just under a single gram. Get the lithium into
the bulb and maybe heat to around 185C or just leave it for a while (lithium will melt at 185 and react much faster, but make sure the tube can take
the pressure fluxuations). If you remove the bulb and extra lithium/lithium compounds ampoule style, the whole process may be much more achievable
OTC, so to speak. This would also be neat for, lets say an oxygen discharge tube where you merely have to calculate how much lithium you need to bring
the pressure down and add to a tube full of O2.
This right here is mad science. 
Edit: Sauron, having followed up before I could post has pointed out that you don't need full vacuums for these signs. The principle still holds
though, just adjust the measurements.
PS: This is just my off-center mind running on autopilot at 5:40am. 
[Edited on 3-26-07 by UnintentionalChaos]
Department of Redundancy Department - Now with paperwork!
'In organic synthesis, we call decomposition products "crap", however this is not a IUPAC approved nomenclature.' -Nicodem
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Sauron
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Oh. That's not hard vacuum, or even medium vacuum. That is low vacuum, anything from 1 toru up is low.
Any good mechanical pump will get you there, even a single stage one. You can find second hand pumps for not a lot of money, the Bell Jar has aricles
about using HVAC or refrigeration compressors plumbed backwards but I think that is false economy myself.
http://www.belljar.net/index.htm
Have fun. That's the Bell Jar link
What chemists usually think of as "hard" vacuum is merely the lower end of low vacuum. Physicists have a very specific meang for low/medium/high/ and
ultrahigh. By the way get used to dealing with Pa as they are the proper units now although many of us old fogeys cling to the torr.
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indigofuzzy
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@Sauron: This suddenly sounds almost trivially easy
Hypothetically speaking then, if I were to fill the globe to be evacuated with a liquid that has a low vapor pressure, and connect a similarly filled
(but very long) tube to that, take the globe up to the third floor, and leave the free end of the tube sitting in a bucket of whatever low vapor
pressure liquid I use, and let the liquid run out of the globe..... (I'm taking this from something I read many years ago about how mercury barometers
work, and how the residual mercury vapor above the column could be made to glow... But I'm sure there are much safer alternatives to mercury.) Is
this setup reasonable? Or are there gotchas that I may have missed? (Other than the fact that said plasma globe will be filled with some vapor at the
vapor pressure of the liquid in question.)
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not_important
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As already stated a mechanical pump will do. The was a trick used a century ago that allowed experimenters without mechanical pumps to reach that
pressure range. The system would be evacuated with an aspirator pump (AKA water pump), then filld with CO2. Repeat several times, letting it sit for
awhile after each filling before pumping it down again. Finally, after pumping it down a bulb containing solid potassium hydroxide was attached, the
system sealed off, and then left alone for some hours on up to a day or more. The KOH bulb was then sealed off from the system, having absorbed the
CO2 and reaching a decent vacuum.
A good mechanical pump is a lot easier.
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Eclectic
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Indigofuzzy's mercury pump idea is basically a Geissler vacuum pump, as used in the 1850's-1900's
http://www.sparkmuseum.com/GLASS.HTM
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Twospoons
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So you want to do something like this?
Thats me and my mates goofing off on a friday night with a belljar ($5 second hand), a 15kV neon transformer (dumpster-dived for free) and the
compressor from a junked refridgerator (also free) operating as a vacuum pump. No fancy gases, just air or butane
So you can see you can have a bit of fun for very little outlay.
One comment about the converted fridge pump - put an oil-mist trap on the outlet ( $10 or so) , and return the collected oil to the inlet.
Locally I can buy a 2 stage refridgeration service pump that will go down to 50mTorr, for about $NZ 400.
Helicopter: "helico" -> spiral, "pter" -> with wings
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jpsmith123
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Someone is selling two stage, 4 CFM mechanical pumps (apparently from China) on Ebay for 122.95; there's also a 2.5 CFM model for 99.95.
http://cgi.ebay.com/ebaymotors/FJC-4-CFM-Vacuum-Pump-HV-AC-R...
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Bander
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| Quote: | Originally posted by indigofuzzy
I was wondering if it's possible to achieve a 2-7 torr vacuum without spending several hundred us dollars. I'd like to experiment with low pressure
gas discharges (and make things like plasma spheres, "neon" signs, etc.) |
Yes, ~$20 and a few hours. Try a linear peristaltic vacuum pump as in:
http://www.tinkerhack.com/vac6.htm
http://www.tinkerhack.com/vac8.htm
You can get down to 1/2 torr.
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chemrox
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| Quote: | Originally posted by indigofuzzy
I was wondering if it's possible to achieve a 2-7 torr vacuum without spending several hundred us dollars. |
I thought he meant 2 to 7 torr .. and that should be easily obtained .. what's the water pump? 30 torr for warmish water (20deg)? .. with a little
water vapor inside the vessel to be evacuated and heat on it - 20 torr?
The homemade peristaltic pump article is awsome!
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indigofuzzy
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WOW. Thanks! I'll start building this one very soon.
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chemrox
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Yeah, I'm going to make one too.. looks easy and fun!
I'm going to try and paste in a pdf I made of a trap. I'm really hoping this doesn't fill up the whole page...
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chemrox
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trap
this may be too big-if so I'm going to delete and try again tomorrow
anyway..it's a sidearm flask say 125-250 ml and 6-8mm glass tubing, rubber hoses, pinch clamp
Attachment: trap.pdf (8kB)
This file has been downloaded 729 times
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Texium
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