vacuum pump for experiments

My 11 year daughter is showing a strong interest in science. We received a Edmunds catalog with lots of cool stuff in it. I have always wanted to play with vacuum stuff when I was a kid and I didn't get much of a chance so I figure now is my chance.
I figure it would be cool try to make a crude light bulb inside a bell jar and things like that.
I looked at ebay and there are many vacuum pumps FA.
Do I need a two stage pump and how big of a pump do I need? The killer is these things are heavy and shipping is expensive.
chuck

If you have an air compressor, you can make a venturi pump (I think that is what it is called) like HF sells. It is simply a block of aluminum with a T intersection angle drilled into it. The air from the compressor blows through the straight drilled holes which creates a vacuum on the remaining leg. I use mine for air conditioner work and it will pull 29 lbs of vacuum.

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Really? With only 14.7lb possible? Did you mean 29" of mercury? <G>
LLoyd

Yes, yes. Another case of hands working faster than brain.

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Or salvage a small AC compressor from an old window unit or water cooler. You will need the run capacitor. Mount it on some type of base and insulate everything so it is safe.
Vaughn
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But don't they need oil which is usually lost when you cut the lines?

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There is enough oil left in the sump for lots of operation, and I suppose you could add some once in a while if you really wanted. I have had mine for about 6 or 8 years of occasional operation without a problem. If it ever goes bad on me, I will simply scrounge another.
Vaughn
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On Sat, 14 May 2005 16:22:30 GMT, "Vaughn"
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If you know which soldered off tubes go into the sump and the ullage space, you can install a simple vinyl-tubing sight glass gauge to monitor the oil level in the hermetic can - the problem being to determine the proper oil level to begin with.
I suppose if you call the manufacturer with the compressor model number from the data plate, they can tell you how many fluid ounces of oil, and what weight and type - mineral, POE or PAG synthetic. Do NOT mix or change the oil, the oil seals inside are different to be compatible with the oil. And you can drain and fill it to make the initial 'full' level mark.
Oh, and put an oil separation filter on the compressed air output to trap the excess oil - in refrigeration they don't care about a little bit of oil entrainment in the output, because it's a sealed system and the oil will be back soon...
--<< Bruce >>--

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The ultimate pressure you can achieve is very much affected by the oil in the vacuum pump and I'm not convinced that AC oil is good vacuum pump oil.
I think the primary factor with AC oil is that it has to be compatable with the Freon (as well as the seals). In the AC, I doubt very much that they care about the vapor pressure of the oil which is critical in vacuum operations.
Supporting data from one of the laser web sites indicate that they change the oil when they use an AC compressor as a vacuum pump.
Its a moot point for me, because I just bought a welch 1400 two stage pump for 127 off ebay. While that is more than an AC compressor, its a turn key operation and I believe this vacuum pump will perform way better than a converted AC compressor.
chuck

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Or an old refridgerator. I have one that I use for vacuum bleeding brakes. Pulls a very good suction. Lane

TheAndroid posted:
"The air from the compressor blows through the straight drilled holes which creates a vacuum on the remaining leg. I use mine for air conditioner work and it
will pull 29 lbs of vacuum."
One of these devices would be inadequate for Chuck to do the demonstration he wants. A single-stage air conditioning evaculation pump would be perfect, or as we did in the old days salvage an old refrigeration sealed compressor from a junked refrigerator and us it, after cleanup and flushing out, to provide the vacuum.
Harry C.

Chuck, you don't need a two-stage pump. Any motor driven pump capable of achieving 1-5-mm Hg of vacuum should do the trick. You don't need to pump down to the micron region.
An important thing to note is that most lightbulbs are not evacuated -- they contain low pressure argon so that the filament material doesn't evaporate as it would under a high-vacuum.
For a simple demo experiement using a bell jar, I'd pump the bell jar down to about 30-mm of Hg, then turn on the current to the filament. It will start to glow, then as you pump more air out of the jar it will being to glow brighter and brighter, since less and less of the input energy is being lost to air molecules. Of course you will need to use a filament material that is (1) not readily oxidized and (2) has a very high melting temperature.
This is the principle upon which thermocouple vacuum gauges operate.
For such a simple demonstration, I really don't believe that it would be beneficial to get into issues like the rarified argon backfill, unless of course you want a bulb that will burn for hundreds to thousands of hours.
Harry C.

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Will a two stage pump work faster or slower ?
Will I need a higher vacuum for things like neon lights? I assume you remove most of the air and then inject the gas, so the initial pressure isn't critical??
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Thanks I was wondering about using an inert gas.
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Will Nichrome wire work reasonably well?

how about a piece of tungsten??
walt

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well I have some tungsten rod for tig welding, but its much to big. Can you buy tungsten wire or do I have to break a light bulb to get some?

you can buy tunsten wire: http://www.ebsstore.com/control/product/~category_id=G7/~product_id=TU-T2 I also found a great article on light bulbs at: http://science.howstuffworks.com/light-bulb2.htm
I believe bulb manufatcurers use rhenium filiments instead of thoriated tungsten. I think 2% thoriated wire would work but i'm not an expert. Good luck, sounds like a fun project!
walt

Chuck Sherwood wrote:
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I have a spool of .002 tungsten wire that you can have some of. The advantage of .002 is that it will take very little voltage to heat. The disadvantage is that it'll be hard to handle. "Hard to handle"? Hell, it's hard to see!

Chuck Sherwood wrote:
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Break a "new" light bulb for the metal. The new tungsten is still flexible and can be formed but an old one will be brittle. Also you may want the metal to glass seal that is hard to make your self (I think).
Bill K7NOM

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Two stage pumps will go down to a lower pressure.
Typcially five or ten microns (1 micron = 1 millitor)
A single stage pump might do 40 or 50 microns ultimate pressure.
Jim

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Related question: what does "gas ballast" mean in the context of vacuum pumps, and what is it for?