vacuum pump for experiments

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.

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

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

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.

But don't they need oil which is usually lost when you cut the lines?

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.

Two stage is necessary to do a lot of interesting stuff. Some of the really interesting stuff requires a much harder vacuum than you can get with a mechanical roughing pump, and you have to go to a diffusion (oil, they used to use mercury) or turbomolecular pump. Suggest you get ahold of some of the old Scientific American Amateur Scientist columns (especially from the fifties and sixties) for ideas before you decide on one. Also check the availability and cost of rebuild kits. I got a brand new yet ancient (1965?) US government surplus one from a dealer in PA some years back. Still in the original crate, with two copies of mil-spec manuals sealed in foil, and with a new/old bottle of pump oil.
Best regards, Spehro Pefhany

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??
Thanks I was wondering about using an inert gas.
Will Nichrome wire work reasonably well?

I think they also require a cold trap. I have the amateur scientist book and CD Rom. I also have a book from lindsey that is much to deep for my needs.
chuck

This pump is close enough I can drive to pick it up. Will it do the job??

how about a piece of tungsten??
walt

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?

I had one of those. It'll pull good vacuum, but the CFM is so low it'll take time to evacuate a moderate sized space.
I upgraded to a brand new robinair pump. A little more money, but worth it. I use mine for vacuum processing and casting of urethanes, pulling hard vacuum fast is a good thing. This thing pulls a 12qt bowl down to 29" + in less than 30 seconds.

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

A two stage pump is usually just two identical pumps on the same shaft connected in series. It will pump air at exactly the same rate as a single stage pump of the same displacement (usually pumps are rated in litres/min or m3/hour) but it will be able to achieve a higher ultimate vacuum. Naturally if the pump can attain a higher ultimate vacuum, it will take longer to reach its ultimate vacuum.
I think neon signs do require a pretty high vacuum if they are to work well and stand the test of time, but I'm not certain of this. Perhaps someone here can confirm? There's also a short article about vacuum pumps for neon signs here, which suggests that the ultimate vacuum required is pretty high: You can make a glow discharge tube without neon; just reduce the pressure of a volume of air and apply a high voltage across it. A single stage pump should be fine for this. See for more details.
That pump you spotted on eBay looks quite nice. The Welch 1400 is a 25 litre/min two stage pump. It should serve well for small science experiments. The pump looks in good condition but of course it's hard to tell without seeing it in person. You might want to ask the seller if he can test the pump's ultimate vacuum and also tell you what it has been used for. It does appear to be without a belt guard but I believe these are available for A lot of useful information about vacuum pumps is available on this page, including the specifications of common Welch pumps:
I also have a service manual for Edwards pumps which gives a lot of useful hints about rebuilding, should you need it:
Best wishes,
Chris Tidy

you can buy tunsten wire: also found a great article on light bulbs at: 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

By the way I'd recommend a belt driven pump. These are generally much better made than the direct drive pumps, and should you need a new motor there are a wide variety of replacements available.
Best wishes,
Chris

Thanks guys. Some excellent info and so FAST!

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!