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# A good deep vacuum pump? Answered

I'm looking for a vacuum pump for use with projects like the fusor included in make magazine, a laser tube, or a plasma ball.  It doesn't need to move huge amounts of air, but nothing below 1.5 lpm.  It also needs to have an ultimate vacuum of at most 25 microns.  I have been unable to locate any pumps which meet these standards, all of them have ultimate vacuums of around 100 to 75 microns or are completely unlisted.  Do any of you have some experience with these and some advice.  I don't need something for pumping out heating/cooling units daily, just a very deep vacuum a couple times a month.

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I actually have this vacuum pump, and I was curious about this quote of 25 microns.  I mean could it actually be true? It seemed kind of hard to believe, and since not everyone has a gauge that will measure pressure that low, how would anyone discover if the quote was actually a typo, or advertising hyperbole, or just a naked lie?

Anyway, I actually bought a used absolute pressure transducer from somebody on eBay, one transducer that measured from 0 to 10,  torr full scale, and hooked it up to this vacuum pump.

The darned thing is, I don't remember exactly what the measurement was.  I think it was either around 3 or 0.3 torr.   Both those numbers are higher than 25 microns, since 25 microns = 25 millitorr = 0.025 torr, erm right? To give you a better guess than that, I would have to either find any notes, or pictures, I might have taken, or just pull all that junk out, and re-take the measurement.

Also, there might have been some some volatile substance(s) in my plumbing, i.e some substance with a room temperature vapor pressure of around 3, or 0.3 torr.

Yeah... so it is 1 micron = 10^-3 torr = 1 millitorr.  A micron, in vacuum-speak, is like a micron (10^-6 m) of mercury.  One torr is one millimeter (10^-3 m) of mercury.  I'm glad I got that right.

Regarding the measurement story, I couldn't find any notes, but I did find one picture. In that picture, the voltmeter says about 3 volts, and I think this transducer produced an output voltage of 1 volt per torr, with 10 volts, or 10 torr, being the full scale.

So the measurement was around 3 torr, but there was a whole bunch of other stuff attached to this setup besides the just the vacuum pump and the sensor, lots of chemicals like  paint, RTV silicone seal, soldering flux, and also lots of soft vinyl tubing.

There could have been tiny leaks too.

In summary, I did not get a vacuum anywhere near 0.025 torr = 25 micron, but I have not proven that it cannot be done with this HarborFreight(r) brand vacuum pump.

Actually I was mostly pleased with this pump.  It pumps quickly, and for easy stuff like a homemade purple plasma globe, i.e. a glow discharge in low pressure air, an absolute pressure of as high as 10 torr will work.

BTW, one of the things that displeased me about this pump, is that it does not hold back pressure when turned off.  That is to say it is necessary to have a check-valve, or ordinary valve, in between this vacuum pump and the volume being evacuated. Without this measure in place, when the pump is turned off, the outside air pressure will slowly push the all the vacuum pump's oil into the newly evacuated volume, and this makes an awful mess, and it's very discouraging because the vacuum pump oil is this exotic, hard-to-find stuff, at least in my neighborhood.

Anyway, I should wrap up this long-winded apology, and point to the related column there
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for those of us who are still trying to make our vacuums harder.

Highly recommend this setup, this is roughly a 25 to 30 minute vacuum.

You need to get yourself the archive of the Scientific American "Amateur Scientist" column - I am pretty sure there were plans for a diffusion pump in there. A fridge compressor will usually work as a roughing pump,

I think this is the archive you are speaking of:
http://jesseenterprises.net/amsci/index.htm
http://jesseenterprises.net/amsci/indexes/indices/1920-fs.html
http://jesseenterprises.net/amsci/indexes/indices/1930-fs.html
http://jesseenterprises.net/amsci/indexes/indices/1940-fs.html
http://jesseenterprises.net/amsci/indexes/indices/1950-fs.html
http://jesseenterprises.net/amsci/indexes/indices/1960-fs.html
http://jesseenterprises.net/amsci/indexes/indices/1970-fs.html
http://jesseenterprises.net/amsci/indexes/indices/1980-fs.html
http://jesseenterprises.net/amsci/indexes/indices/1990-fs.html
http://jesseenterprises.net/amsci/indexes/indices/2000-fs.html

The same content is also sold (by the legitimate copyright holders, I guess)  as a CD-ROM titled, "The Amateur Scientist Collection",or something like that.

I am guessing a  compressed archive of the same is ?probably? also available via bit-torrent or various file-sharing sites.  I think the whole bundle (html and pictures) is only about 300 Mb in size.

I mention this for anyone reading this post who might be interested in obtaining a copy.

Too much from the vacuum pump I mean, 25 microns, is that out of a 2-300 dollar budget?

I don't need to build my own, I have just been unable to locate a pump which meets these standards for me to order online. Am I expecting too much?

Sorry, but that is impossible, that is a negative vacuum because 1 atm is approximately 29.92 inhg. 30 is the standard maximum vacuum because nobody is going to put ~29.93 on their guage. 29.92 inhg, the lowest inch rating I can find is 100 microns (.1mmgh) anything below that is a subunit of 29.92 for example 29.925. Once you move on to 29.23 you are in the negative and you can't have a negative vacuum. If you reached 34 microns, I am quite impressed and would like some more information haha.

The deepest vacuum I have seen from a fridge is 27 inches or several hundred microns.