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Author: Subject: High vacuum CHEAP and dirty!
Mendeleev
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[*] posted on 24-9-2004 at 15:06


Yeah, them physics people love absolute vacuum, fictionless sufaces, and massless ropes and pulleys, etc :). The job of applying it to real world is up to the engineers. Also while looking wikipedia at that vacuum pump article you mentioned Hermes, I also found this:



Ion pump
From Wikipedia, the free encyclopedia.
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Sputter ion pumps are a class of vacuum pump designed to operate in very low pressure (i.e. very high vacuum) conditions. They operate by ionizing a gas within a magnetically confined cold cathode discharge. The events that combine to enable pumping of gases under vacuum are:

Entrapment of electrons in orbit by a magnetic field.
Ionization of gas by collision with electrons.
Sputtering of titanium by ion bombardment.
Titanium gettering of active gases.
Pumping of heavy noble gases by ion burial.
Diffusion of hydrogen and helium into titanium.
Dissociation of complex molecules into simple ones for pumping ease, e.g., CH4 breaks down into C and 2H2. Hydrogen is pumped separately. Carbon is no longer part of the residual gas and resides in solid form.
Burial is the basic means of pumping heavy noble gases. Argon ions neutralized via glancing collisions with a sputter cathode impact the pump wall and are coated with sputtered titanium. Triode pumps are specially designed to maximize the kind of collisions that produce energetic neutrals.

Argon is permanently pumped on the wall behind the cathode in these pumps. The wall area receives titanium for inert gas burial but, because of a retarding electric field between the cathode and the wall, it is not subjected to ion bombardment and thus gases are not resputtered.



Not too shabby eh? :D




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Jen
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[*] posted on 8-11-2004 at 11:32


For electron microscopy which requires ultra-high vacuum we use a rotary pump for 'roughing', then a diffusion pump, and usually several ion getter pumps and a cold trap (running on LN2) to keep things going. The rotary pump is turned off when the pressure is low enough for the diffusion pump as it creates too much vibration.

http://lerch.no-ip.com/atm/Trap.htm - home-made cold trap, using Rp22 instead of liquid nitrogen as the coolant.
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basicelectromechanic
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[*] posted on 7-12-2004 at 15:08


As mentioned earlier by Democritus, the compressor from a junk refrigerator can be used as a cheap ( or free ) improvised vacuum pump. In my line of work we bench test used compressors and one that pulls a respectable vacuum against a test manifold ( and does not lose vacuum to discharge valve leakage after power is removed ) is considered fit for use.
This method will not pull a deep vacuum though, 50 to 80 torr ( 50 000 to 80 000 microns ) at best. Old beat compressors are not capable of pulling down like new due to piston to cylinder wear and valve wear.
With hermetic and semi hermetic pumps the vapor, lubricating oil and motor windings all are enclosed in the same space. Polyolester lubricant used with the newer HFC's is VERY hygroscopic. Long term use for this duty for which the pump was not designed will result in pump failure, particularly if you intend to vacuum distill strong acid, as this will atack the motor winding insulation resulting in " hermetic burnout ".As a short term cheap solution to a breif exercise where high vacuum is not required such a pump fits the bill.
For a deeper vacuum a two stage rotary in good maintain can pull 20 microns or better. Routine change of vacuum pump oil really makes a difference in ultimate vacuum achieved. Such pumps are available new from a refrigeration wholesaler or used and very cheap at auction.

If you intend to use an old compressor as a vacuum pump select one from your junkyard that has the start components attached and intact, if those components have been removed or disassembled odds are that the pump is failed. Also look at the paper or plastic label ( if present ) on the can, if it is browned instead of white this can has run at elevated temperature for a long time and should be passed by.
A compressor designed for use with R134 or R12 in a domestic unit will pull a deeper vacuum than one designed for use with R22 in an air conditioner. Coupling compressors together in series as shown on the previously mentioned website is not advisable, at high vacuum the motor winding insulation can fail.

[Edited on 7-12-2004 by basicelectromechanic]
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[*] posted on 12-12-2004 at 22:01


I am interested in buying a vacuum pump for common lab applications like filtering and vacuum distillation. Much of the previous discussion in this thread is about diffusion pumps and high vacuum. This seemed beyond my needs. So I have done some looking in catalogues. I soon realized that there are many types of pumps, even just for lab use. I felt I needed to establish my criteria, so, without much experience to base this on, here is what I have:

1. ultimate vacuum: 5-20 mm Hg absolute

2. noncondensable gas capacity: ~1 cfm

3. reasonably quiet

4. reasonably compact

5. relatively maintenance free

6. <$200

I think I'm reading that if I want to reliably get down to the vacuum of my criteria I will need a 2-stage pump. Then I must choose:

1. piston type, or
2. rotary vane

And then:

1. oil-less (PTFE/Kalrez), or
2. oil-filled

I would appreciate comments and recommendations, especially from those with some experience. If I have failed to list all needed criteria or missed some pump types, I'd like to hear about those too.

Even to meet my relatively modest needs it seems a new pump will be over $1000. This is beyond my budget. But there are a great deal of used pumps advertised on the internet for much less. I'm hoping I can get one for <$200 that won't require a rebuild right off the bat. Does this sound possible? Thanks for any help you can give.




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Mendeleev
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[*] posted on 12-12-2004 at 22:11


I would go to ebay. I got welch duo type vacuum pump for $75 1.2 cfm and a max vacuum of under 1 mm Hg. It's one of those old oil pumps that has a separate motor turning the pump using a rubber belt. It's not the prettiest pump but it gets the job done.



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Magpie
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[*] posted on 13-12-2004 at 22:43


Thank you Mendeleev. I have seen the Welch belt driven single stage pumps advertised. Apparently the belt drive provides low pump speed. This in turn allows reaching a 1 mmHg vacuum with only 1 stage. It also decreases pump wear.

It looks like selecting a lab vacuum pump is about as confusing as trying to decide what type of boat to buy. They all have their pros and cons. :D I think it signigicant, however, that the mother of all lab supply houses only offers a 2-stage, oil-filled, direct drive pump (rotary vane I presume) for their "high" vacuum.

[Edited on 14-12-2004 by Magpie]




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[*] posted on 3-6-2005 at 20:33


This is similar to a water jet vacuum I believe. I was wondering what type of vacuum one of these was able to pull. It is a pyrex type glass vacuum. 20 torr maybe?

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=1...
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[*] posted on 3-6-2005 at 22:20
Aspirator


Like any aspirator, it should pull down to the vapor pressure of water at 17 mm Hg (@ 20 C) as
Unionised stated in an earlier post.




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[*] posted on 14-6-2005 at 08:06


Sorry for my ignorance but would a hand pump such as this one that produces 25inHg vacuum produce 634mmHg after you convert inches of Hg to mmHg? That seems like a really good vacuum for a hand pump. I was just wondering, the idea of vacuums interest me.

http://sciencekit.com/category.asp_Q_c_E_439390




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neutrino
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[*] posted on 14-6-2005 at 15:24


Yes. That does look like a good vacuum, but I doubt you could quickly evacuate something with 15cc strokes. Also remember the ridiculous price tag... I've seem better aspirators on ebay for <$2.:D
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[*] posted on 14-6-2005 at 17:17


I got the same thing for much less a few years ago at Harbor Freight. Cynmar sells it for $20. $10 without the gauge. The second that I got it out of the plastic packaging, I noticed the flimsy handle and wondered how long it would last. Not long at all, it soon broke above the fulcrum despite some epoxy reinforcement. You get what you pay for. It is convenient for mobile application but a tire pump is at least less fragile.
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MadHatter
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[*] posted on 14-6-2005 at 20:47
Vacuum


I have a couple of those hand pumps. I find they're great for vaccum filtrations through
an Erlenmeyer flask, but for acid distillation you really do need an aspirator because of the
constant vacuum needed. I have a rotary vane pump(pulls down to 29" Hg) but I limit this
to air conditioning systems because acid vapours are quite corrosive. Stick with those cheap
little hydroaspirators for distillation work. There's not much(hydogen fluoride excepted) that
will hurt them. They're definitely worth the low price ! Besides, they'll wash any noxious
vapours down the drain as well.

For me price comparisons as follows:

Hand pump - $30
Hydroaspirator - $11
Rotary Vane pump - $200

It really depends upon your needs, but I'll stick with the hydoaspirator for most of my work !

[Edited on 15-6-2005 by MadHatter]




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[*] posted on 20-6-2005 at 03:59


The only downside to a little hydroaspirator for vacuum distillation is the vacuum is pretty weak. It does have many pros, but is the weak vacuum it pulls really going to have a noticeable effect on the BP of the substance in your distillation setup?
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[*] posted on 20-6-2005 at 12:37


Have you read the thread?
If so, did it occur to you that, if a water aspirator can drop the pressure to the vapour pressure of the water running through it, then it can boil water that is marginally warmer than that cold water?
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[*] posted on 23-6-2005 at 15:20


Reread the thread, thanks for the help. I think I'll experiment with the hydroaspirator and examine the results.
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[*] posted on 9-1-2009 at 18:57


http://books.google.com/books?id=msgAAAAAYAAJ&pg=PA77

This aspirator-assisted Sprengel pump, which recirculates the liquid mercury (not a novel idea, but a good design that one can improvise easily), makes me wish that I had some. The vacuum produced is on the order of vapor diffusion pumps. OMFG MERCURY! posts in 3,2,
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watson.fawkes
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[*] posted on 10-1-2009 at 08:14


Quote:
Originally posted by S.C. Wack
This aspirator-assisted Sprengel pump, which recirculates the liquid mercury (not a novel idea, but a good design that one can improvise easily), makes me wish that I had some. The vacuum produced is on the order of vapor diffusion pumps.
You could use the same basic design with an oil designed for a diffusion pump. It's not as dense, so your loop should probably be taller, but the same tuning principles apply. You'll need to find a tubing diameter than matches the surface tension of the oil. You need to adjust the head losses in the pumping loop (that's what the stopcocks are for) to get the right pulse widths of oil and gas. And there's less worry about containment, contamination, and cleanup.
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[*] posted on 28-2-2009 at 16:58


Quote:
Originally posted by sarcosuchus
Well sir; I'll admit when I'm being educated.

I am still trying to figure out the practical aspects of how to make the conversion from refridgerator compressor to vacuum pump and had to buy a small commercial version in the interim.

I have so many questions about these freezer compressors that I don't know where to start.

How did you convert yours?

H Tris.


Just cut the copper tubing with a hacksaw to vent the refrigerant (you wicked puppy!), cut the wires from inside the fridge and unbolt the compressor.

You will have to short out the wires that came from the thermostat, but that's not hard to figure.

Leave the compressor running for a few hours to purge it of lubricating oil, and there you have it... either a vacuum or a source of compressed air. The air might need a filter to catch entrained oil.

I've done toluene-based Dean & Starks that boiled from the heat of my hand.

[Edited on 1-3-2009 by Paddywhacker]
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[*] posted on 1-3-2009 at 01:42


I have a water aspirator, an aspirator pump, two, two vane mechanical pumps. My "high vacuum" requirement is 0.5 mm Hg. While an oil diffusion pump would help me get past some leaky connections, it is cheaper to track down the leaks and seal 'em up.
Cheers,
CRX




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[*] posted on 4-3-2009 at 13:03


Quote:
Originally posted by watson.fawkes
You could use the same basic design with an oil designed for a diffusion pump. It's not as dense, so your loop should probably be taller, but the same tuning principles apply.


If I'm not mistaken, your loop will need to be something like fifteen times taller. Between that and the viscosity of the oil, it's not at all clear to me that this could be made to work.

It might be possible to make it work with indium-gallium eutectic, but that sticks to glass, and would probably be quite messy. It's also a lot more reactive than mercury, which could be a problem depending on what you're pumping down. And, of course, it's many times more expensive.
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[*] posted on 7-3-2009 at 05:26


Quote:
Originally posted by chemrox
I have a water aspirator, an aspirator pump, two, two vane mechanical pumps. My "high vacuum" requirement is 0.5 mm Hg. While an oil diffusion pump would help me get past some leaky connections, it is cheaper to track down the leaks and seal 'em up.
Cheers,
CRX


A diffusion pump couldn't keep up with a leak anyways rox, they are for molecular scavenging to micro torr vacuum, and always require a good two stage roughing pump on the front end.
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[*] posted on 5-9-2009 at 14:08
Air compressor into vacuum pump


Here's a neat and tidy mod which claims to produce a 25 inch mercury column.
If run in tandem with the oulet to an aspirator it should pull near absolute vacuum.

" convert a tire inflator-type air compressor into a vacuum pump "

http://www.instructables.com/id/convert-a-tire-inflator-type...
http://www.instructables.com/id/SKBL98OF23Z39WW
http://www.instructables.com/id/SU2KRUEF23Z39XI
http://www.instructables.com/id/SA9XE0TF23Z39Y0
http://www.instructables.com/id/SQ9X01JF23Z39Y7
http://www.instructables.com/id/S1E11W9F23Z39YB
http://www.instructables.com/id/SGA4GN7F23Z39ZU
http://www.instructables.com/id/SRHI634F23Z3A00
http://www.instructables.com/id/SCP3YEGF23Z3A07
http://www.instructables.com/id/SWI4ACRF23Z3A0G

.
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Eclectic
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[*] posted on 5-9-2009 at 16:56


or you can pick up a vacuum pump cheap at Harbor Freight.

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnu...
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