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Author: Subject: Vacuum pump suggestions
Raptureisbliss
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[*] posted on 21-12-2011 at 16:22
Vacuum pump suggestions


For those who have invested in a vacuum pump, what type do you use and how much did you pay for it?
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overload
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[*] posted on 29-12-2011 at 18:21


Harbor Freight. This has already been discussed.
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entropy51
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[*] posted on 29-12-2011 at 18:36


Precision Scientific model D-25. Cost $150 (in 1970). It still pulls less than 1 millitorr. With a small oil diffusion pump it pulls 1 X 10^-6 Torr. If you see one on eBay, buy it.
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Magpie
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[*] posted on 29-12-2011 at 22:06


Quote: Originally posted by entropy51  
It still pulls less than 1 millitorr. With a small oil diffusion pump it pulls 1 X 10^-6 Torr.


What type of gauge do you use to measure such low pressures?




The single most important condition for a successful synthesis is good mixing - Nicodem
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watson.fawkes
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[*] posted on 30-12-2011 at 06:25


Quote: Originally posted by Magpie  
What type of gauge do you use to measure such low pressures?
You can use an ionization gauge to measure pressure in that range, although it will go even lower. It's a kind of triode whose emitted filament current is interfered with by residual gas. It has to operate in the ballistic flow region to be accurate, so it's rarely the only gauge in a system.
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jamit
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[*] posted on 30-12-2011 at 06:29


eBay.
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peach
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[*] posted on 30-12-2011 at 09:41


I have a Leybold Penning cold cathode ionisation gauge head around here (somewhere) if you're in need of one magpie. It's definitely here, since I know I haven't sold it, but it's playing hide and seek.

[Edited on 30-12-2011 by peach]




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[*] posted on 30-12-2011 at 14:24


Thank you peach for that generous offer. But I was just curious what instrument(s) entropy was using for his low pressure work.

Here's the reason I ventured into the world of low pressure measurement: Previous to my fractional distillations of fatty ethyl esters I didn't have much interest in measuring low pressures (10-50mmHg) accurately. But to perform accurate separations of these fatty esters one must know the distillation pressure accurately, say within +/-2 mmHg.

I have a Bennert manometer which by all accounts should serve this function well. The one I bought from eBay came without mercury, but in all other respects was in excellent condition.

I bought some mercury from Elemental Scientific and proceeded to fill the manometer per the instructions on the Ace Glass website. This requires evacuating the manometer to a "hard vacuum." Using an Inficon "Pilot Plus" (pirani) gauge I determined that my "hard vacuum" was about 0.2mmHg. However, after filling the instrument twice, I still end up with about a 3mmHg "pre-load." This shows up when a hard vacuum is pulled on the Bennert and it reads -3mmHg. If no preload exists the Bennert should never read less than 0mmHg, a perfect vacuum.

There are several scenarios that I can imagine that may be the cause of this:

1. My Inficon pirani gauge is not reading accurately (not likely).
2. My Bennert glass was (is) contaminated.
3. My mercury was (is) contaminated.

I have learned to live with this preload. I have developed a quadratic equation which tells me the amount of pressure correction I have to add to each reading. For example, at a reading of -3mmHg I add 3mmHg. At a reading of 36.1mmHg, I add 3.9mmHg.

I could always attempt a cleaning of the mercury and the manometer. But there is a certain amount of effort and risk associated with this.




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entropy51
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[*] posted on 30-12-2011 at 17:19


Quote: Originally posted by Magpie  
Quote: Originally posted by entropy51  
It still pulls less than 1 millitorr. With a small oil diffusion pump it pulls 1 X 10^-6 Torr.


What type of gauge do you use to measure such low pressures?
I have a Hastings thermocouple gauge good to about 1 millitorr.

For lower pressures I use an ionization gauge driven by a Granville Phillips Model 280 controller. The controller cost about $ 1K. The guage tube is something over $100 IIRC.

Most of my good vacuum equipment was purchased from Dunniway Stockroom.
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[*] posted on 11-1-2012 at 01:20


The name "back pump" is sometimes used to refer to the mechanical piston pump or other type of pump used for developing a pretty excellent vacuum but not condition of the art.




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