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LAN_403

Physikfan

Mon Apr 24 2017, 04:48PM

Registered Member #60240 Joined: Mon May 16 2016, 07:01PM
Location:
Posts: 304

I have built up a vacuum system for gas discharge tests.
It consists of a two-stage rotary vane pump Leybold D2A, with 240 V single-phase, with new oil filling, the oil change was not a pleasant work.
The previous minimum pressure is controlled by the two electronic manometers 4 x 10 ** - 2 mbar after approximately 20 minutes of operation of the pump.

P1030980400x306

Can someone give me a hint how I could get a better vacuum?

Proud Mary

Mon Apr 24 2017, 06:58PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Hello Physikfan! :)

1) If you want to evacuate a glass vessel, you could place a getter inside, and then activate it when you have got to the lowest pressure that you can get with the pump. Fox example, if you put some magnesium or calcium metal on the inside of the glass, when you heat it from outside, the metals will form their oxides, and so remove oxygen from the gas remaining in the system. This is what they do with thermionic valves (US: tubes)

2. If you first flush your system with helium balloon gas, and then evacuate it, you won't get a lower pressure, but you will get a longer electron free path useful in many experiments

johnf

Mon Apr 24 2017, 07:24PM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

That pump should do better than that
Have you gas ballasted the pump for half an hour
When you do you should use an oil mist trap or vent the pump outlet outside.
the pump is working well enough to back either a turbo or diff pump that would get you to 10^-6 millibar easily

klugesmith

Mon Apr 24 2017, 07:28PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

In a chamber that size, 4e-2 mbar (a.k.a. 30 microns) with a 2-stage rotary pump should not be a big disappointment.

What happens if you connect the vacuum gauge, and nothing else, directly to the pump inlet?
If you are lucky, the indicated pressure will be substantially lower than you got in the full system.
Then the system might benefit from having a fatter, shorter suction hose. Are you familiar with the "vacuum conductance" of pipes and hoses?

Sulaiman

Mon Apr 24 2017, 11:57PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

a two-stage rotary should get you to mean free path lengths of 10 to 100 mm

for longer free paths, ion and turbo pumps are good, but expensive,
I've not tried yet but a Sprengel pump looks to be the easiest hobby approach

e.g.

johnf

Tue Apr 25 2017, 02:01AM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

By golly
if we where to go with esoteric pumps then the
Toepler pump would be a contender
saw one on a video of home made vacuum tubes by a "french man" i think

Sulaiman

Tue Apr 25 2017, 03:42AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I am currently looking for 1+ m of 2.5mm i.d. glass tubing to make a Sprengel pump, as I have the other requirements,
I just need to order a sufficient range of tubing to justify postage cost of one tube.

After watching the Cody's Lab video I decided to make one for myself,
a dual-stage rotary for roughing and a Sprengel for the lowest pressures.
(I have no way to measure these pressures yet)

As a hobbyist this seems the cheapest.easiest route for me to get vacuum suitable for 4HV-type experimenting.
The only practical Toepler pumps that I have seen require rather large quantities of mercury, and/or glass float valves,
but if anyone knows of a better diy mPa range pump, please tell us.
... the sooner the better for me.

Physikfan

Tue Apr 25 2017, 09:16AM

Registered Member #60240 Joined: Mon May 16 2016, 07:01PM
Location:
Posts: 304

Hi Sulaiman

Please, could you show me the link to
"After watching the Cody's Lab video"

Do you have the possibility to do such experiments outdoors or in a glovebox?
As you may know, very small, almost invisible mercury globules can spoil the air in a closed room for years.
As a teenager I once had a mercury poisoning through a broken fever thermometer in the bedroom.

Physikfan

Tue Apr 25 2017, 09:42AM

Registered Member #60240 Joined: Mon May 16 2016, 07:01PM
Location:
Posts: 304

Another point:

The limiting factor of such pumps is the vapor pressure of the liquid media.
Normal vacuum pump oil about 10 ** - 3 mbar
Mercury at 20 Â° C also about 10 ** -3 mbar

Physikfan

Tue Apr 25 2017, 09:56AM

Registered Member #60240 Joined: Mon May 16 2016, 07:01PM
Location:
Posts: 304

Hi Johnf

Please could you give me the link to
"Toepler pump would be a contender
saw one on a video of home made vacuum tubes by a "french man""

Regards

Physikfan

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