New Turbomolecular Vacuum System - EXT75iDX

Every fusor and fusion system seems to need a vacuum. This area is for detailed discussion of vacuum systems, materials, gauging, etc. related to fusor or fusion research.
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Nathan Marshall
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New Turbomolecular Vacuum System - EXT75iDX

Post by Nathan Marshall »

Hello all,

I've been working on a new high vacuum system based on a turbomolecular pump. This is intended to be a general purpose high vacuum system for numerous experiments. I took a scientific glassblowing class this past semester, and I've been highly interested in making glass to metal seals for vacuum tubes. I am also interested in ion gun experiments.

The roughing pump is my trusty Edwards E2M2. It is connected to a KF-25 T-flange through a valve. One arm of the flange is connected to an Edwards APG-M Pirani gauge to monitor the roughing pressure, and the other goes to the outlet of the turbo pump. The turbo is an Edwards EXT75iDX, a cute little 60 l/s pump that I got a good deal for on eBay.
The whole system
The whole system
Roughing pump connection
Roughing pump connection
The T-flange and Pirani gauge
The T-flange and Pirani gauge
Turbo pump mounted underneath the table
Turbo pump mounted underneath the table
Inlet connection above the table
Inlet connection above the table
The outlet of the pump is an ISO-63 flange, and I adapted it down to KF-25 for convenience on the high vacuum side. I'm not planning on attaching large volumes to this system, so I'm fine with the loss of pumping speed. The system roughed down to a pressure of about 20 microns according to the Pirani gauge, and I'm sure it would drop lower as the water is pulled out of the system over time. I interfaced with the turbo electronics using a DB-15 breakout connector. Power was supplied with a cheap 24 VDC switching power supply from Amazon. By using another DB-15 breakout connector and a Serial-to-USB cable, I was able to control and monitor the pump with a serial connection via PuTTY on my laptop. The pump reaches its top speed of 1500 Hz in a little under two minutes. The high pitched hum of the turbo gets cyclically louder and softer as it spools up through different resonances. It settles at a moderately loud high pitched hum which is still quieter than the roughing pump. I was worried that the hum might indicate worn bearings, but the power consumption of the pump was 8-10 watts at full speed and it took 24 minutes for it to come to a complete stop. I think this indicates that the bearings are in decent shape and not about to blow. The hum might just be a mild resonance. I've also read in previous posts that these smaller Edwards pumps can be noisy right out of the box.

The roughing pressure rose to about 30 microns while the pump was at full speed. I have no reliable high vacuum measurement at the moment, so that is my next goal for this system. I had the inlet blanked off with another valve that I had on hand. Not an ideal blanking situation, but I didn't have another KF-25 blank. Does anyone have any objections to me leaving the valve on the inlet and simply attaching the rest of my vacuum system on the other side? Having the ability to shut off the flow into the turbo would be convenient and a good safety measure when sealing off glass tubes. I am not sure if these valves are meant for roughing connections only or if they are alright for high vacuum as well. I'm open to any other critiques on the current setup.
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Joe Gayo
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Re: New Turbomolecular Vacuum System - EXT75iDX

Post by Joe Gayo »

Have you ever tried to drink a milkshake through a coffee stirrer straw?
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Nathan Marshall
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Re: New Turbomolecular Vacuum System - EXT75iDX

Post by Nathan Marshall »

Point taken! Perhaps a less severe adaption ratio would be better in the long run. However for my vacuum tube experiments the tubes will be attached through a tiny glass stem so the adaption ratio doesn't make a huge difference. The stem will be the bottleneck.
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Richard Hull
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Re: New Turbomolecular Vacuum System - EXT75iDX

Post by Richard Hull »

A rather clean and simple rig. It should be OK for your purposes if you can keep it well sealed without opening to air a lot. Thanks for sharing.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Finn Hammer
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Re: New Turbomolecular Vacuum System - EXT75iDX

Post by Finn Hammer »

Nathan,

The reduction to KF25 is a minor problem on so short a distance. I did the same thing on my pumping station, and it comes down to xE-6 in no time.

But with the kind of jobs you have in mind, please be aware that you could benefit from an addition to your setup.

Suppose you have evacuated some fancy glass container connected to the TableTopValve (TTV) at the tabletop. Now you want to pump down another vessel and you close the TTV and switch vessels.

What can you do next?. If you open the TTV, the turbo will be exposed to the atmospheric pressure within this vessel, which it wil not like, and even if you open the valve *VERY* slowly, the turbo will see a pressure rise that it will not like, so you need another couple of valves, to isolate the turbo, while you rough down the vessel.

You can see some solutions to this challenge
here:
viewtopic.php?f=10&t=13708
and here:
viewtopic.php?f=18&t=13936 where you can also see a graph of successive pumpdowns done in short order.

The foreline pressure holding vessel is a great addition, it comes highly recommended, because once the low pressures are established, you can turn off the roughing pump for periods of time, and work in silent bliss. Your pump has a drag stage, and could benefit from a diafragm pump, but anyway, it will accept foreline pressures up to 5-10 torr and once pumped down to 20 microns it will take a long time to reach 5 torr.
I envy you the glass blowers class.


Cheers, Finn Hammer
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Joe Gayo
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Re: New Turbomolecular Vacuum System - EXT75iDX

Post by Joe Gayo »

@Finn

Just so you are aware - "The reduction to KF25 is a minor problem on so short a distance." is not correct when it comes to conductance reduction. That valve is a massive loss in conductance and still would be if it was a thin orifice plate.
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Richard Hull
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Re: New Turbomolecular Vacuum System - EXT75iDX

Post by Richard Hull »

When I designed fusor IV setup in 2004, I allowed for a technical vacuum take-off and valving as part of the setup for GM manufacture and other vacuum related experimental projects where 10e-6 was just not needed. 10e-3 torr was just great. I attach an image of the setup. The two valves isolated the "micro-maze" which allowed the mechanical pump to come close to 10e-4 torr at the connecting valve. Isolation was demanded as the need to use the heater built into the micro-maze to release all the evils it captured to be evacuated on occasion via the 5 CFM Precision mechanical pump. Thus, one more valve was needed to keep the, then, diff, and now, the turbo pumps in the high vac lines clean.

Richard Hull
Attachments
IV const 04 (9)anno.jpg
IV const 04 (27) anno.jpg
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Finn Hammer
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Re: New Turbomolecular Vacuum System - EXT75iDX

Post by Finn Hammer »

@Joe,

You are the master, I am the student. Of course you are right, thanks for reminding me about that.

Cheers, Finn Hammer
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Maciek Szymanski
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Re: New Turbomolecular Vacuum System - EXT75iDX

Post by Maciek Szymanski »

For me the easiest way to think on the effect of the restrictions in the high vacuum system is the "hole to the perfect vacuum system". Just consider the turbo or diff pump as the perfect vacuum space of unlimited size. The pumped volume is connected to this space through some conductance. The pumping speed and ultimate pressure will be just the function of the conductance and the volume and outgass/leak rate of the pumped volume. In fact the pump throughput do not matter - the pump inlet is usually sized to match the throughput so it is possible to reach the nominal throughput only when the pump is connected without any restrictions. For other hand the low volume, low outgassing vessels (like glass vacuum tubes) can be pumped with very small ducts, especially if provided with some kind of getter to reduce the outgassing.
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland
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Richard Hull
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Re: New Turbomolecular Vacuum System - EXT75iDX

Post by Richard Hull »

Yes, for fusor work, choking a 3" or larger turbo or Diff pump throat to 1.25-inch C.F. 2.75 common piping is no big issue.

The typical choker we see is the neophytes with their first mechanical pump run a 3 foot length of 1/2" I.D. plastic tubing to their first chamber effort. Maciek's comment about tiny volumes (vacuum/GM tube) being rather free from simple restrictions is true. Large volumes to be pumped to 10e-5 torr or better demand rather unrestricted high conductance flow. That last little bit of deep pumping between the item pumped and the throat of the diff or turbo is strictly done by ricocheting molecules in Brownian motion. They just as soon ricochet back into the chamber as towards the pump at some point in any pump down by any means, regardless of conductance. At the deepest levels far beyond what we will ever do, gettering of some sort is the only way to go lower. 90,000 RPM turbo blades can only "whack so many molecules" until they are so scarce that any form of pumping just, effectively, ceases. In the end, it is up to the vacuumist as to what they call a real vacuum. One man's vacuum is another man's horrid gas load.

We often laugh at the guy who reports he has a 29-inch vacuum. Others in the big biz end of vacuum laugh at us working a 10e-4 torr as a base level vacuum to start admitting deuterium.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Nathan Marshall
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Joined: Wed May 08, 2019 8:13 pm
Real name: Nathan Marshall

Re: New Turbomolecular Vacuum System - EXT75iDX

Post by Nathan Marshall »

Thanks everyone for the useful comments and suggestions on this system! Finn, I intend to rig up the system with another valve to connect the roughing pump to the high vacuum side for quick cycling like you mentioned. It will be very nice to be able to cycle the high vacuum without having to spin down the turbo each time.

I finally had some time this past weekend to measure the high vacuum with a Granville-Phillips 343014 Mini Ion Gauge that I purchased from eBay. I also got a couple more bellows valves, so I traded out the constriction-inducing straight valve for a bellows valve. I interfaced with the gauge through a DB9 cable and breakout board.
The vacuum gauge attached through a bellows valve
The vacuum gauge attached through a bellows valve
Model number information
Model number information
The gauge couldn't be simpler to use! It requires 24VDC to operate. After connecting the ON/OFF pin to ground with the gauge under vacuum from the turbo, the analog output from pin 3 indicates the vacuum. It goes from 0-10V and logarithmically indicates the pressure with 0 volts (theoretically) corresponding to 1E-9 torr, 1 volt = 1E-8 torr, 2 volts = 2E-7 torr, etc. With the turbo at full speed, the gauge initially showed 4.2 volts indicating a pressure of about 2E-5 torr. The pressure dropped to the xE-6 range only 30 minutes after turning on the turbo. After allowing the system to pump for four hours, the analog output read 3.8 volts corresponding to 6E-6 torr. I am satisfied with this considering that the chamber was open to atmosphere prior to the run! I'm sure the pressure would continue to drop if I let things outgas longer. However, a pressure in the XE-6 range is sufficient for my vacuum tube experiments already.
Graph from the gauge manual indicating pressure vs. voltage
Graph from the gauge manual indicating pressure vs. voltage
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