Vacuum Chamber Construction

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Mark Rowley
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Re: Vacuum Chamber Construction

Post by Mark Rowley »

A+ in my book. Here’s why.
You are not a professional welder but your effort and extreme focus are far beyond 99.99999% of most people here who attempt this type of work on their own. ESPECIALLY at your age! Bravo to the extreme!

In the realm of amateur work (which is what this group is about) you get top scores.

Mark Rowley

Ps...yes, there’s always room for improvement. But that’s true for even the best cut diamond in the history of human existence. Doesn’t mean an A+ isn’t warranted.
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Nicolas Krause
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Re: Vacuum Chamber Construction

Post by Nicolas Krause »

I am now working on the pumping system of the fusor. In addition to the previous vacuum sensor and uC board I've mentioned earlier I also have a surplus vacuum pump and turbopump without controller. I haven't turned on the vacuum pump since I bought it, nor have I monkeyed around with the turbopump. At this point I plan on working on the vacuum sensor circuit, and then building a controller for the turbopump. At that point I can start to troubleshoot the complete vacuum system.
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Rich Feldman
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Re: Vacuum Chamber Construction

Post by Rich Feldman »

Nice looking sock there.

How 'bout starting with vacuum gauge, then characterizing the rotary vane (backing) pump?

Scratch-building a turbo controller would be a learning exercise much bigger than making and welding your chamber parts.
You can go a long way without it.

I would plumb up the chamber, with or without turbo pump, and learn the pumpdown behavior.
Then light a plasma using any old HV power source.
I don't remember whether NST-powered plasmas typically extinguish at pressures attainable with rotary pump (50 micron? 10 micron?)
They sure do in pencil-size glass tubes.
All models are wrong; some models are useful. -- George Box
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Nicolas Krause
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Re: Vacuum Chamber Construction

Post by Nicolas Krause »

Funnily enough I feel a lot more comfortable on the electronics side than the machining and welding side. I've got an electronics technologists diploma, I'm 3/4 of the way through an electrical engineering degree, and I've done some embedded programming for a small oil company up here in Canada. I think like a lot of people I initially joined up here hoping to be the one to crack the fusion puzzle, but reality sets in and instead this project has turned into a tremendous self-education tool. While I think building a motor controller would be tough, it's exactly the kind of thing I'd like to learn how to do.

All that being said, I think I'm going to take your advice Rich and test the backing pump with my vacuum gauge first, then I can move on to the turbopump controller.

I do have a couple options for high voltage supplies, I bought a high voltage transformer a few years back and I've got a busted spellmann supply I picked up off of eBay last year. Finally the quickest route might just be to grab one of those precipitator supplies off the internet.
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Mark Rowley
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Re: Vacuum Chamber Construction

Post by Mark Rowley »

Nicolas, the precip supplies work ok with the 2.75" system but will not put out the current needed for a large spherical arrangement like yours. Theyre cheap enough and it could be worth a try, just dont expect much from them with your arrangement. They may be ok for plasma / vacuum testing as long as you keep a sharp eye on the current draw.

Rich, it's been my experience to have NST driven air plasma extinguish well within the range of a decent roughing pump. Going below 35mTorr things quickly get flickery.

Mark Rowley
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Re: Vacuum Chamber Construction

Post by Rex Allers »

from Nickolas:
"All that being said, I think I'm going to take your advice Rich and test the backing pump with my vacuum gauge first"

Yes indeed. One step at a time when possible and that is a basic good one. What kind of gauge have you?

If your gauge is good and on the pump input that sets the best you can expect at the pump.

I'd use just that one pump first onto your chamber to see if you can prove there are no big leak problems. If things look good seal the chamber off and see how it holds the max vac you achieved. That should define any minor leak issues.

Last, go on to add the secondary pump (Turbo or diff). At lower levels outgassing issues might look like leaks.

Issues along the way need to be worked on or thought through. You are doing great.
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Nicolas Krause
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Re: Vacuum Chamber Construction

Post by Nicolas Krause »

Hi Rex,

I have two, a pirani and a thermocouple. I'm going to start with the thermocouple, build a circuit and write a little program to measure the pressure and update a spare LCD I've got lying around. As far as my vacuum pump. there are a couple issues, first it has no electrical chord atm, and second its leaking a decent amount of oil from the black tube out its back. The pump is a leybold trivac D4A, I've managed to find a copy of the manual online, so I'll go through that next.
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Re: Vacuum Chamber Construction

Post by Rex Allers »

Re:
" its leaking a decent amount of oil from the black tube out its back"

That sounds unusual to me. Not sure what to make of the black tube or this leak of oil. Not something I would expect from a normal working pump.

Key, I guess, is what vacuum you get at the input of the mechanical pump. Second, does it have other issues -- like: what's with this oil you mentioned.
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Richard Hull
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Re: Vacuum Chamber Construction

Post by Richard Hull »

Is this leak in between the motor and the back of the pump body? If so, you have a shaft oil seal leak.
A year ago I purchased an Edwards speedivac 2 at a yard sale for $5.00. It leaked oil, copiously, at the seal. I finally got around to ordering the oil seal ($32.00 - Duniway). Installed it and cleaned the fairly clean looking guts. It had a decent vacuum to the thumb over the inlet once new oil was in it. After the fix, I had zero oil leak. Unfortunately, it would not indicate on a TC gauge. So, I hooked up to the normally shunned, but telling 0-30-inch gauge and it would pull 26-inches!!! (~40 torr!!). I was hoping for at least 20 microns, (0.02 torr). I now have a perfectly sealed, worthless pump. Oh well....

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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Nicolas Krause
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Re: Vacuum Chamber Construction

Post by Nicolas Krause »

After reading parts of the manual and inspecting the pump visually the leak appears to be from one of the oil drain ports. So it may not be a leak after all! A black rubber tube is hooked up to a brass fitting and it is this tube that the oil is draining out of. I'm not sure if anyone can explain the reason for this hookup, in the manual it shows a screw in the drain port. I had to put a new power chord in place, the old one had been chopped off. I'll wait to put the cover for the power chord connection back on until I've ordered a strain relief plug, the current one I have is too small for the chord in question. After making the connections I've plugged it in and the pump turns on and appears to work without too much trouble! I'll have to complete my vacuum gauge circuit next to characterize the pump.
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Richard Hull
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Re: Vacuum Chamber Construction

Post by Richard Hull »

It is obvious that the pump was in a place or position where the pump would not allow for easy draining to replace oil. The tube will go to a drain pan at a lower, convenient location. The hose would have a petcock that could be opened to drain the fouled or old oil.

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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Nicolas Krause
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Re: Vacuum Chamber Construction

Post by Nicolas Krause »

Thanks for the explanation Richard, I do have another question. I'm missing a small number of vacuum parts for the hookup to my chamber, a couple clamps etc. that I plan on ordering soon. Once those parts arrive, if I put my turbopump in series with my backing pump, and leave the turbopump off, can I just turn on the backing pump without causing any problems? Having read a number of other threads I know that if I turn on the turbopump without the backing pump operating I can do some serious damage, but if the turbopump is off will it just act as a bit of additional tubing?
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Richard Hull
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Re: Vacuum Chamber Construction

Post by Richard Hull »

Absolutely! Same goes for a diff pump. It is just another "hanger-on" in any vacuum system if it is not turned on. The proper way to work a system is to open all valves and pump the entire vacuum system down as low as the mechanical pump can go. (hopefully at or below 50 microns) Once at fore-line vacuum, the secondary pump (turbo or diff pump), is turned on and you are off to the races from there.

All of the above assumes that your system is sealed and tested. During testing, a stepped assembly process is demanded to be sure you are sealed at each step. This can be a tedious process. Newbies tend to assemble the entire system and then wonder why they can't get a good vacuum as they now have to hunt for issues and leaks.

System assembly absolutely demands a TC gauge at minimum!!! In the last stage a deeper reading gauge on the reactor vessel would be nice, leaving the TC gauge in the fore-line permanently. This means you need a lot of blank offs, clamps and adapters if seeking to do a professional level job of it.

Note: Valves in a vacuum system are more for isolation, shut-down and compartmentalized tear down or modification of the system without fouling the entire business if one component needs replacement or a new one added in the middle of the system. There is no such thing as too many valves.....(if you can afford good ones.)

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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Nicolas Krause
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Re: Vacuum Chamber Construction

Post by Nicolas Krause »

Earlier this week, my 8020 frame arrived. I had to wait a few days for my M8 Tap to arrive, but on Friday it did and I've completed tapping the ends of the extrusion so it can all be put together. I plan on mounting the vacuum chamber in the center of the frame.
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John Futter
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Re: Vacuum Chamber Construction

Post by John Futter »

Nicolas
I would srongly suggest you do a pressure test on your chamber first.
Ie blank off everthing and feed shop air at 15psi (no more)into your chamber through your gas feed line
Use 50:50 detergent and water and go round every weld and vacuum joint with a small paint brush dipped in the detergent water mix and look for foaming or bubble formation
this method far quicker than trying to use vacuum in the first instance
only when you pass this stage put the chamber under vacuum
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Richard Hull
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Re: Vacuum Chamber Construction

Post by Richard Hull »

As the whole device is SS, I would pressurize it to 30 psi and put it in a 5 gallon bucket of water. Have a grease pencil handy.

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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Nicolas Krause
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Re: Vacuum Chamber Construction

Post by Nicolas Krause »

Thanks for the suggestion John, I'll definitely look at doing that. Currently in an apartment in the great white north, so I'll have to poke around and see if I can find a compressor of some sorts.
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Rich Feldman
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Re: Vacuum Chamber Construction

Post by Rich Feldman »

Add a Schrader or Presta valve stem adapter to one port, e,g. gas inlet hose.
Then use a bicycle tire pump, or haul the chamber over to some gas station or fabrication shop.
All models are wrong; some models are useful. -- George Box
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Nicolas Krause
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Re: Vacuum Chamber Construction

Post by Nicolas Krause »

I'm currently trying to tap a blank for my vacuum chamber, to use as a port for my thermocouple gauge and also for the pressure test. Based on Rich's suggestion I found a 1/8 NPT to schraeder valve fitting that will allow me to hook up my bicycle pump and look for leaks. I used my Machinery's Handbook and grabbed a 5/16 drill bit for the initial hole which went fine. However trying to tap the hole was a no go. My tap would not bite, and looking at the tap itself it says a "Q" drill bit must be used to drill the initial hole. I assume I was unable to start the tap because the hole I drilled was too small, but I believe I've correctly confirmed for my thermocouple gauge that is a 1/8 NPT male fitting. Have I made a mistake somewhere? Did I select an incorrect tap?
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Rich Feldman
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Re: Vacuum Chamber Construction

Post by Rich Feldman »

I've tapped plenty of 1/8 NPT (27 tpi) threaded holes, but don't remember the starting size. 5/16 seems kind of small. Isn't the OD of the pipe, at big end of taper, 0.405"? Consider a tapered reamer with T-handle to adjust your starter hole. Could always practice in a scrap of aluminum or brass.

Schrader valves are held shut by a spring, of course, and open when filling attachment depresses the pin. I think the spring is plenty strong to resist suction when vacuum is inside. But consider the reason metal valve caps with o-ring seals have been available since 19th century.
All models are wrong; some models are useful. -- George Box
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Re: Vacuum Chamber Construction

Post by John Myers »

An online tap & drill calculator shows it should be 21/64 (.3281) which is close to the Q drill bit size (.3320)
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Rich Feldman
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Re: Vacuum Chamber Construction

Post by Rich Feldman »

Yes, drill Q seems to be what they say, except McMaster-Carr's economy carbon steel tap calls for drill R (0.339). If you go a little large and don't get full height internal threads at the small end of taper, would a bad thing happen?

Machinists argue about the value of tapering the hole before tapping pipe threads.
https://www.practicalmachinist.com/vb/g ... ap-151341/
With a straight hole, the big end of the tap does a lot more work than the little end.
I like to use a tapered reamer from the hardware store, with a T handle.
All models are wrong; some models are useful. -- George Box
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Nicolas Krause
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Re: Vacuum Chamber Construction

Post by Nicolas Krause »

Thanks for the clarification! I'd seen Q and R mentioned as well online, but had decided to go with the 5/16 since if it was a mistake I could always make the hole bigger!
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Nicolas Krause
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Re: Vacuum Chamber Construction

Post by Nicolas Krause »

I have some successes and failures to report by way of an update. I've successfully sourced some valves via eBay for my fusor, a ball valve is now attached to my roughing pump and I've obtained a manually operated butterfly valve. With regards to the butterfly valve, the lip for the seal seems different from the knife edge I've got on all my other conflats. Is this just due to a manufacturing difference or does it mean it can't take a copper gasket?
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Secondly I've successfully tapped a blank and put in a bike valve. It took a bit of work but pleased with that effort.
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Finally the failure, while assembling the chamber for a leak test, I accidentally cross-threaded two bolts on one of my half nipples. This is the only nipple on the chamber that has threaded holes, I purchased it at a cheap price off of eBay, all others were ordered new and didn't have threaded holes. While trying to remove the cross-threaded bolts both snapped. I'm now wondering what the best course of action for removing them is and repairing the damage. Can I just drill out the holes with a slightly larger drill bit and remove the threads? Or is it better to get a tap and re-thread them?
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John Futter
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Re: Vacuum Chamber Construction

Post by John Futter »

Take it to a tool making house that has a spark eroder
my local one charged $25 per bolt removal and they did such a good job that putting a tap back down the hole removed no material and the correct bolts worked fine mixup between 5/16 and 8mm bolts
you now need to buy a tub of no sieze grease i prefer the nickel based one as the copper one is a nasty color
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