Single-digit micron pressure in plastic belljar?

[Rich Feldman]

In my volunteer work for a science workshop program, I'm upgrading their vacuum apparatus. Hope to get down to single-digit micron pressures in a plastic bell jar, to support a couple of kids's science fair projects. Last time I worked with vacuum was in the 1970's.

The intended setup is organized like http://www.belljar.net/manifold.htm , with these enhancements:

1. Pump is a donated Alcatel 2015 with KF-25 fittings. Rated flow 5 liters/second, with deadhead vacuum pressure 1e-4 torr. I hope it's in good condition inside.

2. Manifold is made from nominal 1/2" brass pipe nipples & a 1/2" ball valve. (We found a relatively good deal on KF-25 to 1/2" F-NPT adapters). I figure the conductance will be around 2 liters/second, but a wider path would need more money and/or volunteer hours. No TIG-welded stainless steel fabrication this year, unfortunately. Did consider soldering 3/4" Cu pipe to brass KF-25 half nipples, after removing 0.015" interference with a lathe.

3. Hope to use their stock Nalgene classroom belljar and ring gasket, with replacement baseplates. Each kid will get a baseplate & add his own feedthroughs. My plan of record is to use 1/2" hard plastic or 1/4" aluminum, about 8 inches square (belljar OD is about 6 inches), with a hole tapped for 1/2" pipe thread. A short nipple and KF-25 flange, loaned by the science workshop program, complete the baseplate.

General comments & advice would be welcome.
Specific questions:
*Plastic material recommendations? Would be nice if feedthroughs can be bare metal through hole sealed with appropriate "glue".
*Is there a handier way to adapt KF-25 hardware to a homemade baseplate?
*How to retain flat belljar gasket against suck-in? Does baseplate need ridges, roughening, or adhesive? Is neoprene sheet (e.g. from McMaster) suitable?

Thanks. I appreciate your work here; did my searching & reading homework.
Rich

[Chris Bradley]

My suggestions, for what it's worth;

Rich Feldman wrote:
> Hope to get down to single-digit micron pressures in a plastic bell jar
You're at the mercy of the condition of the pump. I managed it with an E2M2 and much worse material than Nalgene

> *Plastic material recommendations? Would be nice if feedthroughs can be bare metal through hole sealed with appropriate "glue".
D'you mean electrical feedthrough, or gas? What voltage?

> *Is there a handier way to adapt KF-25 hardware to a homemade baseplate?
Drill a small hole, 10mm or so, and just hold the KF25 and a sealing ring against the baseplate until the vacuum holds it in place. You may have to 'take the weight' off the hose. Or just get some adapters, KF25-to-reguar hose diameters.

> *How to retain flat belljar gasket against suck-in?
If you're cutting your own from sheet rubber, just cut the hole and the sheet will stay put.

> Is neoprene sheet (e.g. from McMaster) suitable?
the material is OK, but depends somewhat on thickness, hardness and smoothness

> Does baseplate need ridges, roughening, or adhesive?
No. Make it smooth.

[Rich Feldman]

Chris Bradley wrote:
> My suggestions, for what it's worth; ...
Thank you.

> > *Plastic material recommendations? Would be nice if feedthroughs can be bare metal through hole sealed with appropriate "glue".
> D'you mean electrical feedthrough, or gas? What voltage?
Some electrical, maybe up to a few 100 volts. One kid want to run an electron gun from a CRT and follow the e-beam in gassy space. I'm familiar with Paschen's Law. All HV will be current limited. We might want something to coat exposed wires, in cases where they would otherwise spark or glow like Nixie tube cathodes.
Gas backfill will go through a port in the manifold. One project will have a low speed rotary motion feedthrough (hand cranked).
I am going to try PVC or other hard white plastic from a store like Tap. Clear acrylic is a bit harder to fabricate, but would let us see inside a metal vacuum pot.

> > *Is there a handier way to adapt KF-25 hardware to a homemade baseplate?
> Drill a small hole, 10mm or so, and just hold the KF25 and a sealing ring against the baseplate until the vacuum holds it in place. You may have to 'take the weight' off the hose.
Great idea, thank you again. And I remember seeing KF "bulkhead clamps": two C-shaped parts that attach to plate with screws -- could use small wingnuts on fixed studs.
Along the lines of connection kludgery: My first new result is from testing a donated, used Convectron gauge/controller with my DV-85 refrigeration service pump. Gauge came with a KF-16 flange, but its tube ID makes a snug fit with the 3/8" OD of a Tygon tube, esp. with a 1/4" brass sleeve inside. WIth the other end of Tygon held onto the brass flare fitting of pump, I got down to an indicated pressure of 45 millitorr. Serendipity in mechanical fitting is a frequent source of delight.

Looks like I should add some anaerobic thread sealant and some vacuum grease to the shopping list. Any recommendations? As a reminder, we'd be surprised & pleased to reach 1e-3 torr.

[Chris Bradley]

You're free to try all manner of plastics and glues, but I'd suggest avoiding anything you'd not be able to subsequently pick or melt off completely in case you just make it worse. 10 microns with a rotary pump on a small chamber should be expected, units of microns will depend on the condition of the pump and you'll be making a meal for yourself if you do pick a 'gassy' material.

For just a few electrical feed throughs, why not do what is common here - an inside out old spark plug. If you're going for low volts, avoid a supression type plug that'll have a resistive core (I hope they still make non-carbon types, else that's a useless comment. 'Fraid I'm not sure, I've driven diesel cars for the last 10 years!!)

Try PTFE tape before any glues. I'd not consider using anything that permament in case you just make it worse and can't back-track - there is a substance called Apiezon Q which is a vacuum putty a bit like blu-tac in consistency.

Hope that gives a few ideas. It'll be a learning experience in itself figuring out what might leak and how to avoid it.

[Doug Coulter]

I have real doubts that you will ever get close to single digit microns in anything of your desired gas with only a mech pump, even if the chamber is SS, and small. You may touch single digits with all the outgassing products alone(water...air, plasticizer) ... at best, and quite rarely. Which is why to run small 2 digit microns in fusors everyone uses at least a diffusion pump to get reasonable purity of the desired gas.

It's not that no one has tried....just that no one yet has been successful that way. No exceptions I'm aware of.

As that includes a lot of smart and resourceful people indeed, not to mention some with quite a lot more vacuum experience than posted on this thread -- I think you'll need to either degrade the spec or get a better pump system. Just my $.02.

[lutzhoffman]

Hello:

Mmmm Nalgene is a pain to glue, so the baseplate is the key, a few added legs may be nice with a 90 deg. angle elbow. One simple method of doing a feedthough for mass-production, like for an entire class is to use "seal screws". These have an o-ring with its own groove built right in under the screw head. So as long as your surface is smooth and flat, they seal pretty well. I use these for my feed through on oil HV projects.

For KF flanges and fittings, rosin soft solder, with Cu pipe and brass KF fittings works fine for me almost down to DP level vacuum. I just remove the excess rosin with acetone if its a problem. I just do not like the acid based flux for vacuum work, any small trace amount left, attract water, and cause a general mess. Rosin has such a low VP, it can be left in place. The old standard home brew HV sealant was simply a mixture of beeswax and rosin. I think the ratio was 3 parts rosin to 1 part wax, it is applied smoking hot with a small brush. Preheating the part a little helps also.


For sealing threads, I have given up on teflon tape, especialy after I discovered last night that it was the source of my pump testing jig leak. I replaced it with 5 min epoxy, and bingo it works great now. Maybe my teflon tape winding technique just sucks, but I will stick with epoxy for now.

If you have to remove set epoxy, methylene chloride based paint stripper cuts it real well. For a large amount you may have to leave it overnight. I use this to expose epoxy potted circuit boards. The next morning I just pick away the jell.

For the rubber pad, silicon rubber works well for me, and you can get sheets of it cheap from the cooking supply places, due to its high temp properties. Its nice and supple, so it seals well without grease etc.

[DaveC]

Hi Richard -

After reading your project plans, (which sound "reasonably likely" to be successful with the pump you have), I have just a couple additional suggestions to those already made.

First is about the base plate and gasket. The flat silicone rubber gasket material is excellent. Get it about 1/8" in thickness. It will need to be a flat ring, about 1/2' or more wide, and be in a shallow recess, the same depth as the gasket thickness, and about 1/16" wider, in order not to slip out from under the jar. This suggests a little thicker baseplate...say 3/8" thick, rather than 1/4". Aluminum is a good choice for this.

Second - the vacuum connection. Unless you really want or need to go with KF fittings, consider a threaded straight tube with a flat flange and O ring in a groove on the underside of the flange. Mount it going through the baseplate from the inside and tighten up a nut on the tube threads... and you have a very good, removable vacuum fitting, that will take standard flexible vacuum hose- either rubber or plastic.

I am assuming you have acess to some machine tools... some schools may still have the metal shop... depends on the area....etc. So you might even be able to get it made "in-house" for only the cost of the material. It could be stainless steel (304 or 308), but I'd recommend aluminum here, too as it's easy to machine...etc etc...

Third - High voltage feedthroughs. This is more involved, but could be as simple as a piece of plastic rod, (polycarbonate is a good candidate) with a machined shoulder, threads, mounting nut and an internal O ring seal, a la the vacuum connection. The electrical connection can be made the same way, with a flange and O ring on the interior side, and a snug sliding fit, (press fit would be better, but that's dicey for a long fitting.) inside the plastic. A nut on threads on the outside, can pull everything down tight, and this will work for modest high voltages.... at least a few kV.

The feedthrough length either side of the baseplate (which is at ground potential) needs to be long enough to avoid flashovers. At the comparatively high pressures you are describing, and as Chris has already noted, you will be tending to go into glow discharge at voltages much above a couple of kV. This will be your limit, irrespective of the feedthrough itself. So the above should work nicely. Remove all sharp edges for best results.

Lastly - sealing compound....there are some commerical paste type sealants that work much better than teflon tape. SWAK is one brand name... But with O rings and flanges, you might not need any of it,.which is even better.

Just some ideas for you to consider....

Dave Cooper

[Richard Hull]

Doug, I have hit single digits with Fusor II and a mechanical pump only. The pumps capacity was way over kill for the smallish volume pumped. I hit those single digits fast, as well. I used a Baratron so the cal was good on the pressure reading.

The key is keeping the system closed and well sealed, use a high conductance short connection and don't leave the pump on so long that backstreaming of hot oil works its way back to the chamber.

I hit 5 microns and could hold it for about 20 minutes max before the pressure began to rise slowly. The addition of a micromaze by Lesker took me submicron and was the total pumping system used in fusor III when I first achieved fusion. the pump and micro maze worked wonderfully.

In fusor IV, I was determined not to lose or waste D2 and went with the diff pump.

97% of all amateurs here cannot ever hit single digits with their mechanical pumps. This is attributed to neophyte blunders, use of old, wornout, surplus pumps, outgassing, low conductance lines, bad seals, etc.

I would consider a first pass amateur, hitting 20 microns with his mechanical pump as having achieved a major coup.

Old "vacuum heads" can hit single digits with good mechanocal pump gear rather easily if they are on a mission to do so. Few have such a mission or goal as they almost always have a diff or turbo pump in the system as well and there is no big bonus or payoff for single micron mechanical backing.

Richard Hull

[Doug Coulter]

I know I never got there in a 3 qt belljar system with a brand new hi cap 2 stage Welch pump, not even close to "non conducting vacuum", and I was not a beginner at the time. The pump was 2 feet away on 1/2" id Cu pipe, and there was one gasket in the whole system, which didn't appear to leak (by the push some gas on the outside to see if you can change the glow color in there method). Fusor type conditions where you can have 10's of kv across two electrodes, no way! The system was glass, leak tight, monitored by the pirani gages I still use, and it said about twice what I'm (and no doubt everyone else) running fusors at after a hot bakeout, at best.

That gas pressure where a fusor runs stable and in control at high voltage is such a narrow range I'd almost suggest it as a calibration point, itself. It is very reliably the same on the same setup at any rate.

I am interested in how you are saving D with a diff pump -- is it the well known effect of these kinetic pumps (which includes turbos) preferentially pumping the heavier gases? I am lucky to have a lifetime supply of D (about 1000 liters at stp) but hey, I'd love to do a better job of not losing the T and 3He I'm making in there. At this point, with the turbo spun down as low as it will allow, and the forepump line throttled a bit, I'm having to use perhaps 5-10 cc per hour flow.

So I may add a conductance valve and spin the doggone thing back up where the differential pumping effect is larger?

[Chris Bradley]

I hit an *indicated juuust-scraping-under-10 microns* with my chamber in viewtopic.php?f=15&t=7222#p49094 in 'empty' trim and a long pump with fresh lower viscocity pump oil, as shown.

(Oh, yeah, worth checking the pump manual as sometimes they give an indication of what specific oils are recommended to achieve min absolute pressure.)

However, since I put in all the experimental 'gubbins' and opened and closed it repeatedly, I don't get much below 50 microns now with just the rotary pump.

[Richard Hull]

When you have only an 8 micron air/oil vacuum and try to do fusion, you have to waste a lot of D2 to arrive at an 8 micron vacuum that replaced all that air/oil or at least, say, 90% of it with fusion fuel. You have to do this every time you start up. Starting at 10e-5 or 10e-6 torr and pushing in D2 to 8 microns you have about a 99.99% D2 environment instantly.

I often, when using a mechanical only, had to push D2 in for 15 minutes to get to my max fusion point. The micro maze would take it to 0.5 microns , ~5X10e-4 torr, but you still had to push D2 in for a few minutes to get the best results.

With a Diff or Turbo, you get instant gratification.

Remember, back in the early days 96-99 I used only a single mechanical pump. I got real good at it too. I was a VCIW (Don Lancaster's term - voice crying in the wilderness.) zero-point-zero amateurs were doing fusor work then.

The micromaze was a mile high advantage and I recognized that, so, when I built fuosr IV I used one of the small diff pumps I had obtained in the interim. Many of the earliest fusioneers who did fusion used only a mechanical pump and maybe a micromaze.

Richard Hull

[Rich Feldman]

Thanks for all the advice. Here's an update, a bit more info, and 1 more question.

1. There's no hard requirement for single-digit micron pressure or particular gas composition - we'll just see if what we get is enough for 8th grade science fair goals. One project is to unspool Scotch tape & detect x-rays -- Camara et al did their published work at 1e-3 torr, and we know it does NOT happen at 1 atm. http://www.wired.com/science/discoverie ... _xray_tape Another is to generate an e-beam visible in the gassy space. This guy made cold cathode CRTs that work at 150e-3 torr! http://www.sparkbangbuzz.com/crt/crt6.htm . I have a lecture-hall "fine beam tube" that makes a nice purple beam with a hot cathode & less than 100 volts anode potential. Not sure what its fill gas pressure or composition are.

We'll see (this is the kid's project, I just provide vacuum) if an ex-TV electron gun can be fired in belljar without burning up its filament. Perhaps the critical weakness of the plan. We will test the waters with some light-bulb filaments in the chamber (the more W mass, the better. I wonder what else might serve as a getter?)

2. When the KF fittings come in we'll try mating our manifold directly to a belljar baseplate
with 1" round hole (that receives one side of KF-25 centering ring) and a bulkhead clamp -
which could be just a big flat washer loose on manifold pipe, with a few bolt holes in a circle. That would reduce the number of threaded connections & expensive flanges.

3. For gasket material: If I can't find smooth silicone sheets at a kitchen or restaurant supply store, or suitable diaphragm material at hardware store, will face a bewildering array of choices at McMaster. Must choose thickness, chemistry, durometer, etc. and can't afford to buy one of each. How 'bout 1/8 inch, silicone or neoprene or buna-n, about as hard as rubber bands or inner tubes?

[DaveC]

Rich -

They sound like some interesting projects.

1. Silicone Rubber from Mc Master is the way to go. Get it soft as a pencil eraser. Hardness is not too critical, anyway. Durometer of 50-70 Shore A, I think.

2. With the CRT electron guns, these are all tri-oxide cathodes, which are quite possibly irrepairably ruined by their first air exposure. So be prepared for that. A good pump down to usually in the -5s, -6s (Torr) may allow you to dewater them safely with some filament heat... bright red to dull orange temps. Remember, these are LOW TEMP filaments - typically 900 -1100 K. Heater voltages are usually 6.3 or 12.6 VAC.

If the output.is abnormally low, a Tri-oxide Cathode, can sometimes be regenerated, once or twice. It's done by heating up to about 2700K with some extraction voltage on... a few volts on the control grid, and perhaps a few hundred on the "anode" which is part of the lens assembly inside....it's the strucutre connected to a wire feeler that makes contact to the "dag" inner coating on the CRT. The so-called 2nd anode lead connects both to the (+) HV. But pressures need to be below -4'sTorr.

One surprise here is sputtering of the control grid ( brightness) G1 onto the cathode which rapidly poisons it, and that's it. Usually G1 sees only negative potentials for this reason. I think you can go with 0V on G1 safely, however.

Bottom line on CRT e-guns, they do not like hydrocarbons, or high temperature, so have few on hand while everyone learns about them.

3. Bare tungsten needs above 2200 K to begin to emit in a serious way. Tungsten with Thorum, or Zirconium oxide, or Cerium oxide alloys emit at lower temps by up to 500K. (I like the W-2%Th best, but it is slightly radioactive and that frightens some.)

Building a test cathode out of simple flashlight lamp is a great learning experience.... but again, tunsgten doesn't last real long in micron pressures of air. If you can do a N2, Ar or He flush...first... that will lengthen your play time.

But all the improvising is a major part of the fun and excitement.

Check out Richard Hull's notes on Micro-Maze... it may be just the thing you need to get good results all around, with the mechanical pump, only, as long as you don't plan to use H2 or D2.

Dave Cooper

[Carl Willis]

Rich,

Regarding your question about sealing materials, look for an industrial gasket supply company in your phone book. We have two in Albuquerque. Custom flat gaskets in our linear accelerator and related vacuum work are usually cut from Viton sheet from these sources. The sheet itself is surprisingly affordable. We just drive over and pick it up as needed. I have made gaskets at home for diff pump mounting (used to be custom Edwards gaskets) from this as well.

Pre-made standard gaskets, in all kinds of shapes, are available from a local Parker or similar dealer ("Southwest Seal" in Albuquerque). This is an expeditious and less expensive alternative to ordering from Kurt Lesker (or probably McMaster-Carr), although the gasket dealers tend not to know the vacuum hardware standards and you have to order the o-rings by dimension. I recommend Viton. Buna-N also gets used in vacuum gear quite a bit, but lacks the temperature and chemical stability and has worse outgassing and permeation properties. Viton, when dropped, does not bounce. Buna rubber bounces. If you want to get a matching elastomer for a NW, ISO or ConFlat flange, bring the flange to the store, or better yet, an example o-ring. It is easy for them to pick out a suitable alternative from stock, even if they are clueless about the standards for vacuum work specifically. I have no idea if there is a standard, stocked size of flat gasket suitable for a bell jar, but there's a good chance.

Good luck on your educational projects.

-Carl

[Rich Feldman]

This is to report my biggest "Doh!" moment of the year.

Readers of the 4hv.org forum followed my story a couple months ago, about repairing a refrigeration-service pump that my dumpster-diving friend had scrounged. The cartridge was practically seized, so the motor could not start. No damage was evident on disassembly, but a part was missing, so I wasn't the first to open this case. The shaft turned freely, until the bolts holding cartridge to aluminum case were tightened. Had my predecessor disassembled the cartridge too far & got the stack mis-registered? Tapping with a steel hammer while progressively tightening the bolts & briefly running the motor seemed to fix it. Then came February ...

Things were going well with the DV-85 pump. Yesterday was the first time I ran it for more than a few seconds. Had a donated Convectron gauge and was testing some vacuum connection parts & methods for the science workshop program (while waiting for KF-25 fittings to hook up the fancy pump). With a KF-16 ring set between the gauge and the 1/2" ball valve, I reached an indicated vacuum of 20 millitorr after a few minutes for the new & old plumbing (and gauge) to settle down.

That was fantastic, but when I tried to repeat it this morning the motor could hardly turn the pump. The machine had reverted almost to its as-scrounged state. Well I needed it for a Sunday afternoon session at the science workshop. So drained the oil, opened the case, and messed with the all-too-familiar cartridge bolts as reported above. That seemed to do the trick, but after a minute or two of run time (at science workshop) the pump started dogging again.

Back home after a generally productive session, I was eager to tackle the pump, but reluctant to repeat the oily disassembly job. How about running in different positions? With pump standing vertically on its nose (motor end up), it seemed to run a bit smoother. Now stand it on its tail, which had been handy when the case was open to access the cartridge.

But this time the case was closed & full of oil. The bent-pipe handle on these pumps is screwed directly into the "crankcase" and serves as the air exhaust port. Or, in motor-down orientation, the oil drain port. At least the oil drains slowly because air has to glug in through the same pipe -- if the pump is not turning! When I pulsed the motor for a second, it pressurized the case & drove the whole pint of oil out through the handle, in a stream like that from a garden hose. I think this is where Homer Simpson would say, Doh!.

Until I have time to get some more clean oil, I will re-consider what defect of material or assembly could make a good pump (as attested by Lutz on the other forum) turn bad.

-Rich

[Rich Feldman]

Here's a semi-final report -- hope it doesn't break a double-post rule.
Today we got the bell jar down to 20 mtorr, presumably mostly air, using my Plan B configuration. Measured X-rays from unspooling Scotch tape. Observed E-beam from a television CRT electron gun impinging on a shard of broken fluorescent light.
The vacuum system:
8" square of 3/8" HDPE baseplate (deflects about 2 mm). Path to pump is 24" of 1" ID hose with KF-25 flanges on each end. Vacuum gauge connection is a hose barb to 1/8" MPT adapter screwed into baseplate. Electrical feedthrough is 4 pieces of insulated AWG20 solid wire passed through another 1/8" MPT fitting & sealed with 5-minute epoxy.
The x-ray system (in picture): Tape-winding machine made by 8th grader using a DC gearhead timing motor.
The e-beam system: must wait for another post (I have questions for you all).
Thanks for the support.
-Rich

[Doug Coulter]

Looking good! I'd have used a cheap canning jar for this, and probably gotten a bit better vacuum, I've been using them for little, dirty tests for years, no problems -- they are cheap enough to toss in the trash once all gunked up with things. You can even solder a pipe into one of the lids for the vacuum pump, and do similar things for feedthroughs. They are fairly thermal-shock resistant -- not perfect, but they don't break by accident without a heck of a good reason.

[lutzhoffman]

Really Cool!!

Its not just the equipment its the experiments done, I think this was a good choice, anything to keep their minds engaged. Your basplate looks like it could accept a future glass bell jar upgrade also. Good show, you could maybe even try different gasses, like Ar, and He, with a small HF high voltage power supply, for some really cool plasma effects, many students really enjoy this.

Two things that you may consider for a deeper vacuum, if you desire one, are:

1. Add a simple dry ice cold trap, with some activated charcoal. You fill the trap, heat it some, and evacuate to clean it out. Then after cooling you put it in dry ice / alchohol slurry, and your vacuum will drop way down, sort of a poor mans DP. If you can get LN2, then even better!!!

2. Run premium oil with a low VP in the Alcatel pump, Inland 20 will get you way down. Some folks recommend warming the pump a little when doing this, I never needed to on mine though.

Keep going it all looks good.....