home-made vacuum chamber (the real deal)

Pa is pascal and is approx 0.987 atmosphere

Torr is almost 1.33 milibar (ie 0.00133 bar) = 10mm of hg = 0.0013 atm

hope this clarifies

"Pa is pascal and is approx 0.987 atmosphere"

Think that should be 0.00000987.

Also, I believe 1 Torr = 1 mmHg.

Stand corrected (didn't revise the values )

1 pascal = 1N/m² = standard SI unit of pressure

Mega pascal (MPa) = 9.86 bar

and yes 1 Torr = 1mm hg = 0.0013 atm = 1/760 atm

(just as a footnote named after torricelli - the person credited with discovering barometer (principle)

Bar is 10⁵ Pa by definition. So MPa = 10 bar exactly.

Cheers.......Codey

We degass RTV silicone compounds for potting various bits of equipment. These are done at around 750 torr which is about 1 bar.

Any air conditioning equipment supplier should be able to provide you with a vacuum gage. External pressure SHOULD be atmospheric pressure. More critical to the survivability of the vessel is the state it is in- i.e., any corrosion that has decreased wall thickness? Areas that result in stress concentrations? An imploding vessel is a whole lot less scary than an exploding vessel...

One other point- the vessel should be able to withstand external pressure better than internal pressure- the wall material should be in compression, rather than tension.

Not necessarily true - with external pressure / internal vacuum, the limiting factor generally is resistance to buckling - not compressive strength.

Best example of vacuum buckling can be seen in this video.

http://www.youtube.com/watch?v=2WJVHtF8GwI&feature=related

NOTE: That is filmed in real time and not fast forwarded.

Another good example:

http://www.youtube.com/watch?v=Uy-SN5j1ogk&feature=related

Great links. Thanks for that.

You should register.

Actually I am registered, however, I bounce around from computer to computer and neglect/forget/don't bother to log in at all times.

Hello Thornpile:

As previously suggested an HVAC supplier would be a good source for gauges, typically in the industry we work in micron's for deep vacuums although the new digital micron gauges will display in multiple units .

Analog gauges have been the standard for years in the industry and are relatively inexpensive (in the $10 range), however for your needs they're probably not adequate.

The good news bad news is the introduction of new refrigerants and oils required deeper vacuums and consequently the price and availability of accurate micron gauges has improved considerably however there are still in the $200-$250 range.

So if you merely need to confirm a good vacuum the analog gauge will do, generally we evacuate a system until the gauges show a 30 inch vacuum, of course this is a perfect vacuum and of course the gauge is not that accurate.

So to add to the confusion there are about 25400 microns to 1 inch of mercury however I've included a link to a conversion chart that may be of help.

Most good vacuum pumps (clean oil is a must) can pull a 50 micron vacuum when connected directly to the gauge,in reality a 500 micron vacuum is usually adequate, and difficult to obtain.

I've also included a second link that may be of some help.

http://www.lds-vacuum.com/conversion.html

http://www.heatcraftrpd.com/resources/techtopics/Tech%20Topics%20Evacuation.pdf

Gosh! All good but I will install just in case like an pressure safety device in order to try to avoid any chance for an implosion in case the vacuum get down to hard on the vessel. Probably some kind of by-pass that open it up straigh back to the vacumm pump protecting the vessel from collapse. So this way you avoid to pull out more than the safe capacity range.

What else to say? I believe you have the knowledge now big time with the guys expertise here at CR-4 once again. Remember your PPE in any case for safety and keep some distance or a good shield as further personal protective measure.

Impressive Stuff,

MC

don't forget perfect vacuum is -14.7 psi @ sea level, which should not come close to stressing a pressure vessel.

Hello Yosemit3

If a vessel can stand internal pressure 14.7 psig doesn't mean it can take full vacuum. As #11 pointed out, the limiting condition for external pressure is usually buckling.

Not sure what the OP means by boiler (I can't picture that being used as a vacuum vessel) but if it's a hot water cylinder (typically 450mm dia, < 1mm thick) buckling definitely comes first.

Cheers........Codey

It is an old boiler/water heater, the dia is about 450mm, and the walls are about 4mm thick.

Hello Thornpile

According to my calcs (based on Roark), if it's made of copper and very long, differential pressure to cause buckling is ~ 1.9bar. If it's say 1.5m long, figure is 6.6bar.

For steel, corresponding figures are 3bar and 11bar.

So though I'd use a safety factor of about 2 on Roark's figures, it looks like you're OK.

Cheers.........Codey

When I was a prototype machinist, we used the paint pots for a vacuum chamber for degassing our silicone rubber for making prototype molds from SLA parts, worked well, a standard vacuum pump will do fine.

Allen

If you want to get down to really low pressures, you'll have to invest in a decent vacuum pump... Not to worry, they're always available on eBay. Next, you need a multi-stage system. After the pump, you'll need to set up a mineral oil trap to absorb remaining air after the pump. Next, you might want to deploy a cold-trap in your system. After that you need an ion trap - uses high voltage to catch stray atoms trying to get through - again available on eBay (probably).

If you want good ideas and designs for high-vac systems, take a look at what electron microscopes use.

Good luck!

Can you not run 2 pumps in tandem to pull a rough and then final (finish/sustaining) vacuum.

That is how our Alcatel leak detectors operated. You may even find a scrap machine out there some where. The 110 model was really a good machine. I believe we were, on that equip, operating at 10E^-9 We would rough down to 3E and switch to a higher rpm (~10,000) finishing pump. This set up would be left on for days at a time sustaining the desired negative atmosphere environment without a hitch.

Chart recorders and other data collecting devices to record were on the cheaper side too.

My point I guess - depending on the size of the chamber and what all you are trying to accomplish - consider a 2 pump config.

Alcatel is now Adixen

I picked up one of those Alcatel pumps in good working condition on eBay for a cheap price! There's a bunch of 'em out there!

You would use a holding pump (rotary vane or scroll pump) to back a high vacuum pump such a diffusion (not really used anymore) or Turbo pump to get a deeper vacuum. A rotary vane or scroll pump will only pull 1x10-4 Torr, that's all you can achieve with these types of pumps. Your Alcatel (Adixen) Leak detector would be using a turbo pump to reach approx 1x10-6 depending on the leak detector you have. Also to have a rotary vane or scroll pump is relatively inexpensive, cost of operation is low and rebuild cost is ecanomical. A Turbo or Diff pump has a low operating cost but your rebuild cost will be in the thousands of dollars. If your in Canada you there are no Turbo pump repair facilities so good luck with that.

Not recommended - as stated by others (but in the midst of other conversations): if its a central heating or water tank, or indeed any form other than pipework, it will almost certainly buckle. Basically, once you have a usable vacuum for any practical "vacuum chamber" purpose the pressure on the inside will be negligible (as far as the chamber is concerned) and the pressure from the outside will be 1-atmosphere - or 14-lbs per square inch. What's more concerning is that if you try it with a chamber that is grossly inadequate it will be less dangerous than if you use one that is nearly up to the job (the reason is that there will still be a significant amount of air inside that will cushion the implosion - so if you try it and it collapses benignly, you might be tempted to try again with something a bit stronger. Wikipedia gives conversion factors, and there are numerous conversion websites if you want to check your calculations - and most of them are correct

So what if i filled the tank semi full of leca when testing it? The leca would cushion the buckling, but what kind of impact would this have on the rest of the setup?

Thanks alot for all the inputs guys, it has certenly clarified my confused mind. The only thing left for me to do now, is to source some parts, (can be kind of hard in tiny Norway) and build it. I will however reconsider my old boiler as a vessel, and maybe try to get my hands on a large size pressure cooker, and use that instead. (or at least test my boiler with caution). If you guys have any other ideas on what to use as a vessel, please feel free to spell them out

Anyways, thanks again for all comments, it is most appreciated!

Hello, I have re-built an old Kinney Vacuum System and made one from scratch. We used it to experiment with Vapor Deposition. We deposited Tellurium, Bismuth, Silver Gold, Aluminum ect..The System used a roughing pump and a Oil Diffusion Pump(vapor). We made an Ion guage for measuring low pressure. This was accomplished by carefully drilling a hole into the end of an old metal radio tube(triode) and brazing a 1/4" x 1"(length)pipe in the hole and on the other end of the pipe, braze a threded fitting. This fitting should fit your bell jar or better, the base of the vacuum table with access to the vacuum area. Remember, everything must be absolutly clean. No fingerprints or anything that will outgass increasing pressure and contaminating the system. anyway -- look up how an Ion Guage works. The tiode radio tube works well. We callibrated the Ion guage with a known good guage. You must pump the system down -way down before the Ion Guage will work. If you turn the Guage on before the system is at low pressure it will fry. Use a standard mechanical guage to rough the system to low pressure. I think the Ion Guage worked at < .1 micron-Hg. You must remember to turn off all power to the Ion Guage before increasing the pressure and opening the bell jar. You can make a bell Jar from a Al or SS pipe with a welded plate on top and a thick glass window for observation. Or just buy one -check E-bay.

Wow! Sorry about that guys. I thought you were talking about high vacuum systems. I think that when people ask a question in these forums, they should explain what they are trying to do. Trying to out-gass epoxy resin is a lot different than producing a vacuum to vapor deposit material. If you are trying to build a low vacuum system, please disregard all my previous techno-babble.

I suppose it could have been stated a bit clearer, however it is sort of explained in the very first post, if you read through the whole text. Anyhow, going over the top is a good thing, I sure learn alot from it!

Cheers

It rather depends on what quality of outgassing you need. If you are happy to accept small residual bubbles, you don't really need a very low pressure at all. (Small bubbles are maintained by surface tension).

On the other hand, you can get the results you need without a large vacuum tank. I have on occasion used an arrangement similar tot he following:

Glass preserve jars are designed to withstand an atmosphere, so you could replace the glass top with a metal sheet, drill a hole through the sheet and braze a TEE-shaped thick-walled metal pipe to the top. Then attach this to ordinary water valves on either side of the TEE, one between the jar and the pump, and the other vented to the air. You could if you wish put the potting compound directly into the jar, leaving a couple of inches clear at the top (of course you could also place a smaller jar inside). Before starting the pump, you need to open the vent and set the pump-side just open enough to get some airflow. Then you can gradually close the vent, watching that the bubbles never got more than half-way to the top of the jar. Finally (if necessary) start opening the pump-side valve. This way, the compound acts as its own manometer.

If you can't get hold of a proper home-preserve jar, a pickle jar would also do the trick - but the safety margin is rather small, so you will need to route the pipe so that you can have a polycarbonate sheet between you and the valves and the jar.

Whichever you use, you should wear safety glasses while doing this.

Hi Simple1. Had a look at the pump, it's an old pfiffer pump, couldn't figure out the model, but I took a picture of it. Now, I don't know what kind of pump this is, but I would guess it is a direct drive pump, compareing it to other pumps on the net. The pump was left at my dads years ago, by his engineer friend, after one of their many modelairplane projects. I could offcourse call him and ask, but I am afraid he might want it back. And I would very much like to keep it for now, cause i think it is a nice pump. According to the spec plate on the side; It uses 1,2 liters of oil, and sucks 12m3/h. My vessel is about 0.6m3, so in theory, I should have low enough pressure in 3 minuts or so? Which I belive is adequate for most of the resins I will use.

I've seen only one of these before, I would seriously suggest that you don't take this pump apart because I doubt you would be able to get it back together. I had to make many parts from scratch as this pump is completely obsolete. They don't make parts for it anymore and I highly doubt you'll even find someone that can re-build it. From what you have told me this pump should easily do what you want it to do. I would suggest you but good vacuum pump oil preferably a semi synthetic as this pump will run hot with the amount of gas load you'll be running through it. Also I would buy 4 liters of cheap vacuum pump oil and mix it 1 part tranny fluid and 3 parts oil. This will remove many contaminants from the pump thus improving your vacuum pressure and extending your pump life. Run the mix through the pump for about 4-6 hours then flush while its hot and repeat one more time then you'll be ready to use the pump for your application. I've done this before for some pumps and it works relatively well, tho it is no substitute for an overhaul. Do you know of anywhere you can get fluid from were you are? If not tell me where you are and I'll try and stear you in the right direction. Good luck!!

Cheers Simple1, I'll follow up on that advice. There is a service sticker on the side of it, so I'll give them a call, and see if they are still up and running, or try to source the oil somewhere else, know of a few industrial tool stores I believe either should stock it, or at least point me in the right direction.

If not, I'll slip you a mail...

Thanks for all the help.

HELLO Thornpile:

Any HVAC supplier will carry oil for vacuum pumps.

That looks just like the BOC Edwards High-Vac E2M-1.5 pump we use to de-gas potting rubber. Their site lists the oil required, I expect your supplier could offer an equivalent.

hey, you need to measure inches of mercury (inHg), not inches of water, I made the mistake first few gauges I bought off of ebay.

I'm going to post a howto vacuum chamber construction on my website, http://www.castbender.com but you can see most of my process here http://forums.gvillegaming.org/index.php?topic=535.0

Also check out the Bell jar at www.belljar.net A verry informative amature

vacuum website dealing with all aspects of vacuum from electronic (X-ray, electron microscopes) to simple de gassing of liquids.