A vacuum chamber from a pressure cooker

[Allan] needed a small vacuum chamber to get all the air out of clear casting resin. Degassing is a simple step in casting that improves the finished product immensely. The problem, though, is building a vacuum chamber. [Allan]‘s chamber seems easy enough to build, and pulls enough air out to get to 0.1 atmospheres.

After a hole was drilled in the side of the pressure cooker, [Allan] installed a 15mm “speedfit” plastic tank connector. The seal around the connector is neoprene self-adhesive foam. This foam was also taped around the lip of the pressure cooker for the top.

A thick-walled pressure cooker is more than capable of handling the outside pressure when under vacuum, but [Allan] cautions against using acrylic plastic for the top. Acrylic has the tendency to fail catastrophically, so he used a thick sheet of Lexan. Check out the demo video of [Allan] sucking the air out of shaving cream after the break.

Comments

  1. statik says:

    Cool; I did a similar build but was too cheap to buy a sheet of thick lexan so I just made a little window.

    http://thegreatgeekery.blogspot.com/2011/05/pressure-cooker-vacuum-chamber.html

  2. Sci says:

    I’ve made the same sort myself. I doubt he’s going to get a great ultimate vacuum with the foam seal, but as an off the shelf solution it’s pretty clever. I ended up casting my own from PU rubber, which works but has started to crack and degrade. I’ll have to do another in silicone.
    I’d used 12mm polycarbonate on smaller aperture chambers before, but on something as wide as the top of a pressure-cooker I used 25mm as the 12mm already bowed slightly on an aperture about half the width. Less brittle, but I still don’t want to tempt fate using material that’s too thin.

    Also you don’t really need a flange-fitting. I drilled and tapped my holes for screw-in fittings. Just needs a little PTFE tape and it holds fine.

    • Allan Stirling says:

      On the first few pumpdowns, the seal wasn’t actually the limiting factor. I (feel free to shudder at this point) use PVC pipe on my primary reservoir that I really don’t want to stress too much. Also, the compressor I’m using (a standard shop compressor) is really not designed for this – The one-way flap valves have a “good enough” seal against pressure but are not great at vacuum.

  3. Dax says:

    Speaking of vacuum systems, all I can think of is the video of a railway car with a gas tank on it, that the yard workers had cleaned with steam, and then closed the hatches.

    • Nova says:

      Link?

      This is as close as I could find: http://www.youtube.com/watch?v=Zz95_VvTxZM

      • Dax says:

        That’s basically the same thing, but done on purpose. Looks similiar, at least.

        The point being that if you fill a large drum with steam and then close all the ports, it’s going to cool down and pull a strong vacuum as the water condenses.

        No matter how hard the vacuum is though, the pressure differential is just 1 bar at maximum, which goes to show how much force just 1 bar of pressure can generate. If the container isn’t well symmetrical, the uneven forces act like levers that buckle the container walls.

  4. HAD says:

    Depending on the surface, I suggest 1/2″ Polycarbonate (Lexan is one brand of PC) at minimum. Figure the atmosphere is going to exert roughly 14.9 PSI on every square inch of material. PC is quite strong and it doesn’t fail as suddenly as acrylic does but if you are going to go over 6″ in diameter, check your numbers to ensure you have enough strength + a safety factor.

    • Allan Stirling says:

      Absolutely. I didn’t actually run the numbers on this, but did everything I could to make sure it was “safe enough” – I’ve seen designs with the vacuum port on an acrylic lid, which made me shudder.

      In terms of forces, a perfectly round porthole is the best you can get, the pan’s wall thickness is fairly well supported by both the bottom rim and the top rim that’s pressed very firmly against the lexan sheet.

  5. HAD says:

    Also – it is not correct to assume if a chamber can hold pressure, it is also rated to safely hold vacuum.

  6. tz says:

    I have a vacuum marinader – this would probably do much better.

  7. Cal says:

    We use acrylic doors on (much larger) chambers that go down to 0.00007 atm and lower. The doors are anywhere between 1″ and 1.5″ thick.

    A sheet of acrylic with those same dimensions that Allan used would be adequate.

    • HAD says:

      Do note though that the difference between 0.00007 atm or 0.0000000000000000000001 atm is almost 0 psi. Maximum vacuum results in an exertion of about 14.9 PSI against the vacuum no matter how perfect of a vacuum you have – the atmosphere simply can’t “weigh” more than that.

  8. userjjb says:

    Even with a perfect vacuum, 14.7 psi is the maximum pressure exerted on the viewing port.

    PMMA has a Poisson’s ratio of 0.35, poly-carbonate 0.37 The loading case is a simply supported circular plate. Maximum stress will be:

    3/8p*(3+v)*(r/t)^2
    [Advanced Mechanics of Materials, Boresi/Schmidt pg 491]

    r is the radius, t thickness, v Poisson’s Ratio, and p the uniform load (100000 Pa atmospheric pressure)

    Feel free to use that formula for future reference on “porthole” like designs under either vacuum or pressurized conditions. Notably thick portholes may require a modification due to assumptions about plane strain.

    Both PMMA and polycarb have about the same tensile strength of ~50 MPa and elastic modulus. The only difference between the two is that polycarb has about 4 times the impact strength. Assuming the diameter of the pressure cooker is 30 cm he has about a 2x safety factor.

    • HAD says:

      Acrylic tends to “shatter” much more so than polycarbonate though. And make sure you consider the impact of scratches and other defects on overall strength.

      • userjjb says:

        Worrying about minor surface defects like scratches is akin to worrying about stepping in a puddle in the middle of a hurricane.

        A Mode I crack occuring in plane strain can have a maximum depth determined by:

        c = 2/pi * (K/yield)^2

        where K is the fracture toughness of the material (PMMA ~1.2, polycarb ~2.2 MPa/(m^(1/2))

        This is ~2mm for the PMMA and ~7mm for the 12mm thick plate. Those aren’t scratches, they’re cuts.

    • Allan Stirling says:

      Thank you very much for actually running the numbers – Definitely appreciated and something I really should have done before embarking on this.

      I’m more than happy with the result!

  9. Ken says:

    Partial vacuum has been good enougg for the few times I’ve cast small pieces. I was making silicone molds out of SmoothSil something or other.

    Like a good HaD’er, I found an unused plastic canister in my kitchen, drilled a hole in the top and used my wine bottle vacuum [http://www.vacuvin.com/270/Vacuum_Wine_Saver.html]. Yeah, I suppose it could have imploded. But it didn’t.

    • snowdruid says:

      my guess is it would be REALLY hard to make any kind of extended vacuum with this “pump” anyways i cant really immagine a plastic canister implode. more like buldge in?

  10. Hirudinea says:

    Is it just me or do you think this could be applied to confectionery, eg. giant marshmallows?

  11. Guest says:

    Is it just me, or should the tank connector gasket be on the outside, so air pressure will force it against the tank wall?

  12. bob says:

    its not that good…..

    The shaving foam should expand as the gas expands due to the negative pressure, but then it should collapse to a liquid as the bubbles are extracted from the foam.

    That is the whole point of this device…… to remove bubbles from a liquid not expand them.

    Also I would have used an ‘old’ scrap pressure cooker, the new ones are shite as the wall thickness is about a quarter of the older ones. (20-30 years ago)

  13. Sam says:

    Lexan is an acrylic…

  14. joe says:

    Use a turbomolecular pump. These are regularly used in Scanning Electron Microscopes to get the needed 10^-9 Torr needed. Pricey though.

    • userjjb says:

      This is like saying you need to mow your lawn with a 10 megawatt laser; sure it will do the job but is completely unnecessary.

      To even run a turbo pump you needed a backing roughing pump, something like a rotary vane pump for instance. The roughing pump already can draw a good enough vacuum to serve your needs, you would just use that.

      Also good look getting 10^-9 with a random metal pressure cooker, giant plastic porthole, and a big ‘ol lump of resin all of which will be outgassing like crazy.

      You missed the entire point of what he’s doing here.

  15. Ren says:

    Is it really necessary to view the interior?

    Maybe so, to see if all the bubbles have come out, and maybe to see if the casting resin has set.

    But if not, why not use the original lid? It probably has a pressure release hole which can be used to attach the pump (i.e. no drilling).

  16. bothersaidpooh says:

    DIY fusor, backfill with He at <0.01 atm

  17. terpene vacuum says:

    Im making my own vacuum chamber out of an All american 941 pressure cooker. http://www.allamericancanner.com/allamerican941pressurecanner.htm
    Inside diameter – 15 1/4 inches
    Inside Height – 14 1/4 inches
    Overall Height – 19 inches

    I was going to buy a 1/2″ thick sheet of lexan to make the lid but am wondering if it will be strong enough given the size of the opening? Should I go with 1″ thick plexiglas?

    Thanks in Advance,

    TV

  18. Dereck says:

    Assuming that I didn’t need the viewing window, is it not possible to just use the top from the pressure cooker? Maybe not the particular pressure cooker that he used, but one of the more substantial ones with a machined aluminum top.

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