Nearspace Environmental Chamber

vacuum

If you’re going to send some hardware up to 100,000 feet, where atmospheric pressure is 1% of what we enjoy on the surface and temperatures swing down to where Fahrenheit and Celsius don’t matter anymore, you might want to do a bit of testing to make sure everything works before launch. With a few bits of PVC, though, that’s a piece of cake.

There were several environmental conditions to take into consideration; the near vacuum experienced by high altitude balloons would be replicated by a refrigerator compressor, the increased solar flux is simulated by a light bulb, and the cold temperatures provided by a chunk of dry ice.

For a proper high altitude, low temperature environmental chamber the test payload should be cooled down via radiation with tubes filled with liquid nitrogen embedded in the walls. This is the NASA way of doing things, but for the budget of $200, [arko]’s chamber simulates a high altitude environment just fine.

31 thoughts on “Nearspace Environmental Chamber

  1. Stop promoting pressure vessels made out of PVC.

    A tubular vacuum vessel is at a high danger of failing because it’s structurally unstable against the outside air pressure, and when you cool it down with dry ice, you have a fragile shrapnel bomb at your hands.

    1. in this setup it can only collapse!! though that’s unlikely… note that vacuum, unlike positive pressure is just 1 atm (acting on the outside of the chamber not inside duuuh)

      1. The collapse is still a release of energy that will put shrapnel and plastic pieces flying around.

        The difference between internal pressure and external pressure to a vessel is, that with internal pressure the minimum energy situation is when the vessel maintains its shape. All parts are under tension, so when you deform the vessel, it returns back to shape. The deformation acts against the pressure force.

        When the vessel is under external pressure (internal vacuum) the minimum energy case is when the vessel deforms and assumes a lower volume. In other words, the material is under compression and tends to buckle instead of returning to shape when disturbed. The deformations act along the pressure force.

        The end result is sudden and energetic even at low pressure differences. For example:

        With a material that is prone to fracture, such as PVC that is being cooled down to a significant degree, the structure readily implodes into pieces.

      2. Generally speaking, vacuum vessels collapse at a fraction of the pressure required to explode them from the inside out. What makes it dangerous is that unless the vessel has a distinct weak point that breaks first, the buckling of the vessel becomes completely unpredictable.

        It’s like building an upside-down pyramid. If you construct it very carefully, you can grow it bigger and bigger until suddenly the first brick crumbles under the weight and the whole thing comes down at once.

          1. Schedule 40 Tee’s are 127mm outer diameter and 6.35mm, based on that calculator its a theoretical collapse strength of 124.34psi which is approx. 8 times perfect vacuum (~14psi)

        1. http://www.madisongroup.com/case-studies-pvc-pipe.html

          The impact strenght of PVC drops sharply between +20 and -10 C and it drops to about 1/6th going from room temperature to dry ice, so based on that you only have a safety margin of 1.4. The trouble is, without knowing the exact material used, it’s hard to tell what will happen.

          http://www.makeitfrom.com/material-data/?for=Plasticized-Flexible-Polyvinyl-Chloride-PVC-P

          Using that as a reference, a Poisson’s ratio of 0.39 and Young’s modulus of 2×10^5 gives you a collapse strenght of 4.57 bars but we can give it a bit more Young’s modulus because it tends to increases with lowering the temperature, so it’s there and there whether we get any sort of safety margin.

          In any case, the lower working temperature of PVC is generally quoted at -25 C and dry ice is -78.5 C so even that should say it’s not safe for this application.

    2. Wrapping the tube in a few layers duct tape or fiber-reinforced packing tape would probably do a lot to contain any shrapnel in the event of an implosion. Just a thought.

        1. Also implosion protection there are reasons why modern CRTs are shaped the way they are, the glass not exposed to the outside is also real thick once one gets past the neck of the CRT.

    1. Not sure why you’re so butthurt about some realistic and sensible warnings from Dax but supporting youth elsewhere is a good idea even if your reasoning is severely flawed. PVC pipe is a horrible material to make any kind of pressure vessel unless you significantly overspec it. It will turn into shapnel at the slightly provocation if you don’t know what you’re doing.

      1. Even if this version of the altitude chamber was reasonably over-engineered to be somewhat safe, someone is bound to try making a bigger one and think “Oh it’s just going to be a 1 bar difference, it’ll hold”.

        And then it will not.

      2. yet with fairly simple safety precautions it is possible to use, providing a low cost alternative to people who might otherwise not have the opportunity.

        if you have a blast shield you can use that or put it somewhere where you are not, then use remote control and monitoring if needed, or collect your data afterwards.
        even if the worst case scenario happens and the entire vessel catastrophically fails all you would be left with is a mess, not a trip to the ER.
        for someone to actually do that is another matter entirely

      3. so tell this guy he’s risking his life and life of his students

        What if those syringes explode and send shrapnels everywhere ?!?!?!?!?!?!?

        Never been a fan of PVC used for pressurised air, but this is vacuum – it’s less than 1atm difference to start with and EVEN if something happens you’ve got a chamber that collapses on itself and is thus safe. Too dumb to get it ? Then don’t breed.

        1. That is polypropylene, not PVC. PVC shatters into shards, which can still fly out even though it’s collapsing.

          Using PVC is not worth the risk, because imperfections in it can easily mean that a PVC plumbing fitting could not handle the vacuum and cooling.

          1. ohh my god you don’t get it do you ??
            1. very low pressure difference
            2. even if it fails it won’t be dangerous as it implodes not explodes

            You read something somewhere and then you repeat it without thinking. I feel like this guy

    2. A CRT is being used under the conditions for which it was engineered. The PVC T is being pushed far outside of its design spec. It’s akin to saying that driving your mom’s station wagon at 150 mph on the highway is perfectly safe because NASCAR drivers race cars that fast on the track.

  2. Okay, okay, we know about the dangers of PVC under pressure. Time to move on, folks.

    On another note entirely, can someone explain the need of the dry ice for cooling? I was always under the impression that lowering air pressure reduced the temperature because of the reduction of overall energy in a given volume?
    I could be wrong…

    1. It does, but you still need active cooling because heat leaks back in, and the device you’re testing, and the lights your’re using will quickly turn the chamber into an oven.

    2. Low pressure does not mean low temperature, it oily means well insulated. With a bright light on you, you can get very hot in vacuum, even without light, radiation from the environment will warm you a little. That said, water, for example, will cool down because it will start subliming at pressures below 6mbar. If you get to 0.06 mbar, ice surface temperature will be around -40C. By including a small amount of vapor at a known temperature you can counter any radiation and maintain a specific temperature of things in the chamber.

  3. The folks building fusors usually use a blast shield for 1) x-ray protection , and 2) in the event of a catastrophic failure.

    I’ve got a nice glass sphere here which would work well, the catch is that it has bubbles and may or may not be safe especially with beams of energised particles flying around heating up sections of it unpredictably.
    Its well worth building a “beam dump” for your fusor so that any wayward beam(s) can be deflected into a safe area not the wall.

  4. While it probably won’t be much of a problem at that vacuum PVC outgasses at a high rate. A bigger problem is likely that the outgassing will cause it to become brittle over time.

  5. A proper vacuum chamber is very inexpensive and easy to make. You need a pressure cooker and 2″ polycarbonate for your lid and a simple gasket….. Look online and you will see many commercial ones made this way. Much safer and great for de-gassing mold material. No excuse for using PVC

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