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Measuring the internal pressure of a sealed container

  1. Sep 4, 2014 #1
    hey everyone, im a chemistry student and i am currently working on my final year project, which is to find a chemical difference between canned and bottled coke. one of the aspects im looking at is CO2 content, so i need a way of measuring the quantity of CO2 in a container.
    ideally i want to do this before it has been opened, to reduce the chances of gas escaping unmeasured, and so i thught the best way might be to measure the internal pressure of the container and calculate CO2 content from there.
    so far ive thought of testing the force required to deform the container a certain amount, and then creating a standard to check that against by inflating empty cans to known pressures and testing them in the same way. the down side is that i dont think i have the equipment for it.

    since this is a little out of my field, i figured it would be better to ask engineers for advice, so what do you guys think?
    ideally any way that i do this would use as simple equipment as possible, since im not sure how much access ill be able to get to any engineering labs.
     
  2. jcsd
  3. Sep 4, 2014 #2
    oh, also, i thought of weighing the can, gettign rid of CO2 using sonication and then reweighing the can, but past students have gotten results where the difference from one bottle to the next is greater than the difference between bottles and cans, so im hoping to find another method.
     
  4. Sep 4, 2014 #3

    Baluncore

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    Welcome to PF.

    You have little choice since to eliminate comparison errors, the same technique must work on both bottles and cans.

    I would attach a pressure gauge with a fine bore tube to a sharp conical probe that can be pressed through the cap of a bottle or the end of a can. You may be able to do that without an external seal. You could use a hydraulic press or a screw clamp to puncture the container with the conical probe.
     
  5. Sep 4, 2014 #4

    AlephZero

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    Could you catch all the gas my making something that looks like a large diameter water-filled barometer (which would be completely full of water unless you made it 10 meters tall!) and opening the can inside it so all the gas collects in the top?

    You might have to make some device to break the seal on the can remotely, inside the "barometer."

    Some of the gas would dissolve in the water, but I expect a chemist would know how to measure that. (I'm not a chemist!)
     
  6. Sep 10, 2014 #5
    baluncore:
    if the measurement is standardised against other pressures in the same container then the pressure measurement should be accurate, so would comparison errors then be important?

    AlephZero:
    i was thinking that, but im not sure how accurately i would end up beign able to measure it. also i dont think theres an easy way to calculate how much is lost in the liquid
     
  7. Sep 10, 2014 #6

    Mech_Engineer

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    If you know very well the material properties and geometric properties of the can's wall, you might be able to depress a tool into the can's wall and measure the force response. With analytical calculation and/or simulation you could to correlate the force response to a known internal pressure.

    Take a look at these slides on calculation of stresses in cylindrical and spherical pressure vessels: http://fp.optics.arizona.edu/optomech/references/OPTI_222/OPTI_222_W24.pdf

    And this paper on compressing cylinders (including hollow cylinders) and spheres with point and line contacts: http://emtoolbox.nist.gov/Publications/NationalStandardsLaboratoryTechnicalPaperNo25.pdf
     
    Last edited: Sep 10, 2014
  8. Sep 10, 2014 #7

    Mech_Engineer

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    And another idea- what if you put a strain gauge on the outside of the can, and changed its temperature while logging the strain change. Maybe with some knowledge of how the liquid inside will absorb CO2 you could correlate the temperature-dependent strain data to an estimated amount of CO2 in the can?
     
  9. Sep 10, 2014 #8

    Mech_Engineer

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    Or a brute force approach- put the can in another pressure vessel and slowly increase pressure until the pressure outside the can matches the pressure inside the can. If you identified a feature on the can that would behave very differently without a differential pressure you'd be good to go...
     
  10. Oct 7, 2014 #9
    Step 1: Weigh the full, unopened can.
    Step 2: Open the can, let sit until all C02 is gone (completely flat)
    Step 3: Weigh again.

    The weight difference should be the C02. You can probably assume that each can has the same amount.
     
  11. May 31, 2016 #10
    Why don't you just call up Coke and ask them?
     
  12. May 31, 2016 #11
    You can get a sample of vapor and measure the fractions of water and CO2(probably very little water). Then if you know the total pressure, you can use the Henry's law constant to calculate the amount of dissolved CO2.
     
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