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  1. #51
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    Quote Originally Posted by brianmcg View Post
    The purpose of a CO2 cartridge is to get you home. When you get home you should let out the CO2 and fill with regular air from a proper floor pump.
    Right, I'm trying to avoid that.

    Anyways, our resident physical chemist is right, even if he did an awful job of explaining it to me.

    I did the calcs, and found that there's roughly .280 mols of gas in a regular road bike tire, and roughly .36 mols of co2 in a co2 cartridge.

    To put .36 mols of n2 in a co2 cartridge, I'd need to compress it at 89,629psi.

    Which probably isn't going to happen.

    I prefer to not think about this as losing an argument, rather, I prefer to think that the laws of physics and chemistry won.

  2. #52
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    Quote Originally Posted by msrothwe View Post

    I prefer to not think about this as losing an argument, rather, I prefer to think that the laws of physics and chemistry won.
    You fought the law and the law won

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    he did an awful job of explaining it to me.
    Gee, thanks.

    The key point is that you can liquify CO2 under conditions that allow you to carry it in a canister in your bike bag. You can't do that with N2, so you can't store anywhere close to the number of molecules of gas you need to inflate a tire.

    (Is that better?)

    Quote Originally Posted by DaveG View Post
    You fought the law and the law won
    Nah. He just tried to apply the law outside its jurisdiction.

    (The ideal gas law breaks down when the gas becomes a liquid -- which an "ideal gas" never does, by definition.)

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    Quote Originally Posted by wgscott View Post
    Gee, thanks.

    The key point is that you can liquify CO2 under conditions that allow you to carry it in a canister in your bike bag. You can't do that with N2, so you can't store anywhere close to the number of molecules of gas you need to inflate a tire.

    (Is that better?)
    Gah, I meant to put a after my statement, not sure why it didn't work on tapatalk.

    But yes, your second explanation was much better. I failed to grasp the difference in density between a really pressurized but not quite liquid gas and a full-bore liquid co2.

    Quote Originally Posted by wgscott View Post

    Nah. He just tried to apply the law outside its jurisdiction.

    (The ideal gas law breaks down when the gas becomes a liquid -- which an "ideal gas" never does, by definition.)
    Right. Another thing that I messed up is that N2's critical point is far below CO2's, so it will want to be a gas at just about all pressures at a reasonable temperature.

  5. #55
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    You would also need to find green presta valve caps to let everyone know you're riding with nitrogen.

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    Quote Originally Posted by ogre View Post
    You would also need to find green presta valve caps to let everyone know you're riding with nitrogen.
    who uses valve caps

  7. #57
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    Quote Originally Posted by ogre View Post
    You would also need to find green presta valve caps to let everyone know you're riding with nitrogen.
    nitrogen cylinders are typically color coded black, green is for oxygen...

  8. #58
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    Quote Originally Posted by wgscott View Post
    Gee, thanks.

    The key point is that you can liquify CO2 under conditions that allow you to carry it in a canister in your bike bag. You can't do that with N2, so you can't store anywhere close to the number of molecules of gas you need to inflate a tire.

    (Is that better?)



    Nah. He just tried to apply the law outside its jurisdiction.

    (The ideal gas law breaks down when the gas becomes a liquid -- which an "ideal gas" never does, by definition.)
    more of a demonstration of the difference between basic science and applied science. The basic scientist has to know how any why things act as they do in minute detail. The engineer just has to read the chart made by the basic scientist for him to make do

    not everyday do I run into a physchem doctor, though I am married to one

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    Well, I never knew Carbon dioxide leaked out of tubes until confronted with the experimental evidence.

    You have my deepest sympathies, although I married someone who works with live HIV.

  10. #60
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    Quote Originally Posted by BCSaltchucker View Post
    more of a demonstration of the difference between basic science and applied science. The basic scientist has to know how any why things act as they do in minute detail. The engineer just has to read the chart made by the basic scientist for him to make do

    not everyday do I run into a physchem doctor, though I am married to one
    Which, I suppose is why ideal gases exist only in textbooks.

    scott s.
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    Most gases have near-ideal behavior, as long as you don't get too close to liquification temperature or pressure.

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    Quote Originally Posted by wgscott View Post
    It's the Joule-Thompson effect. If would get cold even with a compressed gas (think of those compressed air canisters you get to blow dust out of electronics).
    The adiabatic expansion of a gas causes cooling as you have stated, but the latent heat of vaporization is a much bigger number in the total amount of temperature drop when you discharge a CO2 cartridge.

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    You are right. I thought the JT effect would dominate.

  14. #64
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    Propane would be the way to go... You could probably fill up five tires with a CO2 sized cartridge. Plus you could shoot flames out of your valves if necessary to scare off dogs, small children, the wife...

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  15. #65
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    Quote Originally Posted by wgscott View Post
    You are right. I thought the JT effect would dominate.
    And one more thing: the gas is expanding AFTER it passes through the let-down valve after the outlet of the cartridge - all that cooling takes place in the CO2 device and the inner tube, not in the cart. The ONLY cooling that takes place in the cart is due to the boiling of the liquid CO2.

  16. #66
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    Exactly what gives you the idea that CO2 would leak out faster than N? CO2 is a larger molecule that requires a larger pore to travel through.

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    Why CO2?

    Quote Originally Posted by Tom Kunich View Post
    Exactly what gives you the idea that CO2 would leak out faster than N? CO2 is a larger molecule that requires a larger pore to travel through.
    Actually this is not true. A carbon dioxide molecule is about 116 pico meters, whereas nitrogen is about 300. Which is why bike tires go flat faster when they're inflated with carbon dioxide.


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    Quote Originally Posted by Fireform View Post
    A carbon dioxide molecule is about 116 pico meters, whereas nitrogen is about 300. Which is why bike tires go flat faster when they're inflated with carbon dioxide.
    is this the main reason we should remove the co2 from the tube/tire after returning home and inflate with a floor pump?
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  19. #69
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    Quote Originally Posted by blackfrancois View Post
    is this the main reason we should remove the co2 from the tube/tire after returning home and inflate with a floor pump?
    Yes.


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  20. #70
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    Quote Originally Posted by Fireform View Post
    Actually this is not true. A carbon dioxide molecule is about 116 pico meters, whereas nitrogen is about 300. Which is why bike tires go flat faster when they're inflated with carbon dioxide.


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    From memory a Nitrogen molecule (N2) is something like 300 picometers and a CO2 molecule is 330.

    I was sort of hoping that a materials scientist would chip in with the reason that CO2 leaves a tube somewhat faster than air (which is mostly nitrogen). I am not a chemist but an engineer.

    But CO2 doesn't "leak" out - it defuses through the butyl or latex rubber. This has to do with the make up of the material and not the size of the molecules themselves.

  21. #71
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    Quote Originally Posted by Tom Kunich View Post
    From memory a Nitrogen molecule (N2) is something like 300 picometers and a CO2 molecule is 330.

    I was sort of hoping that a materials scientist would chip in with the reason that CO2 leaves a tube somewhat faster than air (which is mostly nitrogen). I am not a chemist but an engineer.

    But CO2 doesn't "leak" out - it defuses through the butyl or latex rubber. This has to do with the make up of the material and not the size of the molecules themselves.
    I was going from memory too, but I was closer to right. CO2 is 232 pm, N2 is 330.

    http://physics.stackexchange.com/que...carbon-dioxide


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    Quote Originally Posted by Tom Kunich View Post
    From memory a Nitrogen molecule (N2) is something like 300 picometers and a CO2 molecule is 330.

    I was sort of hoping that a materials scientist would chip in with the reason that CO2 leaves a tube somewhat faster than air (which is mostly nitrogen). I am not a chemist but an engineer.

    But CO2 doesn't "leak" out - it defuses through the butyl or latex rubber. This has to do with the make up of the material and not the size of the molecules themselves.
    Physics background myself. While N2 is indeed 300 pm, a CO2 molecule is 232 pm. The C=O bond is 116 pm.
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  23. #73
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    Quote Originally Posted by SauronHimself View Post
    Physics background myself. While N2 is indeed 300 pm, a CO2 molecule is 232 pm. The C=O bond is 116 pm.
    Yeah, but the difference in HTFU/WTF coefficients is negligible.

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    Quote Originally Posted by ogre View Post
    Yeah, but the difference in HTFU/WTF coefficients is negligible.
    And we still do not have any good explanation of why CO2 defuses through hydrocarbon compounds faster than N2.

    We also do not have an explanation of the driving force or energy source of this diffusion.

    In any case the size of the molecule does NOT matter.

  25. #75
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    Why CO2?

    Quote Originally Posted by Tom Kunich View Post
    And we still do not have any good explanation of why CO2 defuses through hydrocarbon compounds faster than N2.

    We also do not have an explanation of the driving force or energy source of this diffusion.

    In any case the size of the molecule does NOT matter.
    Gas molecules do migrate through holes in the polymer. Allow me to direct you to a published article: https://imageserv5.team-logic.com/me..._20et_20al.pdf

    One salient quote:
    "The types of gas affect both solubility and diffusivity of the gas. Diffusivity greatly depends on
    the size of the gas molecule. The smaller the size of the gas molecule, the faster the diffusivity."


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