How much does air weigh?

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  • 10-21-2004
    Spoke Wrench
    How much does air weigh?
    I read somewhere that co2 weighs 51% more than air so naturally my first thought was: "How much does the air in my tires weigh?" Obviously, since I use about 7or 8 atmospheres of pressure in my tires it must weigh 7 or 8 times an equal volume of unpressurized air. Course, if you could put unpressurized air on a scale, wouldn't it weigh zero? I need one of you smart people to 'splain it to me.
  • 10-21-2004
    vjarnot
    Quote:

    Originally Posted by Spoke Wrench
    I read somewhere that co2 weighs 51% more than air so naturally my first thought was: "How much does the air in my tires weigh?" Obviously, since I use about 7or 8 atmospheres of pressure in my tires it must weigh 7 or 8 times an equal volume of unpressurized air. Course, if you could put unpressurized air on a scale, wouldn't it weigh zero? I need one of you smart people to 'splain it to me.

    The density of air is approximately 1.25kg/m3, or 1.25 milligrams per cubic centimeter - of course humidity, temperature, and altitude have a lot to do with this. So figure up the volume of your tires and you can estimate based on the density...
  • 10-21-2004
    Tig
    Quote:

    Originally Posted by Spoke Wrench
    Course, if you could put unpressurized air on a scale, wouldn't it weigh zero? I need one of you smart people to 'splain it to me.


    Sea level air "weighs" 14.7 psi. Scales are zeroed out. Politician air is hot and without content, so it "weighs" the least. Try to capture some presidential candidate's air (doesn't matter which one, they are both wind bags) and pump it into your tires for maximum weight weenie points. I hope that helps.
    ;)
  • 10-21-2004
    Spoke Wrench
    Yup. That's exactly the kind of answer that I was looking for.
  • 10-21-2004
    bikist
    The weight of air is completely negligible compared to other weight variables. It wouldn't even factor in if you were running monster truck tires. The weight of the tires sure as heck would though
  • 10-21-2004
    HAL9000
    It depends on...
    ...the (local) pressure and humidity of the air being put into the tire.

    That being said; for a local temp of about 72 deg F, 30 in Hg, moist air weighs about 1.197 g/l or 0.000043 Lb/in^3 (0.00069 oz/in^3).

    But 14.7 psi is easier(more common in the US).
  • 10-21-2004
    Roadcruiser
    Depends...
    Quote:

    Originally Posted by Spoke Wrench
    ...wouldn't it weigh zero? I need one of you smart people to 'splain it to me.

    ...if you're peddling into a strong head wind, it weighs a bloody ton. :D
  • 10-21-2004
    plus_vite
    Easy way to figure it out...
    Quote:

    Originally Posted by Spoke Wrench
    I read somewhere that co2 weighs 51% more than air so naturally my first thought was: "How much does the air in my tires weigh?" Obviously, since I use about 7or 8 atmospheres of pressure in my tires it must weigh 7 or 8 times an equal volume of unpressurized air. Course, if you could put unpressurized air on a scale, wouldn't it weigh zero? I need one of you smart people to 'splain it to me.

    Take a sample of sealed CO2 cartridges and weigh each one, and then determine average weight.

    Take a sample of emptied CO2 cartridges (it's best if you saved the ones that you weighed before use), weigh each one, and determine average weight.

    The difference of the averages should give you a very good idea of exactly how much the CO2 actually weighs.

    -PV
  • 10-21-2004
    Fogdweller
    A common myth explained
    Quote:

    Originally Posted by Tig

    Politician air is hot and without content, so it "weighs" the least.
    ;)

    I used to think this and used it in debate whenever I could. But, it turns out that politicians expel methane from both ends, unlike the voting public, which turns out to be more dense than air at any altitude. This also turns out to be the origin of phrases such "explosive dialog" and "volatile words".
  • 10-21-2004
    jamesau
    Let's Clear the Air
    14.7 psi is a pressure measurement and nothing to do with density, fellas.

    Now hold on while I put on my pointy headed cap...


    OK, here’s the back of the envelope approximation (went to school for a long time for this)…

    Assuming 700x23c tires at 100 pounds inflation pressure at 20 degrees Celsius.
    Inside bead seat diameter is 622mm
    Tire diameter is 23 mm
    Approximate air volume is
    (Pi * 23*23/4)*(622+23)/1000 = 268 cubic centimeters.

    Assuming ideal gas, gives 0.087 mols.

    So, for CO2, we’ve 44 * 0.087 = 3.8 grams
    And Air (79%N2, 21% O2), we’ve 29 * 0.087 = 2.5 grams

    So the difference is 1.3 grams (per tire). That gives you a burden of 2.6 grams if you’re riding on CO2


    I wouldn’t worry about it.
  • 10-21-2004
    TurboTurtle
    Quote:

    Originally Posted by plus_vite
    Take a sample of sealed CO2 cartridges and weigh each one, and then determine average weight.

    Take a sample of emptied CO2 cartridges (it's best if you saved the ones that you weighed before use), weigh each one, and determine average weight.

    The difference of the averages should give you a very good idea of exactly how much the CO2 actually weighs.

    -PV

    Cartridge CO2 is liquid in the cartridge. This would determine the weight of the CO2 in the tire at whatever the pressure you get with one cartridge, though.- TF
  • 10-21-2004
    jamesau
    Oops
    Approximate air volume is
    (Pi * 23*23/4)*Pi*(622+23)/1000 = 842 cubic centimeters. (Forgot extra Pi)

    That increases your burden to 8.2 grams if you're riding on CO2.

    Now I might start to worry.
  • 10-21-2004
    Spoke Wrench
    Quote:

    Originally Posted by jamesau
    Approximate air volume is
    (Pi * 23*23/4)*Pi*(622+23)/1000 = 842 cubic centimeters. (Forgot extra Pi)

    That increases your burden to 8.2 grams if you're riding on CO2.

    Now I might start to worry.

    Whew! That's more than I thought.

    Now how about this. Is that really rotating weight or does the air just sort of slosh around inside the tire?
  • 10-21-2004
    jamesau
    That'd be rotating weight.

    Now I'll have trouble sleeping tonite (better take in another ball game).
  • 10-21-2004
    HAL9000
    for just the weight of the air we breathe
    which is moist air not dry air, for the most(moist?) part;

    for a local temp of about 72 deg F, 30 in Hg, moist air weighs about 1.197 g/l or 0.000043 Lb/in^3 (0.00069 oz/in^3). Now apply the volume & pressure factors to see the weight.

    But you will find the mass diff to be quite small (CO2<-->dry air<-->wet air) as previously stated.

    Now if you want to use Helium or Hydrogen...don't It'll leak out faster than CO2 (overnight) & cost you more.
  • 10-21-2004
    asgelle
    Quote:

    Originally Posted by jamesau
    Approximate air volume is
    (Pi * 23*23/4)*Pi*(622+23)/1000 = 842 cubic centimeters. (Forgot extra Pi)

    That increases your burden to 8.2 grams if you're riding on CO2.

    Now I might start to worry.

    But don't forget that that's the mass in the tire. That's what you would use for acceleration effects. For calculating the force of gravity, you have to subtract out the weight of air displaced.
  • 10-21-2004
    Kerry Irons
    Interesting math
    You said that you forgot Pi (which you did) and then somehow multiplied 3.8 times 3.14 and came up with 8.2. Which base math are you using? Just for reference, a 23 mm tire contains 0.75 liters of air, and at 8 bar that air weighs 8.7 gm. If it was CO2, the weight would be 14.4 gm. Of course, the CO2 diffuses out pretty fast, so it would soon weigh less :)
  • 10-21-2004
    roberto
    Guys remember to include BUOYANCY in your estimates!
    Seems to me that is necessary to include the buoyancy estimate. Air inside the atmosphere
    tends to be weightless.
  • 10-21-2004
    jamesau
    Quote:

    Originally Posted by Kerry Irons
    You said that you forgot Pi (which you did) and then somehow multiplied 3.8 times 3.14 and came up with 8.2. Which base math are you using? Just for reference, a 23 mm tire contains 0.75 liters of air, and at 8 bar that air weighs 8.7 gm. If it was CO2, the weight would be 14.4 gm. Of course, the CO2 diffuses out pretty fast, so it would soon weigh less :)

    Ok, here we go. I estimated the tire contains 842 cc which equals 0.842L which is close enough to .75L for me.

    I initially (based on miscalc) stated that weight burden of CO2 was 2.6 grams... 2.6 times Pi gives me the 8.2 grams... my oops-revealed correction. 8 bar is around 116psi. Kerry, I'm just doing a scratch on a napkin estimate - your weight estimate sounds high but I won't dispute it; I'm not gonna dust off my thermo texts for this argument.

    By ballpark, I'm ok with all this being up to 30% off. Bottom line is that using CO2 will affect only the conscience of the most ardent weight weenie.

    To Roberto, I'm just lookin' at mass estimates... whether it's air, CO2, Hydrogen, or buckshot.
  • 10-21-2004
    Dave_Stohler
    You could always switch to nitrogen. Air has a mass of about 30g/mole (if I remember my thermo equations), whereas N2 is about 28g/mole, and CO2 is 40g/mole. F1 racing teams have used N2 for filling their tires for years. It seems that N2 is less expansive than air, and cuts gown on blowouts as well. The only reason we use CO2 cannisters on bikes is because they already existed for the whipped-cream industry (and it's cheap, too).

    As for the dumb@$$ who confused mass with pressure: don't offer technical information where you are blatantly ignorant on the subject.
  • 10-22-2004
    Kerry Irons
    Tweeeet! Foul!
    Quote:

    Originally Posted by Dave_Stohler
    As for the dumb@$$ who confused mass with pressure: don't offer technical information where you are blatantly ignorant on the subject.

    To follow this advice would violate one of the fundamental rules of the Internet, and of this forum! :)
  • 10-22-2004
    pr0230
    WoW!
    Quote:

    Originally Posted by jamesau
    14.7 psi is a pressure measurement and nothing to do with density, fellas.

    Now hold on while I put on my pointy headed cap...


    OK, here’s the back of the envelope approximation (went to school for a long time for this)…

    Assuming 700x23c tires at 100 pounds inflation pressure at 20 degrees Celsius.
    Inside bead seat diameter is 622mm
    Tire diameter is 23 mm
    Approximate air volume is
    (Pi * 23*23/4)*(622+23)/1000 = 268 cubic centimeters.

    Assuming ideal gas, gives 0.087 mols.

    So, for CO2, we’ve 44 * 0.087 = 3.8 grams
    And Air (79%N2, 21% O2), we’ve 29 * 0.087 = 2.5 grams

    So the difference is 1.3 grams (per tire). That gives you a burden of 2.6 grams if you’re riding on CO2


    I wouldn’t worry about it.


    Ask and yee shall receive!