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Argon in tyres?

Old 12-04-19, 12:49 PM
  #101  
WizardOfBoz
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Originally Posted by cyccommute
I dislike responding to a 12 year old thread but your statement kind of requires it. A heavier molecule doesn’t necessarily mean that it diffuses slower. There are other factors to consider like the solubility of the gas in the rubber of the tube. Think of it this way: If you made a container out of sugar, it would hold hexane forever. It would only old water for a little while. Hexane has a higher molecular weight and is much, much larger than water but it won’t dissolve the sugar.
Yup. If there were only diffusion, the size would play a pretty strong part - mostly as a function of molecular velocity and not due to a filtration-like effect. The diffusion rate is proportional to the inverse of the square root of molecular weight. But solubility/affinity plays a part in rubber tires. So if a molecule tends to "stick" to rubber more, it diffuses more slowly.

As an example you can make a chromatography column with powdered sugar in a glass tube, with glass wool plugs at both ends. Pour some hexane mixed with a little ether through the column to get it wet. The, take some great tree leaves and grind them up. Dissolve in the same hexane mix, with dry CaSO4 mixed in to absorb all water. Filter, and pour into the top of the column. You'll see green, yellow, and red bands develop due to the different affinity that each of the different compounds (chlorophyll, and different caretenoids and anthocyanins). The speed at which these bands move down the column is very much related to how each type of molecule "sticks" to the sugar. Size doesn't play as big a part.

In a porous substance like rubber, diffusion is modulated by this "stickiness".
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Old 12-04-19, 01:03 PM
  #102  
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Originally Posted by phughes
No, they use nitrogen because it has more a consistent contraction/expansion rate, making it more stable. The tire pressure remains more consistent than it would using regular air. It has nothing to do with the moisture in the gas mixture.
Again, I dislike responding to a 12 year old thread but ...

It's the moisture in the gas mixture that cause the inconsistencies. The inconsistencies would be less if the air used was dry. Water does hinky things with temperature that other gases don't. Dry gases increase in pressure with temperature in a linear fashion (Guy-Lusaac Law). Water's increase in pressure with temperature isn't linear. It's actually exponential.
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Old 12-04-19, 01:06 PM
  #103  
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What ever happened to rest in peace? There is enough hot air in here to float a bike.
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Old 12-04-19, 01:17 PM
  #104  
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Originally Posted by phughes
No, they use nitrogen because it has more a consistent contraction/expansion rate, making it more stable. The tire pressure remains more consistent than it would using regular air. It has nothing to do with the moisture in the gas mixture.
Yes, of course. When you use nitrogen, it will be delivered dry. Sorry, need more coffee today.
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Old 12-04-19, 11:04 PM
  #105  
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Originally Posted by phughes
Sorry, need more coffee today.
More coffee! That's what I need! MOAR COFFEEE!!
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Old 12-05-19, 10:54 AM
  #106  
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If the air is saturated, and you have water condensing (as you might with a tire pumped up with air from a hand pump) the compressibility will not follow the idea gas law very well because an increase in pressure results in water condensing. This is highly nonlinear.

For those that are really interested, there's an equation of state developed for most air. See here: https://www.nist.gov/system/files/do.../CIPM-2007.pdf

An ideal gas is represented as P V = n R T. That is pressure times volume = amount of gas times temperature times the ideal gas constant "R". This "law" is a good approximation at low pressures (say 2 atmospherers or below) and high enough temperature (let's say above freezing). At higher pressures and/or lower temperatures a "fudge factor" called compressibility is added. Denoted "z", so we have

P V = z n R T

Obviously, if z = 1, you have the ideal gas loaw. Just to keep things reasonable, at the pressures and temperatures we see in road bike tires I'd be very surprised if z was more than 0.02 less than 1. So the non-idealities are really, really small. That is to say, it's kind of ridiculous to put all this effort into modulating z with expensive dry N2 or Ar. IMHO. If you want to figure out the compressibility of dry N2 or Ar vs moist air, the article above gives you the tools. Do the math!

In summary, if you are using air from a compressor (where some of the water condenses out and is drained) you are using reasonable dry air and things are the compressibility is pretty linear. Don't think you need dry nitrogen.

If you lived in the tropics were you have high absolute humidity and you currently can only inflate with a manual pump, then dry nitrogen (or getting a compressor) might make sense.

I see no advantage and a couple of disadvantages (high cost, higher weight by a couple of grams, and waste of a limited natural resource) in argon. Using argon in bike wheels is just plan silly.

Last edited by WizardOfBoz; 12-05-19 at 12:09 PM.
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Old 12-05-19, 04:37 PM
  #107  
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Originally Posted by DieselDan
Race cars use pure nitrogen in those tires because ambient air can have too much moisture. When that moisture boils, you can have all kinds of problems. We cyclists never deal with anything close to those kinds of speeds and cornering forces. The same can be said of our street cars.
Actually not true. Some racer types put extra air pressure in their tires to begin with. This is usually done at home in the cool of the morning. Then---------with some moisture in the tire, and the heat of the day on black roads, the pressure goes way up, especially with heavy braking down long hills and he tires simply blow up.
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Old 12-05-19, 04:39 PM
  #108  
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Originally Posted by Wordbiker
From here.
There goes my personal methane recycling idea...

I can forget about NAWS for my bike too.
Oh----------and dont forget cow farts that some are so worried about these days.
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