Argon in tyres?
12-04-19 | 12:49 PM
#101
Generally bewildered
Joined: Aug 2015
Posts: 3,038
Likes: 344
From: Eastern PA, USA
Bikes: 2014 Trek Domane 6.9, 1999 LeMond Zurich, 1978 Schwinn Superior
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.
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".
12-04-19 | 01:03 PM
#102
Mad bike riding scientist
Joined: Nov 2004
Posts: 29,139
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From: Denver, CO
Bikes: Some silver ones, a red one, a black and orange one, and a few titanium ones
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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Stuart Black
Dreamin' of Bemidji Down the Mississippi (in part)
Plan Epsilon Around Lake Michigan in the era of Covid
Gold Fever Three days of dirt in Colorado
Pokin' around the Poconos A cold ride around Lake Erie
Dinosaurs in Colorado A mountain bike guide to the Purgatory Canyon dinosaur trackway
Solo Without Pie. The search for pie in the Midwest.
Picking the Scablands. Washington and Oregon, 2005. Pie and spiders on the Columbia River!
12-04-19 | 01:17 PM
#104
Senior Member
Joined: Mar 2011
Posts: 3,860
Likes: 2,217
Yes, of course. When you use nitrogen, it will be delivered dry. Sorry, need more coffee today.
12-04-19 | 11:04 PM
#105
Insane Bicycle Mechanic
Joined: Mar 2008
Posts: 10,154
Likes: 1,122
From: other Vancouver
12-05-19 | 10:54 AM
#106
Generally bewildered
Joined: Aug 2015
Posts: 3,038
Likes: 344
From: Eastern PA, USA
Bikes: 2014 Trek Domane 6.9, 1999 LeMond Zurich, 1978 Schwinn Superior
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.
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.
12-05-19 | 04:37 PM
#107
Banned
Joined: Apr 2010
Posts: 9,923
Likes: 1,066
From: Lincoln Ne
Bikes: RANS Stratus TerraTrike Tour II
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.
12-05-19 | 04:39 PM
#108
Banned
Joined: Apr 2010
Posts: 9,923
Likes: 1,066
From: Lincoln Ne
Bikes: RANS Stratus TerraTrike Tour II
From here.
There goes my personal methane recycling idea...
I can forget about NAWS for my bike too.
There goes my personal methane recycling idea...
I can forget about NAWS for my bike too.
