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Colorado xxxxx is correct. hoffdano is not. The coefficient of pressure change with temperature is much lower with nitrogen than it is with oxygen. All military and commercial aircraft tires are inflated with nitrogen.
Hi, chemical engineer here.
No, Hoffdano is correct. Your tires don't care what gas is inside them. It could be 3 moles of hydrogen or 3 moles of argon. They will occupy the same volume under the same temperature and the same amount of pressure. If the temperature goes from 30ºF to 80ºF then your the tire pressure will be the same regardless of what gas is inside them.
There is no coefficient of pressure change. There are gas laws, and gas laws don't care about chemical identity or molecular weight. If you want to get technical about it, instead of using the ideal gas law, you can use a compressibility factor (PV=znRT), but at ~2.x atm of pressure (gauge), I should be shocked if the ideal gas law prediction was off by more than 5%
Nitrogen is less reactive than oxygen, true. Nitrogen has a slighly larger molecular footprint if memory serves. I am reasonably certain that the diffusion coefficient for N2 is different than O2.... but all of these are moot points because the normal air going into your tires is 79% nitrogen anyway. This all adds up to negligible differences. The crappy rubber stem valve stems on your tires probably have a much more pronounced effect on your tire pressure than whether they are filled with N2 or air.
As to why the military uses nitrogen over compressed air? I have no idea. My guess is that it is probably cheaper to get dry nitrogen than dry air, but you've have to ask a logistics officer.
As to why the military uses nitrogen over compressed air? I have no idea. My guess is that it is probably cheaper to get dry nitrogen than dry air, but you've have to ask a logistics officer.
And this, is probably the one advantage you can give to nitrogen filled tires. If you use bottled nitrogen, you are pretty much guaranteed to have pure dry nitrogen in there. If you use standard compressed air, unless there is a dryer in the system, you are likely to get moisture in the tire. Its in the air after all and a compressor tank usually has a decent amount of water rolling around in the bottom. I bunch of water in your tires is not a god thing, but unless you are using a really waterlogged compressor and having to add air over and over, its just not going to be that much in there. Plus, most wheels are aluminum now anyway, so the moisture is not going to cause corrosion.
I keep my tires filled with hydrogen, makes the car lighter, some mornings I have to pull the car down from the garage ceiling before I can get it in to drive to work.
Realistically, i'd bet if you did fill your tires with hydrogen, they would be flat within a week.
I'd recommend trying it with helium rather than hydrogen. Much safer
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Quote:
Originally Posted by TechGromit
I keep my tires filled with hydrogen, makes the car lighter, some mornings I have to pull the car down from the garage ceiling before I can get it in to drive to work.
Pretty clear that a lot of contributors to this thread don't even have high school chemistry under their belts.
Tarzanman and Hoffdano are notable exceptions.
The only thing in air that makes it behave differently than pure nitrogen wrt temperature changes is moisture (recall partial pressure of water vapor from high school chemistry, if you had it). If the air is dry, then there isn't any difference.
There is a small difference in atomic radius between oxygen and nitrogen - oxygen has one more proton in the nucleus, so it "pulls" harder on the electron cloud, making an oxygen molecule slightly smaller than a nitrogen molecule. Whether this has any effect on the permeability through car tires I do not know, though my own experience suggests that tires filled with pure N2 as opposed to 78% N2 do lose pressure slower over a period of many weeks (I could be imagining this, or other effects, such as pinhole punctures, make a comparison impossible for the layperson).
Last time I looked at a chart of vapor pressure of water vs temperature, my conclusion was that pressure changes due to moisture in air was not significant for passenger car tires, but were significant for racing car tires, since they operate a much higher temperatures. 1/2lb pressure makes a big difference in spring rates for a Sprint Cup car.
The difference is minimal regardless of what a sales person tells you.
In almost all cases it is minimal. However, I have been in Fairbanks, Alaska; there it's a big difference.
Between October and March temperatures typically range from -45F at night to -10F around noon. To human flesh it's still deadly cold either way. To regular air (79% Nitrogen, 21% Oxygen), that is the difference between a full tire and a flat tire, in the -same- day. No punctures but, the drop in temperature causes the air to contract (shrink) and expand again next noon.
Pure Nitrogen (>98%) expands and contracts at far greater temperature ranges than normally exist outside of labs (i.e. -30F to 110F). If you can do it low cost, I would suggest everybody, at all times, have pure Nitrogen in their tires.
I'm sure some people from Phoenix, with their >80F days and <50F nights, the tires look "tired" at night and, fuller in the daytime.
EDIT: However, in most areas, temperature change is either gradual or relatively constant so regular "air" won't adversely affect the life of your tires.
OK, lots of nit-noid info here...
We, and almost everyone I know, fills their race tires with nitrogen. Keeps the pressure much more stable throughout a race weekend. Something about the nitrogen not bleeding through the thin sidewalls as easily as "air".
look at it this way - when the TPMS goes off because of that temperature drop, it's a nice reminder that you should check your tire pressures anyway. Especially if your TPMS allows for 5+ psi differences.
If it comes from factory as nitrogen, so be it. Just don't pay extra for it or during refills.
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