Nitrogen in tyres

[swbarnett]

Thought I'd start a new thread rather than clog the one this came from.

Pure nitrogen apparently varies in density much less over typical tyre temperature ranges, making it well-suited to inflating tyres when cold and then keeping them at the correct pressure throughout a race meeting, etc.

As far as I know, anyway.

Here's one for the chemists out there.

The ideal gas equation is:

PV = nRT

where P=pressure, V=volume, N=number of moles of gas, R=universal gas constant and T=absolute temperature.

So, for an ideal gas kept at a constant volume (i.e. any gas in a tyre as all gases act pretty much like an ideal gas, so I'm told) as temperature rises so does pressure (at the same rate whatever the gas).

From this it should make no difference which gas is used in a tyre. Or does it? Are people mislead in thinking that Nitrogen expands less than air or am I missing something?

[jrandom]

From this it should make no difference which gas is used in a tyre. Or does it? Are people mislead in thinking that Nitrogen expands less than air or am I missing something?

The key thing, I think, is that the gas equation works on mass as an input variable, not volume, and nitrogen is fairly low mass.

A gas formed entirely of a low-atomic-weight element will obviously require less mass to pressurise a given volume to a certain point than a gas formed of the usual atmospheric mix, which has a fair amount of heavier elements spread throughout.

So if you have a lighter gas at a certain pressure in a certain volume, a given change in temperature will result in less expansion or contraction than what you'd get from a heavier gas which started at the same pressure in the same volume.

[R6_kid:Me]

doesnt each gas have its own expansivity constant? Hence why the equation works for an 'ideal gas' which is theoretical.

[jrandom]

doesnt each gas have its own expansivity constant?

Yeah.

It's in direct proportion to how heavy it is.

[jrandom]

... why the equation works for an 'ideal gas' which is theoretical.

The only difference between the 'ideal' gas and a real gas is the fact that the 'ideal' gas equation doesn't consider friction, which would be a vanishingly small term in the equation in any case.

The ideal gas equation still works on the mass of the gas as an input, which makes it applicable to anything.

[vifferman]

Air is 78% nitrogen anyway.
The critical thing with using nitrogen rather than compressed air may be the dryness of the air. From memory, I think the nitrogen used has a lot less moisture and other crap in it than your average compressed air. The moisture heating up and becoming gaseous or cooling down and condensing will possible affect things more than minor elements like hydrogen or methane or whatever misbehaving and wandering off to have a carcase meeting of the executive or whatever the naughty little molecules get up to.

[CookMySock]

Air is 78% nitrogen anyway.

LOL Chawp!!!

DB

[jrandom]

The moisture heating up and becoming gaseous or cooling down and condensing will possible affect things more than minor elements like hydrogen or methane or whatever misbehaving and wandering off...

Damn your practicality, sir. We were having fun considering that particular spherical cow.

[Badjelly]

Here's a few links found with Google:

http://www.racq.com.au/cps/rde/xchg/...s_ENA_HTML.htm
http://www.tyresave.co.uk/nitrogen.html
http://www.blackcircles.com/general/technology

The last one says "oxygen permeates three times faster than the nitrogen". That's the only explanation that makes any sense to me.

[Badjelly]

The only difference between the 'ideal' gas and a real gas is the fact that the 'ideal' gas equation doesn't consider friction

...and finite size of the molecules, but nothing that matters for oxygen or nitrogen at ordinary temperatures and pressures.

[ManDownUnder]

Filling automotive and aircraft tires^[5] due to its inertness and lack of moisture or oxidative qualities, as opposed to air, though this is not necessary for consumer automobiles.^[6][7]
Nitrogen molecules are less likely to escape from the inside of a tire compared with the traditional air mixture used. Air consists mostly of nitrogen and oxygen. Nitrogen molecules have a larger effective diameter than oxygen molecules and therefore diffuse through porous substances more slowly.^[8]

[Badjelly]

Looking up Wikipaedia, that's cheating!

[jrandom]

Right, Google tells me that the molar mass of atmospheric air is about 29 g/mol, and nitrogen's is about 14 g/mol.

In other words, Earth's atmospheric mix is, on the whole, twice as dense as pure nitrogen. Intuitively, I'd think that that would make quite a difference in rates of expansion.

I have to do some proper work now, so if anyone wants to pull some other numbers out of their arse, plug those into the gas equation and find out what the differences in rates of expansion over typical tyre operating pressures are for typical motorcycle tyre volumes, be my guest.

Otherwise I'll do it when I get home tonight...

[Badjelly]

Right, Google tells me that the molar mass of atmospheric air is about 29 g/mol, and nitrogen's is about 14 g/mol.

The second figure is for monatomic nitrogen (N), not diatomic nitrogen (N2). Diatomic nitrogen is the form that occurs in the atmosphere.

Air is a mix of 80% N2 (28 g/mol) and 20% O2 (32 g/mol), hence its molar mass is roughly 29 g/mol. So the difference between air and pure N2 is not large.

I think ManDownUnder has answered the question for us.

[jrandom]

The second figure is for monatomic nitrogen (N), not diatomic nitrogen (N2). Diatomic nitrogen is the form that occurs in the atmosphere.

Ah. I didn't realise that we were talking about filling tyres with N2.

OK, moisture is the culprit...