I have not used air in any of my 3 vehicles for over a year.

Check it out - I use Nitrogen. Really.
It will allow you to keep the same pressure in your tires regardless of the temperature outside.

What do you use in your tires?<ul><li><a href="http://www.suburbantire.com/tires/nitrogen.htm">Nitrogen tires!</a></li></ul>

jk :)

What a rip. Air is 78% nitrogen anyway.

<a href="http://encarta.msn.com/encyclopedia_1741500785/Air.html">AIR</a>

I know that according to the industry, the oxygen leaks out 3X faster than the nitrogen. I don't feel like doing the math, but replacing the 21% of oxygen in the air I pump into my tires with nitrogen isn't going to give me much more time between pressure checks.

But hey... the hype surrounding this does allow Ingersoll-Rand to sell this <a href="http://www.irco.com/pressroom/newsatir/is/58711.html">nifty nitrogen tire-filling system</a>.

With the "3X faster" stat, they are counting on customer stupidity, thinking that AIR = OXYGEN. Bzzzt, wrong, thanks for playing.

that an oxygen molecule leaks thru faster than a nitrogen molecule due to the size of the atoms.

For this calculation, you need to know the void space (gaps between molecules) in galvanized rubber relative to the size of N2 and O2 molecules.

I would guess that different brands of tires have different sized void spaces. For example, if a jar is filled with marbles, the void space is large and air could pass through quickly; however, if the jar were filled with sand, it would take longer for the air to pass through the void space.

My guess is that they are looking at the largest possible void space in their calculation. With that said, a Nitrogen molecule is about 15% larger at most than an Oxygen molecule.

And I would argue that if the oxygen leaves quicker, then each time you fill your tire, you should have a lower ratio of O2 to N2 in your tire.

Aslo, they never state what the rates of diffusion (evaporation of N2 and/ or Oxygen through the rubber) are across their worst case scenario tire.

This ends our lesson for the day. ;-)

in my line of work, it doesn't pay to think that much about it. "Directionally correct" they call it. ;-) I was figuring a 300% faster leakage (their stat) for the oxygen molecules, of which a tire filled with "regular air" from the atmosphere is 21%. Point taken that each subsequent refillage the mixture would seem to become less oxygen rich, further negating the need for the Nitrogen filling gimmick.

This probably matters if you are in NASCAR and replace tires often. But for the long term tire, air is all it need bear!

I'm a chemical engineer, so in my line of work... it pays to be detailed. Well detailed within a 10% margin of error. ;-P

I'm new to this forum. Just bought red ALMS. While being the owner of my son's Limited Late Model stock car and numerous Racing Karts over the years, we used Nitrogen because bottled nitrogen has no moisture. It's the moisture that turns to steam that causes increasing air pressure. We produced the same results with an air dryer in our shop. We used an air bubble with dry air at the track but offered our air compressor to our competitors.

The link is to TruckTires.com but it is a B-F site.

I'll let you read the site for their opinion, but I do know Costco and some other tire stores are starting to inflate tires with Nitrogen for no extra charge, so they must believe in it since it costs them more...<ul><li><a href="http://www.trucktires.com/us_eng/library/publications/periodicals/RealAnswers/03v8iss3/ra8.asp">Bridgestone-Firestone link</a></li></ul>

Quick calc shows that filling tires every three-months will yield a %N2 in tire of 99.2%. You can see that after the 4-month the N2% is already 96.9% in the tire. This means after 4-months you are already pretty close to pure N2 in the tire.

The moisture is a viable argument but these guys need to leave the Oxygen out of it. It's not destroying our tires.

psig N2 psig O2 psig % N2
time=0 36 28.44 7.56 79.0%
1-month 33.7 28.1 5.6 83.5%
2-month 31.3 27.8 3.6 88.6%
3-month 29.0 27.4 1.6 94.6%
Fill 36.0 33.0 3.0 91.6%
4-month 33.7 32.6 1.0 96.9%
5-month 32.3 32.3 0.0 100.0%
6-month 32.0 32.0 0.0 100.0%
Fill 36.0 35.2 0.8 97.6%
7-month 34.8 34.8 0.0 100.0%
8-month 34.5 34.5 0.0 100.0%
9-month 34.2 34.2 0.0 100.0%
Fill 36.0 35.6 0.4 98.9%
10-month35.3 35.3 0.0 100.0%
11-month34.9 34.9 0.0 100.0%
12-month34.6 34.6 0.0 100.0%
Fill 36.0 35.7 0.3 99.2%

Beware German vehicles bearing hydrogen.


--Tim--

I was just pulling his leg.

It's nearly impossible to see the hydrogen fire in the picture
<img src="http://pictureposter.audiworld.com/7403/hydrogenfire_1.jpg">
you can however see a jet of flame shooting out with some photoshop work.
<img src="http://pictureposter.audiworld.com/7403/hydrogenfire_2.jpg">

Correct me if I'm wrong, but I always thought that the temperature of a gas was relative to it's pressure. This is because temperature is a measure of molecular energy, and pressure is the measure of force exerted by a gas on it's container. Here are my calculations ;):

Pressure (1) * Volume (1) / Temperature (1) = Pressure (2) * Volume (2) / Temperature (2)

We assume volume is constant, or remains the same both times. We also are assuming that gas is not being lost by exiting the rubber membrane of the tire.

Therefore,
36psi / 28degrees = Xpsi / 20degrees or

36psi * 20degrees = 28degrees * x
720 = 28x
25 = x

When the temperature decreases from 28 degrees celcius to 20 degrees, the molecular energy is low enough that the origional psi of 36 drops to 25.

Oh yes, this should be true for any ideal gas, because as you may remember from school, a gas with the same temperature and pressure has the same number of molecules.

However, this is not to say that nitrogen gas is not any better, it may be that it does not exit through the membrane of the rubber tire at such a fast rate. Thus, at the same temperature, volume, and pressure, over time, a tire filled with 100% nitrogen gas will lose fewer molecules than straight air.

Your first equation is accurate (as are your assumptions) but your units are not. You are using AES (American Engineering System) units when you should be using metric units. Instead of Fahrenheit (if that's what you mean by "degrees") use Kelvin, and instead of psig use kiloPascals.

If you convert and use your equation that will be your answer. Of course, you must convert back to Fahrenheit and psig as us silly Americans don't deal in metric (we're funny like that).

Additionally, tire manufacturers recommend you inflate your tires at room temperature, ~70-F, this should probably be the base temperature you use for 36-psig.

I have done the calculation for you below to see how pressure(P=psig) changes with temp(T=Fahrenheit). Please keep in mind that I am assuming ideal gases but in reality there are compressibility factors that must be introduced to accurately describe the real behaviors. This is only an estimate!

Gas P Gas T
29.44 -20
30.78 0
32.12 20
32.65 28
33.46 40
34.79 60
36.00 78
36.13 80
37.47 100
38.81 120
40.15 140


"a gas with the same temperature and pressure has the same number of molecules."

This statement is incorrect. Different elements have different weights (that's how we differentiate between them). Scientists (and dorks like me) take this a step further by converting a set number of molecules, or atoms, into a unit called a "mole".

A mole is simply 6.02X10^23 (that's 6 followed by 23 zeros) molecules/ atoms. The weight of this quantity is the atomic mass for that atom/ molecule. An example of this is Hydrogen- 6.02X10^23 hydrogen atoms weigh 1 gram (Does your head hurt yet).

This means that a gas at a certain temp and pressure will have a different number of moles (and weight) than another gas at the same temp and pressure. This is always so. If you have the same temp and press at the same volume. Your moles must be different. If they are the same-- it must be the same gas.

So in your tire, the 36 psig is split between Nitrogen and Oxygen. The total pressure is split up by their weight fractions. So since air is 79% N2 and 21% O2, by weight, then the 36 psig must be split up so that the N2 accounts for 79% of the total pressure(28.44-psig) and the Oxygen accounts for 21% of it(7.56-psig).

This ends our lesson for the day ;-P<ul><li><a href="http://members.aol.com/profchm/idealgas.html">Click here to learn more about the Ideal Gas Law</a></li></ul>