Multiple drain fill cycles would be needed to realize the benefits of a "pure" nitrogen fill..

It is amazing how much some of these places are charging for a nitrogen fill - nitrogen is an industrial commodity......

Evian water and the like is an analogy that comes to mind.

 

I simply cannot believe how often this subject comes up.

"First, the claim that pure N2 provides more "stable pressure" than air over a change in temperature. Not true. Here's why: as a consquence of Avogadro's law, The ideal gas constant has the same value for all gases. This means that the constant k = P*V/T
where:
P is the pressure of the gas, V is the volume and T is the temperature of the gas has the same value for all gases, independent of the size or mass of the gas molecules. In other words, it doesn't matter what the gas is, the pressure change is going to be the same for a given change in temperature.

So let's apply this to the real world. If you're at a track day, and your tire temperature goes from up from ambient, e.g. 20C to 100C, the pressure is going to go up by the ratio of absolute temperature, i.e. (100 + 273) / (20+273), or a factor of 1.27. So if you start at 30 psi, you're going to be at 38 psi. Might be time to take a break in the pits! This pressure rise is going to be the same whether you've got air, pure N2, or any gas you choose in your tires.

Ok, with that settled, let's look at the benefit in terms of how often you need to add air (or pure N2) to your tires. There actually is a small benefit here, but it's not as big as the nitrogen proponents would have you believe.

N2 and O2 have similar molecular weights, but it turns out that even though O2's weight is higher, it's actually a slightly smaller molecule. This has to do with electron orbits, van der Waals forces, and so on. There some other factors at play such that the permeability rate of O2 through rubber is actually quite a bit higher than N2, namely 10 for O2 vs. 3 for N2. This usually where the marketing scams start for the N2 guys, but let's take the next step.

Air is 78% N2 to begin with, so it's not going to leak out 10/3's faster. For pure N2 the rate is 3...how about for air? It's going to be .78*3 + .22*10 = 4.54. So, N2 will leak out more slowly than air, with a theoretical ratio of (3/4.54) or 0.66. Let's compare to the real world again. In a totally unbiased Consumer Reports article, they measured how much pressure a variety of tires lost over a year. Starting at 30 psi, the air filled tires lost 3.5 psi/year and the pure N2 tires lost 2.2 psi/year. How's this compare to the theory? 2.2/3.5 = .63, so both theory and empirical data are in good agreement here.

So, the ultimate question, is what's this worth to you? If losing 2.2 psi/year vs. 3.5 psi/year is worth $20 to you, go for it. If it's free, take it. I would submit, however, that you really ought to be checking the pressure in your tires more than once a year anyway with a high quality pressure gauge."

However, any claim that Nitrogen provides any handling benefit whatsoever, on the race track or anywhere else, is utter nonsense and totally unsupported by science. If you fill your tires with air, nitrogen, helium, or any other gas, it will expand EXACTLY the same when heated. It's called Avogadro's LAW for a reason.

It would seem the Nitrogen marketers have done an amazing job.
In my opinion, it is a complete waste of money and you'd be much better off donating to your favorite charity.

 

Excellent write up!

And just in case folks want to talk about water vapor in air... please remember water vapor is a gas.

 

Of course, you are correct. Avogadro's Law certainly applies to water vapor.
But even in the Equatorial Rain Forest, the moisture content in the Earth's atmosphere is only about 1%.
However, compressed air is actually quite dry. Most air pressurization systems raise the air pressure to 7 Bar or so. In doing so, the air is heated significantly (Boyle's law). As this pressurized air cools to ambient, the pressurized air is no longer able to hold the original moisture content and the water vapor condenses back to a liquid. Modern air pressurization systems use traps or dryers to eliminate this liquid moisture. The result is that most compressed air is almost totally devoid of water vapor.

 

but afterward, the car does feel different.....same pressure I checked.....

 

The ride felt different, because your wallet was $80.00 lighter.

 

Forged, thanks for taking the time for posting the good info....

Compression of air will result in a greater concentration of water in that volume at a given temperature. COOLING will cause condensation to liquid, with an equilibrium partial pressure of vapor above that liquid at that T. Heating will increase the equilibrium partial pressure of water vapor in the air at a given temperature..........there will always be an equilibrium partial pressure of water in compressed air, unless it is scrubbed......water will lead to non-linear and unpredictable pressure variations.............

I am unsure that the majority of compressed air available to the public scrubs the water, but if you are quite sure, that is good news...........perhaps in the past it was not so prevalent, thus the concern over oxygen corrosion abetted by water under extreme conditions in the past......and still current concern over non-linear pressure variations due to water content by those extreme performance orientated folks...

As an aside, - I saw the CR article - the magazine is a POS, IMHO, - we get it because it is gifted by our in-laws...............sorry, I would not quote them...

Having said that, it should be considered that there is a benefit realized for a pure DRY fill of nitrogen vs. air for those folks who race or are serious about track days-

The average driver, there is likely no benefit as you pointed out.

Here is a very practical techncal bulletin , which should be considered by those interested parties, from the Rubber Manufacturers Association, a conservative body, re. nitrogen in tires:

https://betiresmart.org/newsroom/release.cfm?ID=193

Interesting stuff to discuss !

 

Yes, if there is an unknown quantity of LIQUID water in the tire, VERY small pressure variations on the order of .1 psi could occur as the water evaporated and then condensed. However, these variations are minute from a scientific viewpoint. However, most compressed air systems installed since the 1980s have mechanisms to eliminate liquid water.
As long as the water is Vapor, Avogadro's Law applies.
I stand by my previous post. Nitrogen's PLACEBO effect seems well documented, both in these forums and elsewhere. But Nitrogen's actual scientific benefit in these cases is non-existent.
In military and aerospace applications, Nitrogen's moisture free properties are used to prevent corrosion, even small amounts of which could affect the structural integrity of the component part over time.
What annoys me is the ridiculous claims that the Nitrogen marketers use regarding improved ride quality, improved handling, reduced greenhouse gasses, etc.,etc.,etc.
There could be one small weight-reduction benefit to the Nitrogen use. That is the eighty bucks missing from your wallet. However, that benefit is negated if you used a credit card.

 

I'd add that the water is always vapor unless you do something really silly when airing up your tires with good old compressed air (say, pumping in compressed shower air into your tire thats currently in the oven... so when the tire cools to a normal temp, condensation forms). Normal airing when the tires are cool, results, I would bet, no liquid water (or practically none) getting into the tire. When the tire heats up from driving, any traces of liquid evaporate. The insulation properties of nitrogen over air.. forget them. The tire touches the rim at the bead, and in any case, it only would minimize heat transfer from tire to rim or rim to tire. What does that accomplish? It may avoid using your tires to cool your brakes.. if the alloy wheels work as a good heatsink. I think most of the benefit at a track would be from just 'airing to the proper temperature' for the track, regardless of it being air or nitrogen. The rest is placebo effect.