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Re: Nitrogen

To: "Larry Macy" <macy@bbl.med.upenn.edu>
Subject: Re: Nitrogen
From: "Jim Johnson" <bmwwxman@gmail.com>
Date: Fri, 25 Aug 2006 14:34:03 -0500
On 8/24/06, Larry Macy <macy@bbl.med.upenn.edu> wrote:
> Well, I'll let the expert chime in later, but as I recall Boyle's Gas
> Law (Hope it was Boyle anyway) says pressure and temp and volume are
> related. I don't think the make up of the gas has any thing to do with
> volume (unless it is related to Avragado's number, which is).

Hmmm. Larry is not quite right... Boyle's Law states:

P ~ 1/V     Pressure is inversely proportional to Volume

We would be better served here, with the Universal Gas Law

PV = nRT

Where:

P = Pressure
V = Volume
n =  Molecular weight (Moles) of the gas in question
R = Universal Gas constant for the gas in question
T = Temperature

What this law tells us for ALL gases is that Pressure and Volume are
inversely proportional (ie...Increase(decrease) one the other one
decreases(increases) and that both Pressure and Volume are directly
proportional to Temperature (ie...Increase(decrease) one and either of
the others will increase(decrease) also.

What is important to note is that P,V, and T don't care which gas you
use, they will maintain the same relationship. What WILL change as you
change gasses will be the RATE at which they change with respect to
each other. However, the difference in molecular weight between N and
dry air is quite small (N = approx. 21  -  dry air approx. 28)
there will be VERY little difference between P, V, and T in a tire
filled with Nitrogen and one filled with dry air.

I would look, instead, for differences in how each gas:
1) Reacts with the tire compound (oxidation vs nitridation).
2) Leaks out through the tire via osmosis (relative size of the molecules)
3) Disperses heat.

The claims are that:
a) Nitrogen reduces oxidation of the tires. Obviously true because
only oxygen causes oxidation and dry air is about 21 percent oxygen.
b) Nitrogen molecules are definitely bigger than oxygen molecules so
they should not leak from the tire walls via osmosis as quickly as
oxygen.
c) Pure nitrogen is a slightly better heat conductor than dry air so
the temperature of nitrogen filled tires should cool faster.

In conclusion, it appears that the three above items theoretically
show nitrogen to be better for tire inflation than dry air. Only
practical testing will show if these "benefits" are substantial enough
to achieve the results claimed, make anyone safer, save much money, or
save much gas.

The FAA requires by federal mandate that all airlines use Nitrogen
inflation in aircraft tires for the above three reasons. I have to
assume they have done the testing as they are usually extremely
thorough with this sort of thing.

Therefore I would say that nitrogen tire inflation is a good thing but
the benefits may be small for a single Spridget.  Now....  If you have
Frank's *FLEET*.........    ;-)

Cheers!!
Jim - 68 Midget in Dodge City




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