Why nitrogen?

On Sep 16, 8:25*am, a wrote:
On Sep 15, 6:20*pm, a wrote:





On Sep 15, 6:09*pm, "Viperdoc" wrote:

The thread on brakes raised a question from the past- why nitrogen in the
tires of big jets and heavies? Air is around 78% nitrogen anyway, and the
coefficient of expansion of the remaining 20% that's oxygen can't make that
much difference in volume.

Why not just use dry air? I couldn't imagine that dry air or dry nitrogen
could make that much difference in corrosion, either.

I seem to recall someone giving me the rationale for this a long time ago,
but also seem to remember thinking it didn't make that much sense at the
time.

The legend is that *the oxygen is reacting with the rubber. Even
though compressed air has the same fraction of oxygen as does the
atmosphere, the fact that it's compressed, it's been argued, increases
its reactivity. I can't validate the legend.

I got curious, did a quick check. The process also reduces water
vapor, and N2 does not migrate through the rubber as fast as does O2,
so pressure stays more predictable. So it's more consistent tire
pressure, less corrosion.- Hide quoted text -

Dont see why it would reduce water vapour, dry air has no more water
than dry nitrogen. and migration or porosity is a function of
molecular size. There is not much difference between nitrogen MW 28
and Oxygen MW 32.
Terry

On Sep 16, 6:39*pm, terry wrote:
On Sep 16, 8:25*am, a wrote:



On Sep 15, 6:20*pm, a wrote:

On Sep 15, 6:09*pm, "Viperdoc" wrote:

The thread on brakes raised a question from the past- why nitrogen in the
tires of big jets and heavies? Air is around 78% nitrogen anyway, and the
coefficient of expansion of the remaining 20% that's oxygen can't make that
much difference in volume.

Why not just use dry air? I couldn't imagine that dry air or dry nitrogen
could make that much difference in corrosion, either.

I seem to recall someone giving me the rationale for this a long time ago,
but also seem to remember thinking it didn't make that much sense at the
time.

The legend is that *the oxygen is reacting with the rubber. Even
though compressed air has the same fraction of oxygen as does the
atmosphere, the fact that it's compressed, it's been argued, increases
its reactivity. I can't validate the legend.

I got curious, did a quick check. The process also reduces water
vapor, and N2 does not migrate through the rubber as fast as does O2,
so pressure stays more predictable. So it's more consistent tire
pressure, less corrosion.- Hide quoted text -

Dont see why it would reduce water vapour, dry air has no more water
than dry nitrogen. and migration or porosity is a function of
molecular size. *There is not much difference between nitrogen MW 28
and Oxygen MW 32.
Terry- Hide quoted text -

- Show quoted text -

OK , I will stand corrected, on the porosity, the proper word was
permeation and nitrogen is actuallly a larger molecule than oxygen
despite its lower molecular weight. Attached link is a really good
explanation ..from an expert. oxygen does permeate 3 to 4 times
faster through rubber. I guess leakage through the tire would occur
even faster in an aircraft tire at altitude due to the driving force
of the pressure differential. But still not convinced if its the
primary reason.. I think I am leaning towards the fire risk. Anyways
its a very interesting aviation topic.
Terry
http://www.getnitrogen.org/pdf/graham.pdf