In a message dated 1/31/04 5:08:20 AM Pacific Standard Time,
billiam@erols.com writes:
"...since Nitrogen IS a lighter gas than Oxygen...."
Yes, nitrogen is lighter, but NOT MUCH. For those of us here in the backward
US, the densities (dry, at standard temperature and pressure) are:
Air - 0.07651 lb/cu ft
Nitrogen - 0.075261 lb/cu ft
Thus, air is about 1.6% heavier. That's not much.
So how much weight of air do we actually have in our tires?
First, a quick measurement shows an Amphicar tire has a volume of somewhere
around 1 cubic foot.
Thus, if you take your spare tire, set it on the floor and remove its valve
stem core, the interior of the tire will drop to atmospheric pressure. In this
condition, the tire will have about 0.07651 pounds of air in it.*
Now, atmospheric pressure is usually taken as 14.7 psi (the actual pressure
is what we call "barometric" pressure and it varies with weather and altitude).
Anyway, if we now inflate the tire until our tire gage shows 14.7 psi, we
now have twice as much air in the tire as when it was "flat". Likewise, if you
inflate the tire to 29.4 psi, you will have 3 times the air in the tire as
when it was "flat".
So, lets assume we inflate all 5 tires to 29.4 psi (with air). The total
weight of air in the tires will be -- 5 X 3 X 0.07651 this equals 1.1 pounds!
Or, if compared to Nitrogen, air will weigh about 1/4 ounce more. Take a
quarter out of your pocket and toss it overboard -- this will save you the same
1/4
ounce of weight that using nitrogen instead of air will.
About heat transfer, here the difference will be even less. The only way the
wheel and tire can loose heat is outward into the atmosphere or to the
pavement. As the tire heats up, the air in the tire will heat up of course; but
the
only way the heat can escape the air in the tire is to go back out through
the tire or the rim. Now, even IF nitrogen did transfer heat better, it could
transfer a bit more of the heat of the tire to the inside of the rim, thus
cooling the tire a bit more. But, this effect will be small. On top of this,
as
previously noted, most of the atmosphere is nitrogen. Thus, heat transferred
by air or by nitrogen is going to be extremely close to the same. There will
be no measurable difference in the temperature of the tires if you change to
nitrogen. **
As was mentioned, nitrogen is used in airplane tires. This is to avoid
explosions when the tires get VERY hot (over 250 F -- Do your Amphi tires sizzle
and steam when you first hit the water?
). With the high pressures used
(over 150 psi), and the extreme heat that can occur on landing, the rubber
compounds in aircraft tires can actually spontaneously combust if there is too
much
oxygen in the tires. Your Amphicar is not going to have this problem!
Roger
White '63
Seattle
* Do not confuse the pressure of air in your tire with the weight of air in
your tire. People often say "I put 30 pounds of air in my tire." This is NOT
the weight of air you put in the tire. The "pounds" is short for "pounds per
square inch - psi, this is a pressure not a weight. As shown above, at 30
psi, an Amphi sized tire will have less than 1/4 pound (weight) of air in the
tire. If you don't believe this, then put your spare tire on a scale and let
the
air out to see how much lighter it gets.
** Higher tire pressures will decrease the running temperature of tires
significantly. Thus, the tire store you are referring to could lower your tire
temps by simply inflating the tires to a higher pressure when they switch to
nitrogen.
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