Text 5251, 99 rader
Skriven 2005-01-19 08:43:02 av Robert Bashe (2:2448/44)
   Kommentar till text 5212 av Roy Witt (1:10/22)
Ärende: Tire inflation
======================
Roy Witt wrote to Robert Bashe on Monday January 17 2005 at 09:31:

RB>> Here's what I found, for what it's worth. C&EN = Chemical and
RB>> Engineering News, the member publication of the American Chemical
RB>> Society.

RW> A very interesting read, Bob.

That's what I thought too when I saw the articles.

RB>> As pointed out by reader James R. Jeitler of North Idaho College and
RB>> others, tire outlets around the country are offering to replace the
RB>> air in your car tires with nitrogen for about $5.00 each.

RW> Hmmm. I wonder how they get air out them?

Purging, I would imagine. You could raise the car on a lift and suck all the
air out of the tires, then run nitrogen in, repeat the procedure a couple of
times, and presumably that would be adequate.

RB>> There are a number of rather lofty claims being made about nitrogen
RB>> in tires. These include that pressure inside nitrogen-filled tires
RB>> does not rise or fall with temperature changes,

RW> Wish he would have proved or disproved this one.

No need. The claim is ridiculous. All gasses expand with increased temperature
and contract as the temperature falls.

RB>> that nitrogen is an inert gas,

RW> ????

Of course it's not. There are things that react with nitrogen, although it's
not as reactive as oxygen. The "inert gasses" are things like helium, argon,
krypron and xenon. And even they aren't totally inert, as we've seen in the
last decades... you can make compounds of the above, although it isn't easy to
do and for all practical purposes they really are "inert".

RB>> and that moisture and oxygen in air cause corrosion that shortens
RB>> tire life by 25 to 30%.

RW> This is true.

Is it? How do you measure such things? And if it was actually done and the
statement is not a mere claim, who did it? Remember, C&EN is an ACS publication
for scientists, and scientists demand reproducible test results, with
experimental data.

RB>> Some credence to the idea seems to come from the use of nitrogen in
RB>> tires of race cars, jet aircraft, and some heavy trucks. Although
RB>> tire-pressure advantages are mentioned for these vehicles, another
RB>> reason cited here is that nitrogen will not contribute to combustion
RB>> in case of a fire around the tires, as the oxygen in air would.

RW> A really crazy theory...

Not at all. Compressed air is great for making a small fire a large one. More
oxygen in a given volume.

RW> ... as the fire is fed by oxygen in the air surrounding it. If the
RW> tire were filled with sand, the tire would still burn.

Right. But not at the same rate.

RB>> Nitrogen generators are available for filling tires with the gas. At
RB>> least one company recommends using nitrogen that is at least 98% pure
RB>> in your tires to prevent rim corrosion and road failures. The
RB>> nitrogen is generated using a semipermeable membrane system that
RB>> allows oxygen, carbon dioxide, and water vapor to pass through,
RB>> concentrating the nitrogen. This concentrated nitrogen can then be
RB>> put into your tires.

RW> This seems odd, since the size of the molucules of air are smaller
RW> than those in nitrogen. How does this work?

Frankly, I don't know. But I do know that a semipermeable membrane doesn't
necessarily separate on the basis of molecular weight. It can also be a
question of polarity, and nitrogen is presumably less polar than most of the
other components in air (except for the so-called "inert gasses" - there's a
certain percentage of argon in air too). The way people generally generate
nitrogen is by fractionally distilling liquid air, but of course that's fairly
involved for a small operation.

RB>> Physical chemist David Yang writes to point out that the
RB>> oxygen-oxygen double bond is about 1.2 Å long and the
RB>> nitrogen-nitrogen triple bond is about 1.1 Å. "They may have better
RB>> luck inflating with a really large molecule such as helium," Yang
RB>> jokes.

RW> Someone here suggested that. Seems logical after reading this report.

That was a chemist's joke, Roy. Helium is so small and nonpolar that it would
diffuse out of a tire nearly as fast as you could put it in.

Cheers, Bob

--- GoldED+/W32 1.1.5-0613
 * Origin: Jabberwocky System - 02363-56073 ISDN/V34 (2:2448/44)