Text 5282, 121 rader
Skriven 2005-01-20 15:28:13 av Roy Witt (1:10/22)
   Kommentar till text 5251 av Robert Bashe (2:2448/44)
Ärende: Tire inflation
======================
Hello Robert.

19 Jan 05 08:43, you wrote to me:

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

It enlightened me, for sure.

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

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

I asked my 'all knowing' mechanic buddy and he said that it was too
rediculous to even think about. Especially at $5 a tire.

 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.

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

 RB>>> that nitrogen is an inert gas,

 RW>> ????

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

 RB> Is it? How do you measure such things?

With the eyeball. :o)

 RB> And if it was actually done and the statement is not a mere claim,
 RB> who did it? Remember, C&EN is an ACS publication for scientists, and
 RB> scientists demand reproducible test results, with experimental data.

Tires rot from being exposed to the elements. Tire rot from the inside
isn't as noticable, that's all. Compressed air has water in it, that's the
nature of the beast. And why they put drain cocks on compressor tanks. The
air is usually mixed with the oil from the crankcase and crap that makes
it through the filters, not to mention what's carried in the water that's
naturally in air.  Have the scientist visit a tire shop and have the
workers show them the insides of tires when they remove them from the
wheel. The bead will be the worst off, as it gets moisture from both
sides.

 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
 RB>>> combustion in case of a fire around the tires, as the oxygen in air
 RB>>> would.

 RW>> A really crazy theory...

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

Flat tires are usually the cause of race car crashes. No air in those
tires to contribute to a fire.  They worry more over the possibility of
spilled fuel contributing to a fire.

 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.

 RB> Right. But not at the same rate.

Bob, once compressed air is released to the atmosphere, it no longer is a
threat. That happens as soon as there's a hole in the tire to let it out.
It's not like compressed air is going to stay compressed while a fire is
burning around it.

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

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

Interesting.


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