[Borek]

It's called saturation diving I believe.

Yes.

The "habitat" remains connected to a ship and every night the pull it up replace the air tanks for diving and habitat and then drop it in the morning.

It doesn't seem a good idea to me. It must be anchored at the sea bottom and it is not an easy or safe procedure to pull it up and down on a daily basis. Much easier is to send supplies in a metal basket.

[Ashi_Starshade]

HMmm... I think that it's this reason:
"tire pressure does not change or fluctuate with temperature as much with nitrogen"

It certainly does seem to me that nitrogen would be much closer to an ideal gas than air alone, and it may be that the pressure increases more rapidly with temperature for air.

Howstuffworks.com says that (at least for professional race cars) it's to prevent pressure from evaporation of small amounts of condensed water.  But even if you filled a tire with nitrogen, if it already had water adsorbed in the inner tube, it would still be adsorbed there?  Plus, even if this were the reason, one could use anhydrous air - which is still cheaper than nitrogen (I think).

[Maury]

Nitrogen vs. air for tires

I have seen dry nitrogen being recommended for inflating automobile tires instead of air.  Reasons given are:
1. Air's 21% oxygen deteriorates the tire, while nitrogen is inert.  I can accept this, since I believe that oxygen under pressure and at elevated temperatures accelerates oxidation.
2. Oxygen in air plus moisture in air cause corrosion of metallic parts of the wheel.  This, too, seems reasonable.
3. Nitrogen does not penetrate the tire walls as readily as oxygen and so maintains proper pressure longer.  This I do not understand.  The molecular weight of nitrogen is 28 and that of oxygen 32.  Is the size of the nitrogen molecule *larger* than that of oxygen?  It must be so, since the two gases are separable by the use of a semipermeable membrane.  But it just doesn't seem logical to me.
4. Nitrogen-filled tires run cooler than air-filled tires.   How can this be?  Is nitrogen a better "thermal conductor" than oxygen, thus permitting heat from flexure of the tire's sidewalls to be conducted to the metal hub and dissipated by outside air?

If these are not an appropriate questions for this forum, would you kindly direct me to a more suitable one?

[mike]

1. Air's 21% oxygen deteriorates the tire, while nitrogen is inert.  I can accept this, since I believe that oxygen under pressure and at elevated temperatures accelerates oxidation.

I have never seen a tyre that has oxidised all the way through from the inside to destroy the tire. I think my tread always wears out faster, LOL.

2. Oxygen in air plus moisture in air cause corrosion of metallic parts of the wheel.  This, too, seems reasonable.

So what about the rest of the car? Are people only worried about the metal inside their tyres?

3. Nitrogen does not penetrate the tire walls as readily as oxygen and so maintains proper pressure longer.

Who says? If anything this would be good as the concentration of oxygen would decrease compared to nitrogen

Nitrogen-filled tires run cooler than air-filled tires.

I doubt it. Nitrogen and oxygen would have similar conductance. If there is any difference it would be between dry gas and wet gas as the conductivity of water would be different.

I am happy to fill my tires with "air" rather than "dry nitrogen", how pure is "dry nitrogen" anyways?

[pantone159]

how pure is "dry nitrogen" anyways?

I'm sure it's at least 75% pure.   

[Mitch]

You have a semipermeable membrane with no oxygen inside the tire. I'll be more interested in knowing how long it takes a tire to come into concentration equilibrium with the outside air. Less than a day would be my best guess.

Yes, I agree statement 3 is screwy and if true than Mike's comment would mean there is no need for their product.

[Borek]

3. Nitrogen does not penetrate the tire walls as readily as oxygen and so maintains proper pressure longer.  This I do not understand.  The molecular weight of nitrogen is 28 and that of oxygen 32.  Is the size of the nitrogen molecule *larger* than that of oxygen?  It must be so, since the two gases are separable by the use of a semipermeable membrane.  But it just doesn't seem logical to me.

Not only size matters. O₂ particles are different from N₂ particles and they penetrate through the rubber much faster. Interestingly CO₂ is even faster - faster then He!

http://www2.dupont.com/Vamac/en_US/assets/downloads/vamac_gas_permeability.pdf
http://www2.ubu.es/quim/quimorg/polimeros/trab_pub/gas-meg.pdf

And some reasonable IMHO comments on nitrogen in tyres:

http://www.buyrite-tyres.co.uk/tyrecare-source.html
http://www.bridgestone.com.au/tyres/products/car/care/nitrogen.asp

This subject was discussed to death several times in my presence on different chemical forums. Apart from what laymans (and nitrogen sellers) say, most chemists taking part in theses discussions agree that apart from some highly specialized areas (F1, jet tyres) use of nitrogen is a waste of money.

Most interesting comment I have seen stated that liquid nitrogen is a byproduct in some processes and it must be used somehow

[mrdeadman]

i don't think it really matters, air is made up of mostly nitrogen anyway and your reasoning will just make people lazy.

[syko sykes]

3. Nitrogen does not penetrate the tire walls as readily as oxygen and so maintains proper pressure longer. This I do not understand. The molecular weight of nitrogen is 28 and that of oxygen 32. Is the size of the nitrogen molecule *larger* than that of oxygen? It must be so, since the two gases are separable by the use of a semipermeable membrane. But it just doesn't seem logical to me.

Oxygen molecules are larger than nitrogen molecules despite the increase in mass. This is because the valence electrons on each molecule experience the same amount of shielding because of their location but the Oxygen atom has an extra proton attracting the electrons to the center of the atom and therefore making the atom smaller. This principle explains why atoms on the periodic table typically decrease in size (radius) as you move from left to right.

how pure is "dry nitrogen" anyways?

I'm sure it's at least 75% pure.

i would hope it's more than 75% pure because air naturally consists of about 78% Nitrogen deeming it useless to use dry nitrogen as a replacement.

[billnotgatez]

http://news.yahoo.com/s/ap/20060731/ap_on_bi_ge/nitrogen_tires

just when you think you have heard it all

[Borek]

just when you think you have heard it all

Note how those commercially interested speak about benefits, while those not commercially interested doesn't offer any definitive opinion ("The National Highway Traffic Safety Administration has no opinion" & "Tire experts at Consumers Union, publisher of Consumer Reports magazine, neither endorse nor object to the use of nitrogen in tires").

[constant thinker]

Hey for a tire company it could be a great gimmick. A reason to charge extra for something that costs them very little over what it costs them now. That's the whole notion behind super sizing foods at fast food.

[billnotgatez]

It is interesting that the FAA condones the use of nitrogen in commercial aircraft tires.

[Borek]

It is interesting that the FAA condones the use of nitrogen in commercial aircraft tires.

AFAICT they go at much higher speeds and thus much higher temperatures than average car, plus they get cooled to -40 deg C every time plane gets high enough. These are not standard conditions, I suppose that's what makes the difference. But that's only my guess.