Reply to "Nitrogen in tires?"

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Originally posted by Panterror:

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Originally posted by comp2: Oxygen bleeds down a lot quicker then a gas that can be taken to liquid.

They both liquefy. Nitrogen boils at 77K (Kelvin) and Oxygen at 90K, meaning O2 liquefies at a little higher temp than Nitrogen. They are both pretty close in molecular weight O2=32 gm/gm-mole and N2=28 gm/gm-mole so a liter of liquid O2 would convert nearly as much gas as a liter of liquid N2.



That’s one heck of a bank of gas cylinders Gary.

The most significant benefit of using pure N2 is both the absence of water and oxygen. As mentioned in the thread, most aircraft wheels were magnesium and magnesium oxidizes rapidly in the presence of O2 and water so the use of N2 prevents corrosion because it is chemically much less reactive than oxygen and water completes the circuit, especially if there are dissimilar metals present (galvanic corrosion). Pantera Campys are of course mag as well.

Eliminating any water vapor should make the tire pressure a little more stable over a given temperature range. At altitude in an aircraft tire, H2O vapor will freeze and no longer contribute to tire pressure at all. The presence of H20 vapor will also make tire pressures raise more in racing situations as tire temperatures increase. Fill your tire on a hot humid day with one of those cheepo gas station outdoor compressors you pump with quarters and you will have liquid water in your tires at Winter temperatures.

You don’t have to have N2 to eliminate water. Just a good inline desiccant cartridge will do almost as well and achieve pressure dew points of <=100F and this will probably eliminate the lion’s share of corrosive activity.
In theory pure O2 would diffuse (leak) out more slowly and hold tire pressure better than pure N2 because diffusion rate is inversely proportional to the square root of density (proportional to molar mass). In other words, big molecules diffuse more slowly. But O2 and N2 are so close in molecular weight this isn’t an appreciable effect and air is in between the two so even less difference compared to air.

Other than preventing long term corrosion in mag wheels I think it’s picking the fly sh!t out of the pepper for street cars. Just buy an inline desiccant cartridge for your compressed air line and call it a day Doug.

The claims about N2 making tires last longer are usually over blown. I know, the truck stops put N2 in the over the road truck tires but they recap those things and they cost a bundle so prolonging a tire is more significant. Ozone and Ultra Violet light are the chief culprits in degrading tire rubber. You can get rid of ozone on the interior but it will still be there along with UV on the outside.

For estimating how many tire fills you can get from a compressed air tank, the volume of gas is proportional to absolute pressure. For every atmosphere of pressure (14.7 psi) increase, the volume of gas in the tank goes up x1. Most scuba tanks are referred to by how much gas they hold at rated pressure. Anywhere from 65-120 cubic feet is typical and pressures up to 3000 psi depending upon tank type. 315/35/17 is a popular Pantera tire. They’re about 2 cubic feet internal volume. So to raise pressure from completely flat (1 atm absolute) tire to 30 psi (about 2 ATMs) would take 4 cubic feet out of the scuba tank. So even the smallest scuba tank would likely fill 15 large rear tires from completely flat and would top up a partially full tire a very large number times. This assumes ideal gas behavior but close enough.

Be advised, there is a significant danger in using high pressure gas source to fill tires. As a precaution, you should have a very good regulator, an orifice to limit flow, and a very reliable relief valve on the low pressure (tire) side. Being in close proximity to an exploding tire/wheel can easily kill you.

You also won’t find an industrial gas company willing to put pure N2 into a scuba tank or another tank meant for breathing air for concern it would be stored and/or change hands and someone would assume it is full of air instead of N2 and….well, die. Take the scuba tank out of service, paint it, and identify it as not for breathing air. You will also discover the fittings on scuba (air) tanks, oxygen and nitrogen tanks conform to Compressed Gas Association standards and are all different to discourage filling them with the wrong gas. You will need to make your own conversion charging line to do so but just be careful you don’t nominate yourself for Darwin awards in the course of doing so.

Best,
K