Wikipedia:Reference desk/Archives/Science/2022 October 28

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October 28

Nitrogen tire inflation

Costco tire center[1] states:

Additionally, we inflate your tires with nitrogen, not compressed air. Nitrogen retains tire pressure better over time than compressed air, which can positively impact the life of your tires, as well as the fuel economy of your vehicle.

First I ever heard of anything like that. The molecular weight of nitrogen is slightly less than the average molecular weight of air, so nitrogen doesn't sound likely to diffuse through the rubber slower, right? Do they think that the O2 in compressed air will react with the tire interior somehow? And you have to check your tire inflation and top it up every so often anyway, so won't the gas in the tire soon be mixed with whatever you use for the top-up? That will normally be atmospheric air for most of us. Is this nitrogen thing some kind of dumb marketing gimmick, or is there something to it? Thanks. 2602:24A:DE47:B8E0:1B43:29FD:A863:33CA (talk) 21:04, 28 October 2022 (UTC)

I'm not a physicist, but according to File:Empirical atomic radius trends.png the size of an oxygen atom is slightly smaller than the size of an nitrogen atom. I don't know if that transfers to the size of N2 versus O2, but if it does, N2 would be less likely to diffuse through other potentially porous materials. Dhrm77 (talk) 21:34, 28 October 2022 (UTC)

The molecular diameter of nitrogen is less than 1% larger than oxygen. And since air is 78% nitrogen already, I find it difficult to believe this could make a significant difference. Sounds like a snake oil type gimmick to me. --DB1729^talk 22:38, 28 October 2022 (UTC)

Testing by Consumer Reports shows that there is very little practical benefit. Cullen328 (talk) 22:49, 28 October 2022 (UTC)

According to Graham's law, a gas with a lower molecular mass will escape faster (because at a given temperature its molecules are moving faster). catslash (talk) 22:53, 28 October 2022 (UTC)

Using molecular mass as the criterion, CO₂ is better than air by 17.5%. But mice will escape faster from the cheetah zoo cage than the cheetah, even though they are moving more slowly.  --Lambiam 06:54, 29 October 2022 (UTC)

The idea's been around for most of this century, at least. I listened to Cincinnati, Ohio radio stations until moving away in 2005, and Tire Discounters always advertised their nitrogen inflation with implications that nobody else in the region was doing it. 175.39.61.121 (talk) 04:35, 29 October 2022 (UTC)

Large airplanes of the kind used by airlines usually have their tyres inflated with nitrogen. It is my understanding that this is done because the materials used in construction of these tyres release gaseous materials that can form an explosive mixture when combined with oxygen at high pressure. Tyre pressures are very high so, when inflated with air, the partial pressure of oxygen is high. There are times when tyres and their contents get hot, such as after taxying a long way, after landing, and after a rejected takeoff. If a tyre explodes it is a hazard to ground crew and a threat to the airframe. However, none of this supports the notion that tyres inflated with nitrogen maintain the required pressure longer than those inflated with air. It may be that what lies behind these offers to car owners is simply the idea that if it’s good for airlines it must be equally good for owners of up-market automobiles. Dolphin (t) 06:19, 29 October 2022 (UTC)

It's easier to get high pressure cylinders of dry nitrogen than dry compressed air. CO2 would be a better choice. Also, if the O2 does escape, the remaining air inside will have less O2, so after several top ups even if you start with air you get the same effect anyway. Of course the pros use a hexafluoride and the tires occasionally have to be let down as air diffuses in. Greglocock (talk) 07:11, 29 October 2022 (UTC)

Our article on aircraft tyres mentions this too: simple compressed air doesn't work very well in aircraft tyres, as the large temperature swings cause, via condensation, large pressure swings. One needs at least dehumidified compressed air. And the high partial pressure of oxygen may cause explosive mixtures. Pressure on aircraft tyres is on the order of 1400 kPa, while pressure of car tyres is around 200 kPa (and bicycle tyres around 500 kPa), and car tyres don't see such large temperature swings, so it's not applicable to car tyres. Bicycle tyres are nearly always filled with simple compressed air from a hand pump.

When you fill your tyres with nitrogen, nitrogen diffuses out, but oxygen diffuses in, which may reduce the rate of pressure loss. Indeed, if you inflate your tyres with some hexafluoride, you may see rising pressure as air diffuses in. Those hexafluorides have some disadvantages too, so it may be a bad idea to use it on a large scale. PiusImpavidus (talk) 10:53, 29 October 2022 (UTC)

Greglocock Good information! Any idea about the advantage of hexafluoride? Why do the professionals use it? Dolphin (t) 10:32, 29 October 2022 (UTC)

That was a joke. It would work, but it is not practical.Greglocock (talk) 22:46, 29 October 2022 (UTC)

According to Kinetic_diameter, molecular oxygen is about 5% smaller than molecular nitrogen, not 1%. Granted, it doesn't make much of a difference. The more important question might be: what molecular size is rubber not able to contain effectively? (perhaps only helium and ammonia?) Dhrm77 (talk) 21:36, 30 October 2022 (UTC)

According to my local tyre centre using nitrogen prolongs the life of the tyres by stopping the oxidation of the insides. Martin of Sheffield (talk) 21:46, 30 October 2022 (UTC)

Things may be better for you, but I have often found such businesses in the automotive industry to be less than ideal sources of scientific truths. HiLo48 (talk) 23:44, 30 October 2022 (UTC)

Very true, but it gives an explanation about why they are pushing it, even if the whole exercise is pointless. As the OP says:"Is this nitrogen thing some kind of dumb marketing gimmick, or is there something to it?" While posting here, Greglocock commented that "It's easier to get high pressure cylinders of dry nitrogen than dry compressed air." Such equipment is standard for every SCUBA shop and dive club, so I suspect that cost is not a major factor in the long run. Martin of Sheffield (talk) 08:05, 31 October 2022 (UTC)

Thanks all. It sounds like my suspicion is confirmed and the N2 inflation is bogus. 2602:24A:DE47:B8E0:1B43:29FD:A863:33CA (talk) 22:38, 29 October 2022 (UTC)

Bogosity would explain the warnings I've seen not to add mere garden-variety common air to a nitrogen-filled tyre. —Tamfang (talk) 02:02, 3 November 2022 (UTC)