While this is primarily an anthropology and archaeology blog, I also like to write about skeptical topics as well. I’ve written several bits about pseduoarchaeology in the past, but this topic is a straight bit of skepticism.
My wife and I recently traded in one of our 2000 model Saturns for a new 4-cylinder Ford. Having sold new cars for a living about 8 years ago (that’s how we ended up with two Saturns!), my wife knows the car business and wasn’t about to let anyone sell her any add-ons, after market B.S., extended warranties, and all the other sorts of insurance the dealers really make a fair bit of money on. Indeed they were completely frustrated that she and I wouldn’t even bat an eye at what they had to offer.
But, when the finance manager went into his pitch on Nitrofill. This is essentially a service they provide to periodically fill the tires with nitrogen instead of normally compressed air. The difference, he stated, is that “nitrogen filled tires don’t loose pressure as fast as air and nitrogen doesn’t oxidize the inside of the tire as fast.”
I looked at my wife and saw a bit of hesitation. She was buying it. Literally, if I dindn’t stop her. “Ahem,” I got her attention and she snapped out of it. “Isn’t air already 78% nitrogen?” I asked the finance manager. “Uhh… well I’m not a scientist,” he replied with a sheepish grin. My wife, fully back to her senses smiled broadly, pointed at me and said, “but he is!” I’m not, but having stayed at my share of Holiday Inn Expresses, and having paid some attention in my Chemistry classes, I knew $5.00/tire every time they got low wasn’t an expense I wanted. And, if we filled a tire with air somewhere else, it would be $60 to service the tire and fill it back up with nitrogen. The tire that already had 78% nitrogen.
Needless to say, I we didn’t buy the nitrogen scam. And that’s just what it is. On the surface it sounds good. In fact, if it were free, I’d take pure nitrogen over normal air any day. But I’m not about to let a car dealer or service station sell me the air in my tires.
The claims are this:
- Nitrogen-filled tires maintain proper pressure longer
- The rubber of nitrogen-filled tires last longer
- Nitrogen is less volatile than oxygen and thus safer in a fiery crash
- Cars with Nitrogen-filled tires get better gas mileage
- Cars with Nitrogen-filled tires are better for the environment
The last two claims are dependent upon the expectation that the tires filled with nitrogen are actually at properly inflated pressure more consistently. So let’s set them aside and focus on the first three points.
1. Do nitrogen-filled tires maintain proper pressure longer? The premise for this claim is that nitrogen is a larger molecule than oxygen. It is. Only slightly. But let’s not omit the fact that we’re talking about molecules here and not just the element. Oxygen and nitrogen are both diatomic molecules. Nitrogen actually has less mass than oxygen, so Graham’s Law dictates that it diffuses a bit faster than oxygen. However, since the actual size of the oxygen molecule (O2) is a bit larger than that of a nitrogen molecule (N2), this only applies if the opening from which the molecules are effusing from is large enough to permit the largest of the two. In such cases, N2 will diffuse faster.
The question, then, becomes, are the pores in rubber (assuming there are such pores) smaller than the N2 molecule but larger than the O2 molecule? I don’t know the answer to this. Nor could I find any literature in the few minutes I searched, but if anyone has a citation to an independent (i.e. non Nitrogen Tire industry) study or bit of research, I’m interested. Without digging out my old chemistry textbook, I’m willing to tentatively accept Wiki Answers on the sizes of N2 and O2 molecules: N2 is roughly 300 picometers while O2 is slightly smaller at 292 picometers. I’m open to revising these figures if someone cites a more reliable source, but I can’t imagine that there’d be any reason for the link to be more than slightly wrong.
2. Does rubber oxidize faster when exposed to oxygen rather than pure nitrogen? I’d expect so. The real questions are: a) how to you keep oxygen on the outside of your tires from causing oxidation?, and b) does it really matter to me since every single tire I’ve ever replaced was because of worn tread and not oxidation?
3. Why do I give a shit whether or not the oxygen in my tires will fuel the fire of my fiery crash? If the explosion is powerful enough to consume the oxygen in the surrounding air leaving only my tires as reserve fuel, I suspect I’m going to be a crispy critter anyway.
As for 4. and 5., I’m not that arsed for time that I can’t continue my routine of checking my tire pressure every 3,000 miles when I change my oil. In fact, nearly every time I’ve ever checked my tires at 3k, they’ve either been dead on for the proper psi or just a pound or two off. Whenever I’ve had to fill more than that, it’s been either because of a faulty valve or a nail in the tire itself. I suspect that the resulting points of egress in a faulty valve or pucture would create holes large enough for either O2 or N2 to escape through effusion. So, in that case, Graham’s Law would be in effect and N2 would escape faster than O2.
The bottom line: if nitrogen becomes a free option, easily obtainable (i.e. cheaper and easier than the $5.00 Walmart compressor that I plug into my cigarette lighter), I’ll use it since there’s a very slight chance I won’t need to top off my tire pressure as often. But, as long as I have to pay for it or even just drive to the dealer for it, I call bullshit!
Nitrofill is a scam. Nitrogen-filled tires for general consumers is a scam.
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