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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Hello. You may recall that I posted the below entry on your blog, Hot Cup of Joe, countering some of your arguments against nitrogen tire inflation. I was just curious to know why you decided not to publish it? I was always under the impression that the beauty of blogs was that you could often see differing points of view and readers could make their own determination.
First, in an effort to be as forthcoming as possible, I should inform you that I am involved with nitrogen tire inflation. I am a firm believer in nitrogen tire inflation, not only because of the benefits to the consumer, but the environment as well. I run a blog dedicated to nitrogen tire inflation located here:
Today I came across your blog post about nitrogen tire inflation, and wanted to address a few of your points.
1.) I have no intention of arguing molecular chemistry with you, but will tell you that oxygen permeates through the rubber sidewall of a tire 4-6x faster than nitrogen. If you are looking for non-nitrogen tire market proof, I can direct you to the following reprint from Bridgestone, a major tire manufacturer.
This is a reprint from their corporate publication Real Questions, Real Answers. I have seen supporting documentation from Ford as well that back up these numbers.
Here are bulletins from Goodyear and Michelin that reference the better pressure retention characteristics of nitrogen inflated tires.
It is a well proven fact within the tire industry that nitrogen inflated tires maintain their pressure better than air filled tires.
2.) You bring up a good point of oxidation of the outside of the casing. As John Baldwin from Ford points out in his paper entitled “Effects of Nitrogen Inflation on Tire Aging and Performance”, chemical aging of the rubber is nearly halted when tires are filled with nitrogen.
The reason, no oxygen is permeating through the tire, causing belts to wear and rubber to break down and oxidize internally. By minimizing this internal wear, a nitrogen inflated tire is structurally stronger after 2 years than an air filled tire after 10 weeks (Figure 4).
You will also see, from page 3 of his report, that the Ford Explorer rollovers were partly caused by this issue: “defective in part because the physical properties of rubber in the steel belt area had deteriorated due to oxidative aging.”
3.) Point taken
4 & 5.) NHTSA has stated that 85% of the population does not check their tire pressures regularly and that 30% of cars and light trucks have at least one significantly (25% or more) underinflated tire. These underinflated tires cause the USA to waste 2.8B gallons of gas/year (1.6B cars & light trucks, 1.2B heavy duty trucks). Over $11B in wasted money in the US alone due to underinflation.
By maintaining proper inflation pressure longer, nitrogen inflated tires get better gas mileage and have a longer tread life. Goodyear recognizes that underinflation is a major contributor to poor gas mileage and tire life, and are running a summertime air inflation program to combat these issues. More information is available here:
While I doubt that this is proof enough for you that nitrogen tire inflation makes sense for the common consumer, I would hope you could at least understand the proposed benefits a bit better.
I’ve responded to the comment above as a separate post found at: http://ahotcupofjoe.wordpress.com/2008/07/16/nitrogen-tire-scam-part-3/
Claims about expansive behavior of water vapour are all based on 14 year old school kid physics. A tyre filled from 1bar atmosphere with wet 100RH air at 15Â°C holds 10.7g water / Kg air. 195/60R15 tyre that’s about 1.4g of water. At 32psi the boiling the point of water isn’t 100Â°C, it’s 136Â°C so no boiling and no expansive steam in a 80Â°C tyre or even 100Â°C F1 tyre. The actual increase in pressure at 80Â°C due to vapour pressure of water is about 0.7psi.
All compressors at tyre shops and service stations should have a dryer fitted. Just compressing air to 100psi reduces the amount of water air can hold at 15Â°C to 1.34g water / Kg air. Let it stand and the excess condenses in the tank to be released though the drain tap. Tyre filled to 32psi with air at 15Â°C and 1.34g water / Kg air will show a 0.03psi overpressure at 80Â°C. If you don’t let it stand and cool you will have under pressure problems as the air pressure reduces as it cools to ambient in the tyre. Get a tyre fitted at a busy shop with a hot compressor and you will always have low pressure the next day.
Statement: Diffusion rate for N2 is 3, O2 it’s 10. (3rd para from end)
Air contains 78% N2, assume rest is O2
Total diffusion rate of air = N2 0.78 x 3 + O2 0.22 x 10
= N2 2.34 + O2 2.2 = 4.54.
The N2 is exiting the tyre 6% faster than the O2!
93% N2 diffusion rate = N2 0.93 x 3 + O2 0.07 x 10
N2 2.79 + O2 0.7 = 3.49.
Ratio = 4.54/3.49 ~ 1.3
Assuming all loss is diffusion. A tyre with N2 will have same pressure after 6 months as an air filled tyre has after 4.6 months. All it can possibly do is allow you increase the pressure check intervals by 1.3. Every 9 days instead of every week.
Yet the N2 industry makes claims that it’s 3x better (or in the case of Bridgestone 6x). Seems yet again they can’t do a proper analysis but have relied on school kids.
People that fill with N2 are even less likely to check the pressures as they don’t have the N2 to fill them with (though bridgestone do say top up with air). After all it’s sold on the claim that tyres become virtually maintenance free (I can see some law suits real soon). If it takes a special trip to somewhere with a N2 machine then it’s an environmental disaster due to the fuel used making that trip.
Then there is issue of what the temperature is when you check the tyres pressure. Start with 32psi, the temperature you set the pressure at gives several psi variation in running pressure as does the running temp.
temp Â°C at 60Â°C at 80Â°
0 42.26 45.7
5 41.23 44.6
10 40.25 43.5
15 39.29 42.5
20 38.37 41.5
Doesn’t matter if it’s dry air or dry N2 both obey the Ideal gas law. P x V / T = const.
Then there is there is the “they use it on aircraft” crap. They use O2 reduced air, requirement is 5% or less O2. A plane took off with a brake on. This overheated the brake and tyre a lot. It didn’t blow out straight off but it was slowly cooked by the overheated brake. CAP 747, appendix 1, GR No. 16 Tyre Bursts In Flight – Inflation Media. 2.5Mb pdf doc source on this page, link “open doc in new window”.
Only applies to aircraft with RETRACTABLE undercarriage and over 5700Kg. I’ve yet to see a car or truck driven on retractable wheels.
Sorry it took more than 10 days for me to approve your comment. I’ve been more or less away recently and Akismet caught it as spam since it had several links in it.
Anyway, great stuff! Thank you!
The benefits of nitrogen tire inflation are obvious, and extensive. Nitrogen is an inert gas, so pumping it into your tires eliminates the presence of oxygen and water, both of which are sources of many tire problems.
The presence of oxygen and water are each dealt with in my article. They are insignificant problems when one considers the expense of adding nitrogen to tires. Were it free, I’d agree with you. Since it is a money-making scam, I’m afraid I see no reason to pay for a negligible bit of protection. Most tire wear and tear is external to the rubber, not internal. By the time internal oxygen affects the tire, the external forces have long since required new tread.
Sorry. Science and common sense win over scam and greed.
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We tried nitrogen in big truck tires and the only thing we found was your tires run alittle cooler,but not worth the expence of filling with nitrogen. Didnt notice much differents in handeling either.
Your Graham’s Law point is irrelevant. Your point compares nitrogen and oxygen, when the debate for the tires is between nitrogen and regular air. Since regular air isn’t pure oxygen, and pure oxygen isn’t used in tires, then it doesn’t matter that oxygen molecules are bigger. If can apply the law to regular air, then you might have a valid point. But, for now, you haven’t disproved the fact that N2 diffuses at a slower rate than regular air because oxygen isn’t regular air.
I think the Graham’s Law point is quite relevant when you consider that “regular air” is mostly nitrogen (78%) and oxygen (21%). If “regular air” already is already 78% nitrogen, would you not expect that by mixing it with the larger molecules of oxygen (21%) that the efficiency of regular air at not leaking through microscopic holes to be better?
Oxygen isn’t all that comprises regular air, but regular is 21% oxygen.
Incidentally, the remaining 1% is everything else in the air, such as argon, helium, carbon dioxide, methane, etc.