By Russ and Tiña De Maris
If you remember high school chemistry, you may remember our friend nitrogen, the gas that makes up the majority of earth’s atmosphere. While “plain old air” has been used for years to fill tires, the wrinkle a few years back was to fill tires subjected to a lot of stress with nitrogen. Think race car tires, jet aircraft, etcetera. Why nitrogen?
Plain old “oxygen” molecules are about one-fourth the size of nitrogen. Because of that, rubber tires slowly “leak” air because the rubber is a bit permeable – the air slowly works its way through the tire pores, if you will. Since the nitrogen molecules are so much bigger, tires tend to lose pressure far more slowly. As a result, the tires run cooler and get better fuel economy. Sounds like “N-inflation” is a shoe-in? Hang on, might it lead to a false sense of security? If you don’t check your tire pressure as often, will you likewise fail to look your tires over for damage?
Tire permeation isn’t the only reducer of tire pressure. Witness a statement from tire maker Michelin: “The existence of several other possible sources of leaks (tire/rim interface, valve, valve/rim interface and the wheel) prevents the guarantee of better pressure maintenance for individuals using nitrogen inflation.” The company does not recommend nitrogen inflation, except “in a high risk environment and/or when the user wants to reduce the consequences of a potential abnormal overheating of the tire-wheel assembly (for example in some aircraft applications).”
Notwithstanding, economics may come into play. A nitrogen tire “fill up” can cost you as much as $10 each. If you’re somewhere where nitrogen is unavailable and have a low tire, you can “top off” the tire with ordinary air. But later you’ll be advised to have the tire bled out and refilled with nitrogen.
Revision note: A correction from “air” to “oxygen” related to size of molecules made. 4/7/18 12:38PM MST
Some mis information is given also in the comments.
The Oxigen molecule is only slightli smaller then the Nitrogen molecule and not 4 times as small.
Mistake is probably that O2 difuses about 4 times as fast if partial pressure is the same.
Then about water. It can only rise the pressure 1 bar/ 14.5 psi extra when going from 0 degrC/32degrF to 100 degrC/ 212 degrF in a closed compartiment like in a tire.
Then its even an advantage because it gives lesser deflection so less heatproduction.
Last the Oxigen( O2) % will work its way in the verry long run, to a point at wich the partial pressure is the same as the outside air- pressure.
And that is 21% of 1 .013 bar/ 14.7 psi
This means that a tire filled with 3 bar overpressure so 4 bar real pressure the % O2 will go to 1/ 4th of 21% = 5.25% whatever you fill with.
Even if you manage to get 100% Nitrogen ( N2) in tire, the O2 will difise into the tire until partial is the same inside and outside.
I have read a lot about this and for the money it costs to use nitrogen I just don’t see a real-world benefit in an RV.
The RV is filled with pure nitrogen and usually only needs to to checked/topped off once a year, no matter where we travel And, they do not get as hot during the summer. We are able to measure tire pressure and temperature while driving.
The tire consumer is so susceptible to sales non-sense. See link.
Guys, please — it’s not the nitrogen, or “air”, or oxygen. ALL GASES obey thermodynamic laws, and expand and contract with temperature changes. “Inert” means chemically inert, not thermally inert. Boyle’s Law rules all gases.
It’s the HUMIDITY. Humid air can change state, from water vapor to solid water (condensation) and the reverse. When this happens, water expands (or contracts) by 1600X !!! Even trace amounts can have a significant effect on tire pressures.
This phenomenon is most observable when running tires for a while and then stopping. Air-filled tires (with inherent moisture) will cool more quickly than dry-filled tires. (Drives my TPMS nuts, I often get a “leak” alarm after stopping from a highway run.)
One fact that I have never seen mentioned by others relates to load capacity and tire life.
Tire industry standards for load & Inflation have been developed and followed for many decades and tire performance is based on millions of test miles with tires being inflated with air according to the charts.
We also know that with increased pressure there is an increase in load capacity and even a slight increase in fuel economy. Now for a moment think about the claims from those selling N2. They claim that the pressure will not increase with an increase in tire temperature from running down the road. They also claim improved fuel economy. So exactly what are the magic properties of N2 that go counter to the demonstrated relationship of better MPG with higher pressure?
Don’t you think that tires depend on having higher pressure when subjected to higher load? A physical property of rubber is that its strength decreases with an increase in temperature so is there even more “magic” in Nitrogen that works against the fact that lower pressure with the same load will result in more damage to a tire structure at the molecular level?
I guess it’s time for another in-depth blog post on the “Magic” on Nitrogen
I am far from being an expert – so I ask questions
If normal air is 78% nitrogen and if nitrogen is less likely to escape from the tire then the majority of the gas remaining in the tire would presumably be Nitrogen. so the next step is to top it off again with air that is 78% nitrogen and again most of that which escapes is not nitrogen. So now would it not be reasonable to expect that the % of nitrogen has grown from the 78% in the original fill.
The question now is how many tire pressure top offs will it take before the Vast majority of the tire pressure is made up of nitrogen? So why pay for pure nitrogen?
Just asking questions.
Yes, the % N2 does increase. We have measured the change but it is small as there are diminishing returns. Not 100% of the gas leaking out is N2 but even if it was with tires only losing 1 to 2% a month it will take years for a meaningful increase in the percent N2.
I have covered the topic of Measurable vs meaningful.
I bet our resident Electrical expert can explain that there is definitely a measurable difference between 50Amp @ 240V and 51 Amp at 239 V but the difference isn’t meaningful to the person standing in a puddle and grabbing the electrical line.
You’re making too much sense, stop it, will ya. Fools and their money are soon parted. It’s enjoyable to listen to nincomepoops go on and on about how smart they are. Want to make friends, go up yo those knuckleheads that have that prized green ring on their fill stems, and get em going on how they like the nitrogen fill. Get em to offer you a beer. See!!!!
Add to the mix, when using compressed air, one is adding water and oil to the inside of the tire. Nitrogen’s molecules are larger, less leaking and it does not expand or contract with heat near as much as compressed air.
The remark that air molecule being smaller than nitrogen makes be not believe anything they say. First there is no such thing as a air molecule. Air is a mix of different elements that is 78% nitrogen to begin with if the rest leaked out pure nitrogen would be all thst was left, a couple of refills and you tire would end up with just nitrogen.
Always a lot of speculation and soft science when it comes to the benefits of nitrogen in your tires, but the one thing that is certain is that the De Maris’ are completely wrong with their statement that air molecules are a quarter the size of nitrogen. Air is a mixture of 78% nitrogen, 21% oxygen and 1% trace elements; the atomic weight of oxygen is actually 14.2% heavier than nitrogen; the molecule size is just under 3% bigger than nitrogen. As SCUBA divers know, it’s nitrogen that forms bubbles in the bloodstream because their smaller molecule have been absorbed easier than the oxygen molecule. All gases will react to temperature and pressure (Boyle’s Law), but pure nitrogen reacts less (tighter, smaller molecule) than oxygen or the 78/21% mix of “air”. With “air” in the tires, you may see a 10% increase in pressure when hot and a 5% increase when using nitrogen. However, that means that the pressure change with heat is actually a 50% reduction when using nitrogen, and well worth it for critical applications like drag tires and aircraft landings, and a nice-to-have for our valuable motorhomes.
Pete: Point well taken; we have edited the story. Thanks for bringing that editorial gaff to our attention. Russ De Maris, Senior Editor, RVTravel.com
ALL gases expand when subjected to heat. The law of the dilitation of gases is explained in this article from Scientific American. Believe whatever you want, but no matter if tires are filled with standard ‘air’ or pure nitrogen, they WILL increase in PSI when subjected to heat (like, um, while driving)
While most technical people will believe textbook formulas and laws of physics, many consumers need to see a demonstration to understand how things work. I know this from all my electrical demonstrations. Like Nikola Tesla once I have the formula in my head I know how something works and only do the demonstrations to convince everyone else. But its still good to create a real experiment to make sure you haven’t missed a variable somewhere that could skew your results. To disprove this nitrogen thing one would need a pressure chamber with heating elements and a pressure gauge. So you fill it up with regular, humid atmosphere at 40 psi at ambient room temperature, then heat it up a normal tire temperature while monitoring the pressure. Repeat the experiment with dry nitrogen gas and compare the two pressure curves. If there’s any difference at all then try it with dry atmosphere and compare the graphs once again. Myth Busters used to do real experiments like this until they devolved into big explosions. But as much as I like big booms, they really don’t prove much scientifically.
Since the air we breathe is approximately 78% nitrogen, this is nothing more than another marketing scheme to get your money. As was said earlier, nitrogen is used in aircraft tires do to the environment they live in, i.e. near sea level to thousands of feet above sea level. The temperature changes would wreak drastic pressure differences on regular air, but since car tires stay relatively at the same altitude the pressure remains stable. As an example our motorhome tires were inflated to 100 psi in Dec. prior our trip to FL from TN, today 3 months later I checked the pressure in preparation to leave 4/7/2018 and the pressure was between 106-110 psi, they gained pressure. According to some who are claiming that air will leak more than nitrogen I think my tires would be lower not higher, this kind of dispels the nitrogen marketers claims.
Robert, You failed to mention the change in temperature from December to April. An accurate temperature measurement and accurate and precise pressure measurement will show that tire pressure changes about 2% for each change of 10F. I covered this with the mathematical proof in my Blog post https://www.rvtiresafety.net/2014/03/why-does-my-tire-loose-pressure-simple.html
Re Moisture, Yes that seems to have some effect but the “oil & Moisture can be significantly reduced if that is of concern with a low cost filter as demonstrated here https://www.rvtiresafety.net/search/label/Dry%20air
I agree with Bill. Our Grand Cherokee has a tire pressure monitoring system, all tires Nitro filled. Cold they show 33-34 and increase as we drive. Normal temp warm/hot is In the 37-38 range. Park, and the tires cool, the temp goes down.
“It is an inert gas that doesn’t change pressure with temperature change.” This is wrong.
Inert gases do not react chemically.
Every gas changes volume/pressure/temperature relative to each other.
While the temperature/pressure relationship of gases is well known and documented, I wonder if humid air (like the air around us) will have its pressure increase faster with heat compared to dry all-nitrogen air due to an increased vapor pressure of the water molecules. If so, then a dry 100% nitrogen fill would behave more like the theoretical model. I’ve looked for any white papers or studies on this effect, but can’t find one. If there’s any difference at all in the pressure rise between regular air or 100% nitrogen fill, that’s the only explanation I can think of.
I use nitrogen in all my tires. It is an inert gas that doesn’t change pressure with temperature change. That’s why we use it to leak check refrigeration lines. Pressurize a line on a 70 degree afternoon and it will be the same pressure the next morning at 40 degrees if no leaks. I keep two 60 cubic foot bottles in my work van. Each bottle is about fourteen dollars on exchange. I could probably fill twenty tires with one bottle. No brainer since I don’t have to worry about pressure change with temperature change. What we breathe is about 80 percent nitrogen.
Not sure where you got your info, but in a chat directly with Michelin customer service today I got THIS reply when I asked about their stand on nitrogen usage in their tires: “Tires manufactured by Michelin are designed to deliver their expected performance when inflated with air or nitrogen, as long as the user respects the pressures recommended by the vehicle manufacturer on the vehicle’s placard or by the tire manufacturer.”
Costco sells a bit fewer than a bazillion Michelin tires and ALL are filled with Nitrogen and topped off free as needed.
Other checking I have done is that adding oxygen to nitro filled tires does nothing more than reduce the percentage of nitrogen in the tire and does not need to be changed.
Roy, we used to buy tires at Costco, and they filled them with nitrogen. Free top off is great – as long as you can just drive in to your local Costco. We live in northern Nevada. Once we drive away from town on an RV excursion, that’s the last we see of a Costco. Nowhere that WE go has a Costco, so when air is needed, it’s just plain old air. I’m 72, and I’ve been using “plain old air” in tires all my life. Seems to be working just fine. Of course, some would say I’m just too old to appreciate new technology. Maybe they’re right, but I’ll stick to the air I can pump with my 12 volt cigarette lighter and air pump.
Agreed. It seems w age comes wisdom! Our new tires were filled w nitrogen without asking us, we were told free refills for life of tires. 10 hours later . 560 mi from home , no nitrogen to be found in 2 -3 states, except an occas private mechanic…for $5-10/tire, according to the Firestone repair in Biloxi …. so, fill’er up boys …w good old air, save us the headaches for the rest of our 3600 mi drive. No membership required, no ‘closed’ hours to detain us.
Costco only fills the car tires one time. They do NOT get ALL of the “air” out of the tires they are mounting. I constantly have to go back and have the tires topped off because I know they never were filled with pure nitrogen. Living in Southern California, I have to add more nitrogen when we go to the Oregon/Washington coast for the summer.