Inflate with Nitrogen? Why?

Saw this question on a FB page.

" Anyone researched cost etc of replacing the air in our tires with nitrogen? It's more stable and does not change pressure with temps. All my BMW's have it. I hate checking my pressures every trip.

There were over 10 pages of replies. Some good some not so much. Here is my final answer.

Lot of time and effort in this thread but I am not sure if a consensus will ever be reached.

As a tire design engineer I can say that inflating tires with Nitrogen AKA "N2" will not hurt the tire.

Is it better for the tire? Theoretically yes as N2 is less reactive than O2 but this gets sticky when you ask if the benefits are measurable or meaningful for an RV or other street vehicle in normal operation.

I do not see where any poster of this thread has identified any problems with their tire due to Oxygen reacting with the rubber in their tires.

Let me throw a "fly in the ointment" into this topic. Those advertising the use of N2 offer a claim of improved fuel economy. However I believe this claim is based on a couple of assumptions:
1. That the average driver does not check the air pressure in their tires and with O2 levels decreasing due to the O2 reacting with the rubber which effectively lowers the tire pressure over time.
2. Lower tire pressure means worse fuel economy.
I accept and agree with those 2 assumptions
BUT With the advent of TPMS many RV owners are now properly paying more attention to their tire pressure so they are not driving with their tires at lower pressure, so what is the advantage?


I believe I can even posit a disadvantage to running N2.


If we assume, that tire pressure does not change (increase) as some advocates of N2 claim, then that would suggest tires inflated with N2 will deliver worse fuel economy as the tire running at a lower pressure than tires inflated with air, will deflect more which means it will deliver worse fuel economy. This worse fuel economy would be very difficult to measure but the fact that tires with lower pressure deliver worse fuel economy is established and accepted fact.

For me, as a tire design engineer, I feel that using N2 instead of Air to inflate tires MIGHT be beneficial to long term tire life if no effort is made to ensure you are not introducing excess moisture into the tire air chamber. If you make the minimal effort to control the moisture level.That means your air compressor tank is properly maintained and drained of the excess moisture that can accumulate in the tank, or you use a "tank-less" compressor so the only moisture introduced into the tire is the moisture in the air we are breathing.

I have a post in this blog on how to get unlimited dry air for your tires for a few dollars no matter what compressor you use. Make your own air dryer using components used in auto painting and place it in your air line so all your high pressure air is relatively dry. 

In case people are wondering what I do... I have a large air compressor in my shop. I drain it every time i Use it. I also have an "air dryer" in the line to protect my air tools from rust due to moisture. I also have the "dryer" I made in the link so I see no reason to use Nitrogen in any of my tires.

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Are Nitrogen Molecules Really Larger Than Oxygen Molecules?

According to the "Get Nitrogen Institute" in their paper on N2 effusion
http://www.getnitrogen.org/pdf/graham.pdf

"The correct answer, with respect to 'permeation,' is yes."

So I imagine your question is why don't I support the effort to "sell" the idea of always inflating your tires with just Nitrogen. It comes down to effort and cost VS level of benefit.

Maybe one way to think of this would be to imagine dropping a penny as you walk away from making a small purchase at a store. If you dropped a number of coins you might stop, bend over and pick them all up. But what if you only dropped one penny and didn't discover the fact till you had walked to your car. Would you walk the 20' back to the store to look for the penny? I bet not.

There is no doubt that you would have more money if you picked up the penny but would you consider it worth the effort?

In general, tires lose about 1%  of their inflation pressure each month in laboratory testing. This is almost entirely Oxygen. It is also true that tire pressure changes about 2% for every change in temperature of 10F. This is true for Nitrogen or air.
I haven't tried to run a test but it is also true that every time you use a hand pressure gauge to check your air you let a little air out. How much air do you let out if you use a gauge to check your tires every day? Might it be 1% in a month's time? Might it be more?

Finally, what does it cost to inflate your tires with N2? Even if you have a deal with a dealer and can get your pressure "topped-off" for free, you still have to drive to the store location to get that "free" inflation.

Bottom Line:
IMO the small level of benefit of inflating with N2 just isn't worth the effort and cost. There is also the real negative of not checking your tire pressure simply because you believe that by inflating with N2 your tires will never lose pressure so you don't need to check. What about small punctures or leaking valves? If you don't check the pressure you will not learn about the leak till it is too late.

However, if you want to inflate your tires with N2 I see nothing wrong with doing that. After all, it's your time and money, not mine.



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Top 10 reasons to not put nitrogen in your tires

For those that missed this info in RV Travel newsletter:


The language in this video from Australia is a little off-color at times, but the information is solid. The message: Putting nitrogen in your tires is a waste of money. Watch the 17-minute video.

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Why does my tire lose pressure? Simple and Technical answers

 As people get ready to start traveling again, some are discovering that their RV tires have lost inflation pressure over the past few months while it was parked. There are a number of possible contributors to a loss of pressure.

Some of these have been covered in my posts on leaking valve cores or metal valve O-Rings or possibly punctures or even rim corrosion. This post will cover just two of the possible causes. permeation and Temperature change.

The simple answer is that tire pressure will change about 2% for every 10°F change in temperature.

The complex answer and mathematical proof follows:

1. Normal pressure loss due to gas permeation. This is basically what happens when the molecules of gas "leak" or travel through the rubber of a tire. New passenger and LT tires sold to "Detroit" have to pass tests and meet a specification of a loss rate of less than 1% or 2% per month when averaged over a number of weeks or months, with different companies having different specs depending on the application. TBR (Truck-Bus-Radial) tires as seen on Class-A RVs will have similar performance goals. Low Cost import tires sold at "Billy-Jo-Bob's Cheep Tire Emporium and Bait Shop" may not have any such spec so might lose pressure faster because of the use of lower cost inner-liner rubber. This topic could be an entire post of its own but is quite technical and is not based on just the molecule size of Oxygen.

2. Tire inflation pressure is directly proportional to tire temperature. The "Ideal Gas Law" is a good approximation of what really happens. The general terms of the Ideal Gas Law are PV=NRT
 P is the Absolute pressure, not gauge pressure, measured in pascals
V is the air Volume measured in cubic meters. While tires do change a little bit in volume the difference is not meaningful for the purposes of this discussion.
N is the amount of gas in the air chamber in moles. For the purposes of this discussion, we can ignore this number as we are not changing the amount of gas in the tire when we are comparing the pressure difference just due to temperature difference.
R is the ideal, or universal, gas constant. R for dry air is 287.1 for N2 296.8 and for O2 259.8 for water vapor it is 461.5 so you can see that moisture in your inflation gas can significantly affect the Pressure/Temperature ratio a change from air (79%N2) to 95% N2 is not significant for the purposes of this discussion. I did a post on how to make your own "air dryer" so you can approximate the properties of dry air or dry Nitrogen.
T is the absolute temperature in degrees kelvin.

After discarding the inconsequential terms we really end up with is a simple ratio P2/P1 = T2/T1 and we can even ignore the conversion of psi to pascals as the ratio of pressure is the ratio of temperature in degrees Kelvin. Sorry we do need to use Kelvin but you don't need to do a conversion as you will soon see.

Lets see how this woks out
If P1 is 100 psi and if  T1 is 70°F or 294.261°K and if the temperature drops to 60°F or 288.706 °K we can solve for P2/P1 =  288.706/294.261 which leaves us with the ratio ot the two temperatures as 0.9811 so we had a 2% drop in pressure for a 10°F drop in temperature

30°F would be 272.039/294.261 = 0.92448 or 8% drop in pressure for a 40°F drop in temperature from 70°F to 30°F.

You may see various web pages such as this one from TireRack or Wikipedia saying 1psi for each 10°F change but you need to remember that this information is based on the assumption that the "normal" inflation is about 35 psi in a passenger tire.

How to get dry air for your tires

A good part of the discussion on the advantages of inflating tires with Nitrogen focuses on stable inflation pressure which seems to be of concern to some. Now for race car application this is important and the benefits are meaningful in that application but I do not think that a fluctuation of a few psi due to temperature variation is meaningful in normal highway use. Measurable but not meaningful.

I can accept the fact that no matter what my opinion is, some will still argue that stable tire pressure is desirable, so they feel the expense and complications of using Nitrogen are worth the cost. While there are some theoretical advantages to inflating tires with a gas that is low on oxygen, I don't think they are worth the effort. I have both Argon and a mix of Argon and CO2 available to me in my shop but I still use regular air from my compressor. The one thing I do have with the air in my compressor is the knowledge that the moisture level is low, as I have filters and water separators in my system.

But what do you do if you don't have a well equipped shop in your back yard but still want dry air? I am going to show you a set-up I have, that costs about $20 and can be used to inflate your tires for years to come.

Above is a small air drier system I use in my shop for some special equipment that needs especially dry air, you can duplicate it to ensure the air you are using to inflate your tires is dry.

The dryer is available at 2/$20 at WW Grainger as well as $8 ea at Advance Auto. Be sure the one you buy is rated for the air pressure you need. Harbor Freight units are rated only 95 psi.

•The air chuck is under $2 at Harbor freight
•Quick connector set is $5 a set or less at Harbor freight.

Note my four year old desiccant is purple not blue so I am getting close to needing to change the dryer which is a disposable unit. To help keep your dryer from getting old fast, keep it tightly sealed in a good zip-lock bag or some container with air tight top as I do.

There are too many individual air setups for me to identify all the possibilities but this simple dryer will deliver what you need.

Warning. If you are going to try and "dry out" the air in your tires by changing your "wet" air you need to jack up the tire so it is not contacting the ground. If you let air out don't go too low or you may have problems with losing the seal at the bead. A quick test to see if you have lots of moisture in your inflation air is to remove the valve core. If water spits out you have excess moisture in the tire. You may even need a tire store to service it to get the excess moisture out.

Hope this will save money for those wanting dryer air in their tires.

Inflate with Nitrogen and there will be MAGIC

Ok so I may have exaggerated a bit, but not by much.   

Previously I started a series on proper inflation of tires. Before I continue lets cover the Nitrogen inflation question.

Nitrogen, Symbol  N  Atomic Number 7   Mass 14.01

Important concept: A claim of “up to” 30% savings can be met with a 0.01% savings. Telling the truth but leaving out the whole truth can be misleading.
When it comes to the suggestion of inflating with Nitrogen there are many claims. Am I suggesting all the claims are false? Not at all, but I believe many are based on assumptions not stated or significant exaggerations of the data that does exist.
Would I see some benefits if I inflate my tires with Nitrogen? The quick and short answer is yes. But, would these improvements be meaningful or cost effective? The answer to this question is probably not.
Warning: What follows is a detailed engineering discussion. Only read if interested. Otherwise you can skip to the end summary.
Let’s look at some claims
Nitrogen is a dry gas and does not contain moisture. A more accurate statement might be that all compressed gas purchased in a metal tank at very high pressure could be considered dry. This would include a tank of CO2, Argon or even a tank of Air.
Oxygen in compressed air contains moisture and is known to cause oxidation. It’s not the O2 that contains moisture. The compressed air we get from an air-line at the local gas station contains more moisture than high pressure gas.
Tires lose air pressure at about 1% to 3% per month when temperature and barometric pressure are held constant. This is true.
Tires inflated with Nitrogen lose pressure more slowly than when inflated with air but they will still lose pressure. True, but they will seldom tell you in the sales pitch that tires inflated with N2 will still loose pressure. The big selling point for Nitrogen is that if you don't check your tires it is a better inflation gas than regular air. But no matter which gas you use, you need to check to ensure you do not have a puncture or leaking valve.
Would you accept the concept that if I use synthetic engine oil I don't have to check the oil level because synthetic lasts longer?
Nitrogen-filled tires run cooler and can improve tire life up to 25%. Engineer Speak Warning. The Thermal Conductivity of pure N2 is 9.8 miliwatts per meter kelvin and O2 is 9.3 making the N2 more thermally conductive by 5%. So the heat generated by the tire due to flexing can be transferred 5% better to the metal wheel for dissipation to outside air. What they fail to mention is that if N2 conducts heat better it will also conduct heat from bearings and brakes to the tire.
Some have claimed fuel economy gains of over 20%. Well here is another claim that is hard to justify if you know how a tire works. It is well established that increase in inflation will lower the fuel usage because the tire deflects less. So, if the claim is that N2 run cooler, how does that not result in a lower pressure in the tire than when the tire runs hotter? Doesn’t it follow that if the pressure is lower, the tire will deflect more which results in greater energy loss due to tire deflection? The only way for a tire inflated with N2 to yield better fuel economy is if you compare it to a tire that has not had its inflation pressure maintained.
Nitrogen does not expand at the rate compressed air does while heating up as you travel down the road.OK now we are really getting technical. First the Gas law needs to be considered. As pointed out this is theoretical but "the ideal gas law is a good approximation for most gases under moderate pressure and temperature." The link also points out "If the temperature changes and the number of gas molecules are kept constant, then either pressure or volume (or both) will change in direct proportion to the temperature". Now the tire is a closed system so the number of gas molecules does not change. There is a slight increase in tire volume as pressure increases and of course this would offset some of the theoretical pressure increase but we can assume that is essentially negligible for the few degrees temperature difference we might be considering. This take us right back to the question of the properties of pure Nitrogen vs. Dry air. This also brings up the question of what percent Nitrogen do you end up with even after a purge with N2. The numbers I have seen indicate you would be going from 78% to 95% N2 not 100%.
Some claim you can reduce tire failures by 50 percent. If this were true don't you think the tire companies would all have been inflating tires with N2 for decades? Where's the data?
Some like to point to aircraft tires. They use N2 but for a different reason. The Autoignition temperature for rubber is 260 - 316°F. Unless there is some mechanical failure you will not see this temperature. I have seen many dozens of tires where components have exceeded 390°F and there has been no fire so it takes a special set of circumstances for this to be an issue.
End Summary
Now having said all this, would it be better to inflate with Nitrogen than air? If there was no cost and you could somehow have N2 available all the time, my answer is yes.
The problem is this is not a perfect world and I don't want to carry around a tank of Nitrogen to inflate my tires or run my air tools as I did with my race car. Even if I was given the tank and regulator for free, hauling it around will take up space and cost me fuel, most likely much more than I might theoretically save with the almost magic properties attributed to this "Wonder Gas".
A lower cost thing to do is to inflate your tires at least 5 psi over that needed to carry the actual load while not exceeding the max for the tire or wheel. If you are concerned about moisture buy some inexpensive air dryer and inflate your tires with dryer air in your line just before your air chuck.. You will gain almost all the anti-corrosion benefits by eliminating the moisture and you can keep these pocket size "filters" almost anywhere if kept in their original packaging. The filter #68215 at Harbor freight is OK for 90 psi and costs less than $8 and another $10 or so will allow you to make up the needed air chuck adapters.

UPDATE:5-17-12 I will do some research and try and find similar dryers for 125 psi application and make a new post.

The questions supporters don’t address are; What is the initial cost? How will you refill your tire pressure as time goes on? Will you be faced with the decision to drive on an under inflated tire to get to some place to inflate with Nitrogen?
What is not stated with all the claims is that if you check your tires for damage and inflation and adjust as you should ie before each trip, you will not see the performance improvements claimed.