Filling Tires: Air versus Nitrogen

Scott Sherer

This letter is in response to Erich Rempert's article on filling tires with nitrogen:

To Erich Rempert,

The Earth's atmosphere is a layer of gases surrounding the planet Earth and retained by the Earth's gravity. It contains roughly (by molar content/volume) 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.038% carbon dioxide, trace amounts of other gases, and a variable amount (average around 1%) of water vapor.

Since shop air should pass through a dryer, I would hope it is pretty dry. Also if the molecules of nitrogen are really that much larger that the other components, then the actual percentage of nitrogen in the tire will steadily increase though out the cycle of leaking and refilling.

Based on cost, convenience and actual performance benefit, I don't think nitrogen is worth it. A much better use of your money would be to buy a good tire-pressure gauge and check your tires frequently. This is a good idea even if you have a tire-pressure monitoring system in your vehicle be it auto or airplane. The warning lights aren't required to come on until you have less than 25 percent of the recommended tire pressure. Having the correct tire pressure will get you many of the benefits of using nitrogen and will ensure that your tires last longer.

So, although what you say is mostly true, there is really an extremely small practical benefit to using nitrogen versus shop air. And you will have to explain in much more detail how using shop air in my tires will damage struts.

Joe

Scott Sherer

Here is a response from Erich:



Joe,


95% of all shop air I have ever seen does not pass through a drier, and it is not dry by any means. Even air that does go through a drier is no where near as dry as nitrogen in a cylinder from a gas supplier.



Over the winter I checked the pressure in the tires of my Navion (which are filled with nitrogen) they were at the same pressure I had set them at 3 months earlier, I could normally expect them to be down by a 1/3 using air.



My vehicles warning lights come on at 30psi in my experience. And my cars are new enough to actually read the pressure on the dash, since draining and replacing the air with nitrogen I have found the pressures to be noticeably more stable both during use and with the change of the seasons.



We rarely charge anyone for nitrogen, and in many cases it's more convenient than air because the cart can go anywhere I push it, shop air is limited to the length of the hose. Plus in the case of struts, the tank is,already high pressure and you don't need to mess with a strut pump.



I suggest you look closer at the piece and then you'll realize I was referring to the gas "inside" the struts, as well as a few other tid bits it seemed you may have missed.



Of course you should have a good pressure gauge (Mine is calibrated every 12 months), and check your tires often. Maintaining proper pressure is #1, using nitrogen makes that easier to do.



Once a guy taxied up in his Cardinal and asked for air for his flat nose strut. I offered nitrogen and he refused insisting on "air". I was willing to service his strut with nitrogen for him free of charge, but once he insisted on air I handed him the air line and watched instead....



You can lead a horse to water, but you can't make him drink...



Hopefully you never experience the unexpected result of high pressure atmosphere and hydraulic fluid mixed in a shock strut like I have. If you don't feel the minimal cost or effort to use the proper material is worth it to you, that's your choice,



I have found through personal experience, and anecdotally through the experience of others, the performance of nitrogen vs shop air to be of immense value given the cost and availability of nitrogen. In fact, I have paid more to fill my tires with damp shop air at a coin operated gas station dispenser than I have ever paid for the same amount of nitrogen!



Blue skies, tailwinds, and cheap avgas~



Erich Rempert, A&P Consultant

Scott Sherer

Hi Erich,

Absolutely nothing wrong with using nitrogen, you just overstate the benefits. The biggest argument for you in real world is that racing teams use only nitrogen but remember a race car wants ever possible advantage, so they do things above and beyond what is needed for every day driving. If your shops don’t have a good dryer on their air system, spend some money on that. All your tools and your air system will last longer as well.

Check air pressure daily not seasonal! I am no expert on tires and tubes but the tubes I use do not seem to have the problem of pressure loss to which you refer. But I did have 172 once that one tire lost air quicker than the others and I could not locate a leak, so I assume the tube was just not as good as others I have had. I probably lose more air checking them than just sitting. As for temperature change with heat, each 10 degrees Fahrenheit results in about 1 PSI change in tire pressure.

Pilots been discussing such things as crab or slip, stall or wheel landings with tail draggers and numerous other things forever. Guess Nitrogen is just one of them.

Have a great day!
Joe

HESSAERO

The oxygen molecule is about 11% lighter than the nitrogen molecule. Oxygen will diffuse through rubber a fraction faster than nitrogen. But air is already about 80% nitrogen. If the oxygen molecules escaped even 30% faster, .3 x .2 is .06, or six percent faster pressure loss. That doesn't seem terribly noticeable. The water molecule is half the mass of a nitrogen molecule, and moisture in fill air is likely to get absorbed into the rubber tube. Pure rubber won't absorb more than about 0.1% water by weight, but the rubber in our tire tubes is anything but pure, and may absorb a few percent. Absorbed moisture is bad for rubber stability. That's why I prefer to install new tubes when mounting new tires.

For our light airplane tires, there are two kinds of tubes, natural rubber and synthetic rubber. Air (or nitrogen) diffuses through natural rubber substantially faster than through synthetic. So synthetic tubes hold air better. I favor natural rubber tubes anyway, because they appear to have better tear resistance. In my experience a puncture on a motorcycle tube with a natural rubber tube will cause a slow leak, but the same damage on a synthetic rubber tube is more likely to cause a long tear and sudden total air loss.

I totally agree that servicing tires or struts with moist air is a bad idea. And most airplane owners who have small compressors in our hangars do not have driers to reduce the humidity in the supplied air. If struts are serviced repeatedly (because of nuisance leaks) with moist air, it's very likely that moisture will accumulate under the oil in the strut, and that is a recipe for corrosion. In my view, the real problem there is continued monthly or weekly adding air to a bad strut. What needs to happen to prevent water collection is fixing the strut seals. Filling with nitrogen will help reduce corrosion potential, but it isn't a fix.

pjmoser

The oxygen molecule is both heavier and larger in volume than the nitrogen molecule. Another constituent of air is Carbon dioxide, which is a much larger molecule than Nitrogen. There are very small amounts of small molecules, Hydrogen, Argon, etc.

As for the pressure variation with temperature, all these gases nearly follow the ideal gas law, so their behavior is nearly identical.

Nitrogen is not inert. It readily forms molecules with other atoms, such as nitrous oxide, ammonia, etc.

Water vapor can contribute pressure variation of only a few percent of air pressure over usual operating temperatures.

I will save my money, and continue to use air.

Jim Moser

HESSAERO

There might be a third path. If you want to minimize moisture in struts/tires/whatever, but don't want to spend the dollars to put a dedicated N2 bottle in the hangar. I think I might fill our strut and tires with air from a scuba tank. My wife and I dive, so we have several scuba bottles at home. Scuba air is quite dry - the standard is a dew point of -65F. Still contains oxygen, of course, but darned little moisture.

One of my dive buddies always takes a spare regulator first stage on dive trips, with a small blow off hose that he uses to blow any moisture off underwater camera cases prior to opening. The normal intermediate pressure from a scuba first stage will be between 115 and 145 psia. That's plenty for filling tires or single engine Cessna struts.

planewrench

this is on every strut on probably every 135 and 121 aircraft as it is on the A330 nose gear here. AD driven from older days,,, corrosion from moisture caused wheel halves to explode. Collateral damage and deaths if i remember correctly.

carl