N2 FAQ List:
Do nitrogen-filled tires lose pressure more slowly?
This seems to be the main selling point for nitrogen vendors, i.e. you donít have to check/top-off your tire pressures as frequently as you do with air. N2 vendors claim that N2 leaks out anywhere from 3 to 6 times slower than air. At first glance this seems incredible, since air is about 78% nitrogen.
But then again, there may be something to it.
There are two commonly used techniques for separating nitrogen and oxygen from atmospheric air. One is cryogenic: you literally cool the air down until the oxygen condenses into a liquid, while the nitrogen remains a gas. The other technique is membrane technology, explained here. Interestingly enough, this is one of the ways that tire shops obtain nitrogen: they run compressed air through an on-site membrane tube bank (click here, see page 4). The only way this technology works is if O2 permeates through a polymer membrane a lot faster than N2 does. Molecule size is one factor (N2, atomic radius = 75 pm, diatomic bond length = 110 pm; O2, atomic radius = 66 pm, diatomic bond length = 121 pm), but the physics of gas permeation seem to be considerably more complex. Click here for a more in-depth explanation, and take a look in particular at Table 1 to see how oxygen and nitrogen permeate differently through different materials; note how the selectivity (the ratio of O2 and N2 permeation rates) varies by material, indicating that molecule size isnít the only factor at play.
This source offers permeation coefficients for still other materials. Check the bottom of the first table, where we see values describing the diffusion of four common gases through butyl rubber, a material with relatively low O2 and N2 permeability thatís commonly used for inner tubes and as the inner liner on tubeless tires. Of particular interest are the values shown in the far right column, expressed in grams per meter per second. They show that for oxygen and nitrogen under the same pressure, oxygen diffuses through butyl rubber at 4.66 times the rate of nitrogen, i.e. the O2/N2 selectivity is 4.66 (on a mass basis; on a volume basis, itís 4.08). However, for air, the partial pressure of O2 is far less than that of N2. Using the values in that table, we can go through some basic algebra to come up with an estimate of the relative leakage rates for air and pure nitrogen:
Suppose we have a tire filled to X psi with pure nitrogen, and we define the leakage rate as ď1Ē. Now we estimate what the relative leakage rate would be for the same tire filled with air. Using air, the partial pressure of nitrogen would be 0.78X, so the relative leakage rate of nitrogen from that air-filled tire would be 0.78. The partial pressure of oxygen in that air-filled tire would be 0.21X, much lower than that of the nitrogen. However, the oxygenís propensity for diffusion is 4.08 times (on a volume basis) that of nitrogen; the combined result is that the oxygen portion of that air-fill diffuses out at 0.21 * 4.08 = 0.857 times the rate of the nitrogen-filled tire. With nitrogen and oxygen both leaking out of the air-filled tire at the same time, the total leakage rate for the air-filled tire is approximately 0.78 + 0.857 = 1.637 times that of the nitrogen-filled tire. This is considerably less than the ď3 to 6 times fasterĒ that some tire stores claim, but itís not far off from Ingersoll-Randís claim that oxygen - not air - leaks out 3 times faster than N2. (Interestingly enough, even IR canít keep their facts straight: over here, they claim that O2 leaks out only 30-40 percent faster than N2. Probably an editing error, since the previous assertion Ė 3X Ė is in closer agreement with the O2/N2 selectivity I found from the other source.)
In 2007 Consumer Reports published a comparison of air-inflated tires and nitrogen-inflated tires to see how fast each type lost pressure (article here). It was a pretty thorough test, involving 31 different models of automotive tire inflated to an inital pressure of 30 psi, with the loss of pressure measured over a period of one year. Detailed results are shown in a spreadsheet, but what it all boils down to is that on average the air filled tires lost 3.5 psi, and the nitrogen-filled tires lost 2.2 psi. 3.5 divided by 2.2 equals a leakage rate for air that is 1.59 times as great as nitrogen. That's remarkably close to what the theory predicts.
If you check out that spreadsheet (see link, previous paragraph), you'll notice there's a wide variation in pressure loss from tire model to tire model. Some tires lost a fraction of a psi, while the BF Goodrich tire lost around 7 psi. So if your buddy notices his new nitrogen-filled tires are hardly losing pressure at all, ask him if they're the same brand as his old tires. There may also be a difference in leakage rates between brand new tires and tires that have racked up thousands of miles.
OK, so someway, somehow, it seems that air DOES sneak through a tire faster than pure nitrogen; go figure. But by my estimates, the difference isnít anywhere near as much as is commonly claimed. Having said all that, is it worth the extra expense/hassle of using nitrogen instead of air? If you operate a trucking fleet and each of your drivers spends half an hour every two weeks checking/topping off his truckís eighteen tires, then yeah, it may be worth it to buy/use a nitrogen generator and cut that tire-checking by 40%. Me? Iím still checking my motorcycleís tires before every ride, whether they leak down slowly or not; regular checks can help you catch small leaks before they become a big problem (e.g. 50 miles from home).
Note also that if you fill and top off with air, the percentage of N2 will gradually increase over time; at first you lose about half O2 and half N2, but you top off with a mixture of 78% N2 and 22% O2. So if you fill your tires with air and they initially lose about a psi a month, then after a year of this, itís down to mostly N2 inside the tire, and the leakdown rate should slow down. For many motorcyclists, tires donít make it through a season before replacement, so the effect is negligible; but if your car tires last four years, then by using air in them youíve actually gotten about 3/4 of the benefit of a N2 fill, without the expense.
Questions? Comments? Email me!
©2009, Mitchell P. Patrie