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Originally Posted by rx7vert
gain pressure. When it gets cold outside, you can lose up to about 8 psi in your tires, having nitrogen would minimize the difference to about 3 psi.
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I'd be interested in a reference to support that.
You're already running nearly 80% nitrogen with air. Nitrogen's properties are not much different than oxygen. Nitrogen has an atomic weight of 14, 7 protons, and oxygen has an atomic weight of 16, 8 protons. Both turn from liquid to gas at 298k.
In comparison with helium, helium is far less compressible than oxygen, and the result is that pressure variations as a result of temperature differentials far exceed that of air. When filling a scuba tank, we'll typically go 10% over our target pressure on helium to get the mix to come out right after it cools.
I mention helium because helium is lighter than both oxygen and nitrogen, and nitrogen is lighter than oxygen.
(P1 x V1)/T1 = (P2 x V2)/T2 (temperature in K)
If temperature increases, either pressure or volume (or both) must increase.
The primary reason nitrogen is used by the military is because it is inert. It will not promote corrosion and it poses no combustion hazard. Keep in mind that the SR-71 runs at 600psi. A blown tire in a wheel well with hot brakes and with the blow tire taking out the brake lines could fan a serious fire. Nitrogen will quench it.
If low humidity were the issue, it would be much cheaper for the military to use moisture separators and filter stacks, much as we use for scuba cylinders (our air must be VERY dry).
Nitrogen inflation is NOT a bad thing, but it's not a big deal. If the shop uses nitrogen, it's fine. It's not worth paying extra for. There's a lot of hype, voodoo, and misinformation. I had a Costco tire jockey tell me that "Ya, the atmosphere is 80% nitrogen, but oxygen is heavier (he's right), so it's all down here, and the nitrogen makes up the upper 80% of the atmosphere".
The kid honestly believed that he was breathing pure oxygen.
One thing about topups... TRY to get your air at full-service stations, rather than the coin-op units at gas-only or gas/quickie mart stations. The full-service stations also run their tools and other equipment from that compressor, so they are more likely to drain the tanks every night and deliver dryer air. Who knows how well the coin-op units are maintained. I've encountered some that blew visible mist and droplets out.