Nitrogen?

Might we be getting into the realm of spontaneous combustion there? I think not, but it wouldn't make me real comfortable.

 

One doesn't need pure gas to have an interest in the diffusion rates of different components of a mixture. Anyone who attempts to compute pressure loss will need to know the differing rates for each portion of the blend.

 

...but 4x loss rate on O2 is big enough to make 100% N2 better than even 21% O2 for pressure maintenance. Not to mention possible improved tire life and reduced tire failure rates. But the adoption of N2 for passenger tires is apparently going slower than truck tires because the benefits are less obvious for cars, and I would add N2 handling safety as a possible barrier (re: asphyxiation hazard).

 

No it's not. Who cares what the RELATIVE diffusion rates are? The thing that matters is the ABSOLUTE diffusion rate. If N2 lost 0.001 psi per month, and O2 lost 0.004 psi per month, would you care?

 

...hmm I thought the Curmudgeon was budgin'

 

Corwyn brings up a good point. Pneumatic tires are designed to contain gases, not let them diffuse at a high rate. Loss of tire pressure comes from changes in temperature and leaks, not diffusion, at least to any significant degree. As pointed out above, if diffusion were the major factor, tires would quickly self "nitronize" as the oxygen portion dropped to atmospheric levels. Likewise a tire filled with 100% nitrogen would quickly absorb oxygen until the partial pressure of O2 inside the tire equaled that on the outside. To do this, tires would need to be thin permeable membranes, not thick slabs of rubber.

Another seldom mentioned point is that "nitrogen filled" tires are almost never filled with pure nitrogen. Generally the supplier uses some vague term such as "nitronized", meaning air with a slightly higher percentage of nitrogen. Given that ordinary air is mostly nitrogen, boosting the concentration doesn't buy you much.

Tom

 

Yeah Tom, It all goes back to the same thing... "Snake oil_ _ _ pure snake oil" (lol)...

 

That's absolutely understood but, one can't take this difference and use that info to try to convince us of the benefits of using 99% N2 over 78-80% N2 (which is what I think is trying to be presented here). The real info that should be quoted/used is the diffusion rate of 99% N2 vs 78% N2. I guarantee you its not 3-4% and that diffusion rate would decrease over time (as the O2 diffuses faster than the N2).

My point is; If we're going to make comparisons to support our points, lets make sure we make an apples to apples comparison and not an apples to oranges comparison. Even better, lets compare Gala apples to Gala apples.

 

Here is some accurate info from Goodyear on temperature effect and altitude effect on tire pressure.
We should expect 2% tire pressure change for every 10 deg F change in tire temperature.

Example Calc-
Prius tire at 40 psig what happens if outside temp drops from 70 deg F to 30 deg F?
Psig@30F = (Psia@70F x (1-(40/10) x .02) - 14.7 = ((40+14.7) x 0.92) - 14.7 = 35.6 Psig

So 40F temp drop gives about 4.4 psig pressure drop.
This would be same for dry Air = dry O2 = dry N2

Moisture impact is small in this example (moisture is a bigger factor at >100F).
At 70F, moisture if present contributes about 0.35 psia to your tire pressure.
So if all the water condensed at the 30F in the example , your 30F psig would be 35.6-0.35 = 35.25 Psig

 

The old 1 psig per 10 degrees F rule of the thumb is nearly as good and is simpler. Both methods are approximations and close to typical tire gauge accuracy under normal conditions. I also use 0.5/psi per 1000 feet of altitude instead of 0.48. Slightly less accurate but the error is less than a persons ability to read a typical tire gauge. Nice thing is, as you gain altitude, it typically gets cooler so the lower atmospheric pressure and temperature drop tend to cancel each other. As someone who has gone through 8,000 to 9,000 foot altitude changes twice a day an many trips, I tend to ignore the altitude pressure differences.

 

Yes you are correct XS I was trying to check the rule-of-thumb because I thought it was off. Turns out it was pretty good. But also I wanted to show moisture is minor impact on it.

 

Moisture gets dragged into these discussions because it is an important factor for race cars and aircraft:

1) Race cars are driven at the ragged edge of control. The closer to the edge the faster the car. Because of this the people setting up the cars and driving them want predictable tire pressures. Note that I said predictable, not invariable. An indeterminate amount of water vapor would make it harder to predict tire pressure increases during a race. Even though small, the variation is significant for racing.

2) At high altitudes aircraft are subjected to very cold temperatures which can cause water vapor in tires to condense and then freeze. Aircraft tires also go from zero to high RPMs at touchdown - a lump of ice at the bottom of a tire would create a serious out of balance situation.

Neither of these situations apply to the family chariot. Regardless, the moisture card is generally played by those wishing to sell high priced inflation gases.

Tom