At 20 C, room temp, the saturated vapor pressure is 17.5 Torr (0.3 PSI).
At 60 C, the saturated vapor pressure is 149 Torr (2.9 PSI).
At 80 C, the saturated vapor pressure is 355 Torr (6.9 PSI).

I'm not sure what the operating temps for tires are, but water vapor could make a big difference if there is sufficient liquid water in the tire. I'm not sure what the internal volumes of most tires are, but we are probably looking on the order of 10 ml of water to saturate the water vapor at 80 C.

I would think so as well.

 

But aren't these the saturated vapor pressures at atmospheric pressure, not at tire pressure which is (cold) roughly 3-4 atmospheres? I would expect the vapor pressures to be lower at higher air pressures.

Also, if there is insufficient water in the tire, one would not reach these vapor pressures. And even if there is a LOT of water in the tire, the vapor pressure would not go over these values. IOW, water would evaporate until these pressures are reached (if there is enough water in the tire to do that). Once that pressure is reached, any additional water would remain as a liquid.

 

It doesn't matter what the total pressure is. If there is enough water present, the partial pressure of water will reach its saturation vapor pressure for its temperature.

The boiling point of water will change based upon the total pressure. The boiling point is when the vapor pressure of the liquid is equal to the atmospheric pressure. When this happens, you get the bubbling. This is why water boils at a lower temperature at higher elevations. For example, in Denver, the pressure is quite a bit lower than at sea level (~630 Torr versus 760 Torr). For water to boil in Denver, the water only has to be heated to a temperature to obtain a vapor pressure of 630 Torr. Obviously, this is lower than the temperature required to obtain a vapor pressure of 760 Torr.

All of this is correct.

 

How's this for nitrogen tires:

 

Consumer Reports did a one-year study on this, releasing their report just this past October. Their analysis - nitrogen (when the tire is properly purged and filled to 95% N2) does indeed leak slower than standard air out of tire rubber. However, the difference is minimal (on average, N2 filled tires leaked 1.3 psi less than air-filled tires a year). Their advice? If you check and maintain your tire pressure often, there's no need for nitrogen.

The report:
http://blogs.consumerreports.org/car...nitrogen-.html

The Q&A:
http://blogs.consumerreports.org/car...en-tires-.html

The table:

 

I can recommend a Campbell Housefeld rechargeable compressor/power supply that i bought at Walmart 5 years ago. It works great and the battery charge lasts for a long time and i use it all the time. It has a good amount of pressure and never needs to be plugged in either! It only cost like $45 when i purchased it. I check my tire pressure at least once every other week.

 

1 or 2 psi over a years period of time ?? That's not very much at all! And getting 100% Nitrogen is not easy. I worked on F-16's for 15 years and i know they always serviced the tires with Nitrogen, but I'm not convinced it would be worth while in a passenger vehicle?? I know it was essential for the tempature extremes and pressure changes of flight.

 

I have tire pressure caps and fill the tires if they show low. However, it seems I need to check them every 2-3d tank, 6 weeks or so.

Bob Wilson

 

I have never heard of tire pressure caps. That's a great idea. Where did you get them?

 

I got my first set via Ebay but you can Google them up. One caution, they seem to have a quality problem so I'd recommend getting five. Also, it is important to put them on tight but not over tight. Still, for me, they have worked out fine and give me a quick reminder when the pressure gets low.

Bob Wilson