Join Date: Mar 2011
Location: Mid Hudson Valley, NY (USA)
Re: Tire Pressure Monitor
If you can get a 'nitrogen' fill for free at your local shop, take it. But knowing what they are trying to sell you, and with all the variables as Brian described above, it is of dubious value if you have to pay for it.
Remember the cup or so of water based tire bead adhesive and mounting lubricant they slopped on the tire and rim before mounting? That's a significant source of your water! In a closed environment, where is it going to evaporate to? Some of it becomes bound up in the latex thru a chemical reaction. Much of it stays 'wet'. And when that freezes and drops out of the air mix, you get a marked change in pressure. The Ideal Gas Law was never a perfect way to model a multi-component system at the best of times, but something that changes state really messes with the math.
If you can find a tire shop that actually has a tall (like 10 ft) cylinder of N2 out back, filled from an Air Products or other mobile truck, you are getting pure stuff. Otherwise, as Brian said you are getting 'something' from a device that is similar to the Oxygen Concentrator you might have seen in your grandmother's house. Air forced thru a semi-permeable filter medium that permits passage of one sized molecule, but partially blocks the other.
If you believe the tire industry literature, N2 stays behind as the other components leak out. Using that logic, over time, the smaller stuff (CO2, O2) diffuse out, leaving a higher concentration of N2 behind. So your tire is naturally acting as a N2 concentrator.
Current: '15 Sienna Limited Premium FWD, '14 Subaru Outback, '13 Honda CRV AWD.
Past: '08 Sienna LE (FWD), '02 Subaru Outback