Under Pressure

Friday, May 21, 2010 by Cody Rollins
Being the designated car guy in the family, I always get asked to "check my tire pressure," from friends and family. I usually answer "not right now" not because I'm a mean guy but because the question comes usually a couple hours after they arrive.
4" Deluxe Air Pressure Guage
I'll explain that I'm really a nice guy and would be happy to check their tire pressure but we need to wait until the morning or at least give the tires enough time to cool down to their ambient temperature. Sometimes i get a puzzled look from them thinking I just want to put it off a little longer and hope they forget about it. That's when I'll bust out the chemistry, and remind them of the basic gas laws we all forgot from high school. 

A little gas law is all that is needed to justify my apparent laziness. "Pv=nRT" I tell them. As they were driving to my house the deflection of the tire created heat and since the tires volume is consistent as well as the mass of air, then the pressure must have gone up. Basically the car manufacturer's recommendation in the door sill is a "cold pressure recommendation" so we can't check it right after a drive.

We need to wait until the tire's air temperature is the same as the outside to get a correct air pressure setting. The next morning when I bring out an Intercomp 4" Deluxe Air Pressure Gauge they can rest assured their tires will be operating at the correct pressure on the drive home.

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