by That HP Chap
Question by Not So Smart Guy: What pressure will your tire gauge now show?
On a cool morning, when the temperature is 11C , you measure the pressure in your car tires to be 30psi . After driving 30miles on the freeway, the temperature of your tires is 54C .
What pressure will your tire gauge now show?
Answer by Philip J
A perfect gas expands in proportion to the Kelvin temperature.
30 psi x 327 K / 284 K.
However, that assumes that the volume inside the tires remains constant, which is not true. Hotter rubber both expands and stretches more. So the pressure is less than what the perfect gas formula indicates. The amount of this effect varies according to the construction of the tire. Steel belted tires probably expand less. If the cold tire contains condensed water which evaporates on heating, that will further complicate the problem.
So the correct answer is, “Only experimentation with the particular tire can determine the pressure, and it is necessary to control the humidity when filling the tire with air.”
Know better? Leave your own answer in the comments!