Posted by Indiana Ken on January 06, 2013 at 17:39:13 from (66.249.232.131):
In Reply to: Tire pressure posted by 37Chief on January 05, 2013 at 12:06:00:
For dry air, using the Combined Gas Law for a constant volume (assumes the volume of the tires do not change) we have: Initial pressure x Final temperature / Initial temperature = Final pressure. Where the pressure is absolute, psia (add 14.7) and temperature is absolute, degrees K (convert to centigrade and add 273). You did not state the two temperature points so we will use 80 psi at 70 degrees F and compute the pressure at 32 degrees F.
For example:
Initial pressure is; 80psi + 14.7 = 94.7 psia.
Initial temperature is; 70 F = 21.1 C + 273 = 294.1 K
Final temperture is; 32 F = 0 C + 273 = 273 K
We have:
94.7 psia x 273 K / 294.1 K = Final pressure = 87.9 psia and subtracting 14.7 psi results in 73.2 psi. The 73.2 psi is what is read on the tire pressure gage at 32 degrees F.
In conclusion:
Yes, depending on the two temperature points the tire pressure can vary significantly. A rule of thumb is 1 psi per 10 degrees as other have posted. This rule may be more accurate in real world situations, I do not know - it is certainly easier to remember. Remember, it is the temperature of the air in the tire not ambient air temperature - if the sun is shining on the tire it can be much hotter than the outside air temperature. Maybe your gage is ok after all - hope you have not thrown it away.
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