Posted by Indiana Ken on January 06, 2013 at 17:39:13 from (188.8.131.52):
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.
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
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.
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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