TPMS - Tire Pressure Increase

ktmrfs wrote:
This week on our trip I took a look at tire pressure/temp. In the morning before starting one tire 63psi, 55F. After about 30 minutes of driving and outside temps up to 65F, that tire was at 78psi, and 90F. Other tires basically same pattern. Now the "sunny side" of the trailer had tire temps noticeably hotter, 10F or so. and higher pressure as well likely from heat absorption from the sun hitting the black tire.

Now if outside temps hit 90F, I would expect a slight increase in pressure, most of the pressure increase is due to friction induced heat.

Now, the other unkown is how accurate is the temp reading for a TPMS?? Is it measuring rim temp? air temp? or some combo.

And then factor in how dry the air in the tire is. If it is dry, Boyles law works nice. if there is moisture (water) especially if it condenses, pressure will bump up noticeably once it become a vapor. which is a strong function of temperature.

First, water: Unless you have a pool of water in the tire, you can treat ALL the gases in the tire according to the Ideal Gas Law:
PV=nRT (Note Boyle's Law is about the pressure-volume relationship and does not include temperature as part of the law. Unless the tire is expanding, you shouldn't be using Boyle's Law.)

For practical purposes you will hardly ever see liquid water inside a tire. Not only does water evaporate, but the rubber is permeable to water vapor - and the partial pressure equalizes to the outside air. It takes time for gases to migrate in and out of a tire, so even when it is raining, the amount of water vapor in the tire only SLOWLY rises. Meaning you can neglect the effect water might have on the equation.

The rule of thumb is that tires don't like more than a 10% pressure buildup excluding ambient temperature effects (2% for every 10°F).

Doing the math. the ambient temperature ride from 55°F to 65°F results in a 1.3 psi in a 63 psi tire. So deducting the ambient temperature effect, you have a pressure build up from 63 psi to 77 psi = +22%. Not good.

Best guess is that the temperature sensor is reading the temperature where it is located - not the average temp of the pressurized air, nor the ambient air - but you should check with the TPMS manufacturer to be sure.