TPMS - Tire Pressure Increase

CapriRacer wrote:

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.

the tire pressure on my denali truck doing the towing saw a slightly lower but more than 10% increase in pressure. rears started at 70 front and 65 rear and rose to about 82-85 rear and 75 front. This is typical rise I see towing or not.

And similar % increase on our Acura MDX hybrid.

And with constant volume boyles law indicates pressure increase with temperature. PV=nRT. V stays constant, T goes up therefore P must also go up. But remember T is in Kelvin, so "room temp" is about 275K, and 100F is about 300K, or if V changes with constant temperature P must also change.

Now ST tires have a pretty stiff sidewall, not sure how it compares to my LT LRE truck tires, but the stiff sidewall likely results in an increase in rolling friction compared to a non ST tire= higher temps= higher pressure % increase

Now, it's clear the tpms temperature is not a good indicator of what the actual temperature of the tire air temp, it's definitely higher.


On another note, temp outside today has been pretty constant at 75F. We are back from a trip and cleaning out the trailer and I checked tire pressure getting it ready to put away. Adjusted them all to 65psi this morning This afternoon, the tires on the sunny side the pressure had risen to 70, tire sidewall is definitely very warm to the touch. Cool side of the trailer is still at 65/66.

Which is why I check tire pressure either after the vehicle has set in the garage overnight, or if outside only after the tires have cooled and been in the shade for plenty of time.