OAT and Altitude vs. Tire Pressure

June 16, 2011, 11:24:27 pm

Yahoo Message Number: 71906

Hi Country Coach Owners,
Consider this.

The Outside Air Temperature may not be the temperature of the air in the tire after air is added.
Here are a few factors.

The compressor raises the air temperature with its compression cycle as well as the receiver tank as it is filled to a higher pressure.
This is countered with a loss in temperature as the added air expands in the tire.
The true temperature in the tire maybe the same as the OAT, higher or lower.
Altitude is also a factor.

Gauge pressure is relative to the outside pressure as apposed to absolute pressure. As you ascend in altitude the outside pressure goes down but the tire volume is almost constant so the inside pressure goes up.

The pressure equations p1v1=p2v2 and pv=nrt are classic.
An analysis would involve all the above but does it really matter - just look at msg 37201 for using the right pressure based on loading and maybe ignore any altitude and temperature compensation.
As a comparison, buying a climbing rope with a working load of 300 lbs, it is tested to 3000 lbs.
When I come home from a 2500 mi trip, the tire pressure is the same, if it isn't, something is wrong and needs immediate correction on my part.

Fred Kovol

2000 Allure 30593

Re: OAT and Altitude vs. Tire Pressure

Reply #1 – June 16, 2011, 11:34:18 pm

Yahoo Message Number: 71909

I agree with Fred. Though, one could assume that to a first approximation V is constant in a tire at different temperatures and pressures and the key relationship, with n,R constant is P1V1/T1 = P2V2/T2;

but with V constant;

P1/T1 = P2/T2.

To be sure, as Fred says "When I come home from a 2500 mi trip, the tire pressure is the same, if it isn't, something is wrong and needs immediate correction on my part."

Cheers,
Lee
--

Lee Zaborowski

07 Intrigue 12153, Cat C-13

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