Tire Pressure-Altitude v/s Temperature??

[David Brady]

At Ernie Ekberg's request, I'm posting an email discussion
to this forum that we had on the DieselRV's forum. The discussion
we had addressed several questions regarding altitude and
temperature on tire pressures. So I humbly put forth the following:

These questions were asked on the DieselRV's forum:

1) As I go up in elevation, which has more effect on tire pressure, the
altitude or the colder temps?

2) At altitude, will my pressure raise or lower?

3) Higher temps probably mean higher pressures? True??

4) Colder temps, Less pressure. True?

And the reply was as follows:

gauge pressure = psig
absolute pressure = psia

gauge pressure is the difference between the internal
absolute pressure of the tire and the ambient atmospheric
pressure.

If we pump a tire up to 100 psi gauge (psig) at sea level,
the tire actually has 114.7 psi absolute (psia) pressure; i.e.,
100 psig plus 14.7 psia atmosphere.

If we hypothetically ascend to 10000 feet, where the ambient air
pressure is 10.2 psia, then the gauge pressure of the tire will
read higher: 114.7 psia - 10.2 psia = 104.5 psig, read as gauge
pressure.

However, the absolute pressure in the tire has not changed,
it's still 114.7 psia. The amount of air molecules in the tire has
not changed. The tire will hold 114.7 psia even if it were placed
in a vacuum.

If one considers that a tire is not a rigid container, then the tire will
expand under a lesser ambient atmospheric pressure. This will increase
the tire volume, however slightly, with the result that the absolute air
pressure in the tire will decrease to something less then 114.7 psia.
This contribution may be exceedingly small however.

So, the tire pressure will read higher in terms of gauge pressure, psig,
while the absolute pressure, psia, in the tire has actually decreased.

Tire absolute pressure obeys the Ideal Gas Law: PV = nRT. Pressure
and Temperature are in absolute terms, psia and degrees Kelvin.

Tire absolute pressure is linear with respect to changes in T.
Tire absolute pressure is also linear with respect to changes in V.

Tire absolute pressure is proportional to T, an increase in temperature
increases tire psia.
Tire absolute pressure is inversely proportion V, an increase in volume
decreases tire psia.


So to answer the questions:

1) As I go up in elevation, which has more effect on tire pressure, the
altitude or the colder temps?

The answer is: Temperature, however the effects of temperature and
altitude tend to cancel one another out. For example:

Consider our tire which was inflated to 100 psig at sea level,
if we ascend to 10000 feet, where the ambient atmospheric air pressure is
10.2 psia, then the gauge pressure of our tire will read higher: 114.7 psia
minus 10.2 psia = 104.5 psig. A 4.5 psi increase.

The air temperature at higher altitudes will be less than at sea level.
We can
easily have 70 deg F at sea level but 35 degrees F at 10,000 feet. In Kelvin
terms this is a shift from 294 deg K to 275 deg K, or a decrease of 6.4%.
Our tire psia will decrease by 6.4% from 114.7 psia to 107.2 psia. Adjusted
for gauge pressure at sea level, our psig will be 107.2 psia - 14.7 psia =
92.5 psig. A 7.5 psi decrease.

Taken together, our combined 10000 foot altitude and 35 deg F temperature
will result in a gauge pressure of 107.2 psia - 10.2 psia = 97 psig, and our
gauge pressure will range from 97 to 104.5 psig as temperature moves
from 35 deg F to 70 deg F.

So, while temperature prevails as the primary air pressure motivator, in
practice the combined effects of temperature and altitude tend to cancel
one
another out.

2) At altitude, will my pressure raise or lower?

The tire pressure will read higher in terms of gauge pressure, psig,
but the absolute pressure, psia, in the tire will actually decreased,
however slightly.

A tire is not a rigid container; it will expand as atmospheric air pressure
decreases. This will increase the tire volume, and this will cause the
absolute
air pressure in the tire to decrease to something less then 114.7 psia;
however, this contribution can probably be considered negligible.

3) Higher temps probably mean higher pressures? True??

True.

4) Colder temps, Less pressure. True?

True.

David Brady
'02 LXi, Smokey
NC








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