Thank you, David. Very interesting.
Ernie Ekberg
83PT40
Livingston, Mt
************************************** See what's free at
http://www.aol.com.
[Non-text portions of this message have been removed]
Don I'm running the Pressure Pros on mine this year to. On my way home
from FL I started with 100 psi in the front in the morning & they went to 118
psi. The tire temp was 124 degree with my temp gun. The out side temp was
only about 70. I want to see what it does when it gets hot. I didn't notice it
changing in the mountains of VA
Don
89 SP 36'
Butler, PA
run tire pressure monitors that can show me the pressure under way. Not
uncommonly, tires inflated to 100psi at 70 degrees will show running pressures
of
120psi. if that is entirely due to temperature rise, the temps would have to
go up 20%, is that true? That would be from 294 degrees K to 353 degrees K,
or 176 degrees F. Is it possible that the interior of my tires get that hot,
or is something else at work?
The tires on my toad typically increase only 10%, from 33 to 36ps, which
would correspond to running temps of 122 degrees F.
Don Bradner
90 PT40 "Blue Thunder"
Eureka, CA
************************************** See what's free at
http://www.aol.com.
[Non-text portions of this message have been removed]
Pete- hows the wildflowers in the hill country this year? Wish we could have
made the rally in San Antonio
Ernie Ekberg
83PT40
Livingston, Mt
--where we had snow all day- crud!!
************************************** See what's free at
http://www.aol.com.
[Non-text portions of this message have been removed]
In a message dated 4/19/2007 6:30:09 P.M. Mountain Standard Time,
rogerwwebb@... writes:
All this tech talk while sipping cocktails is what makes this forum
the best .. where else can you get this kind of intellectual
stimulation?sti
Roger Webb
Ahh- the plan comes together.
Ernie Ekberg
83PT40
Livingston, Mt
--sometimes Texas
************************************** See what's free at
http://www.aol.com.
[Non-text portions of this message have been removed]
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
[Non-text portions of this message have been removed]
OK, all makes sense, but I wonder about this:
I run tire pressure monitors that can show me the pressure under way. Not
uncommonly, tires inflated to 100psi at 70 degrees will show running pressures
of 120psi. if that is entirely due to temperature rise, the temps would have to
go up 20%, is that true? That would be from 294 degrees K to 353 degrees K, or
176 degrees F. Is it possible that the interior of my tires get that hot, or is
something else at work?
The tires on my toad typically increase only 10%, from 33 to 36ps, which would
correspond to running temps of 122 degrees F.
Don Bradner
90 PT40 "Blue Thunder"
Eureka, CA
The heat generated by the rolling resistance of the tire is the
reason the pressure reading increases. Tires are manufactured with
this in mind -- you should not reduce the pressure of a 'hot' tire as
it will be under inflated and will generate even more heat in operation.
The large tires on a truck or Blue Bird generate considerable heat
when in operation. Perhaps someone who has one of those infrared
thermometers would care to comment.
As for the earlier comments ... I suspect that there was a concern
that the tire pressure should be reduced at altitude -- but the
difference is rarely significant enough to worry about. Remember,
rubber tires are used on airplanes, mostly stored in unpressurized
compartments -- and they don't "blow out" due to changes in altitude
ranging from near sea level to 30,000 to 40,000 feet.
Pete Masterson
'95 Blue Bird Wanderlodge WBDA 42
aeonix1@...
On the road at Johnson City, Texas -- home of LBJ!
On Apr 19, 2007, at 5:55 PM, Don Bradner wrote:
> OK, all makes sense, but I wonder about this:
>
> I run tire pressure monitors that can show me the pressure under
> way. Not uncommonly, tires inflated to 100psi at 70 degrees will
> show running pressures of 120psi. if that is entirely due to
> temperature rise, the temps would have to go up 20%, is that true?
> That would be from 294 degrees K to 353 degrees K, or 176 degrees
> F. Is it possible that the interior of my tires get that hot, or is
> something else at work?
>
> The tires on my toad typically increase only 10%, from 33 to 36ps,
> which would correspond to running temps of 122 degrees F.
>
I don't have a temp gun, but I've heard similar things from others. I can't
reconcile your small (10%) temp rise with the large (18%) pressure rise. That
doesn't agree with Gay-Lussac's law, which says that "The pressure of a fixed
amount of gas at fixed volume is directly proportional to its temperature in
kelvins."
That doesn't even count whatever expansion is allowed to occur by the elasticity
of the tire, which works to keep pressure down.
On 4/19/2007 at 8:26 PM dspithaler@... wrote:
>Don I'm running the Pressure Pros on mine this year to. On my way
>home
>from FL I started with 100 psi in the front in the morning & they went
>to 118
>psi. The tire temp was 124 degree with my temp gun. The out side temp was
>only about 70. I want to see what it does when it gets hot. I didn't
>notice it
>changing in the mountains of VA
>Don
>89 SP 36'
>Butler, PA
>
>run tire pressure monitors that can show me the pressure under way. Not
>uncommonly, tires inflated to 100psi at 70 degrees will show running
>pressures of
>120psi. if that is entirely due to temperature rise, the temps would have
>to
>go up 20%, is that true? That would be from 294 degrees K to 353 degrees
>K,
>or 176 degrees F. Is it possible that the interior of my tires get that
>hot,
>or is something else at work?
>
>The tires on my toad typically increase only 10%, from 33 to 36ps, which
>would correspond to running temps of 122 degrees F.
Don Bradner
90 PT40 "Blue Thunder"
Eureka, CA
All this tech talk while sipping cocktails is what makes this forum
the best .. where else can you get this kind of intellectual
stimulation????
Roger Webb
91WBWL
Cedar Rapids, IA
--- In
WanderlodgeForum@yahoogroups.com, David Brady
wrote:
>
> 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
>
>
>
>
>
>
>
>
> [Non-text portions of this message have been removed]
>
Yes:
I enjoy reading the tech talk and learning how it works. Back in grade school I
pumped my bike tire up to high on a hot day and it exploded. Since then I have
assumed higher ambient temps mean higher pressures and visa versa. Davids post
seems to veryify that.
NASCAR drivers support my experience. They start with lower tire pressures,
drive like hell around the track until the tire pressures come up, then you have
a race. Until that happens, more often than not, a junkyard is established on
the track. Talladega next weekend. Can't wait. LOL
Leroy Eckert
1990WB-40"Smoke N Mirrors"
Niceville, FL
----- Original Message -----
From: rogerwwebb
To:
WanderlodgeForum@yahoogroups.com
Sent: Thursday, April 19, 2007 8:29 PM
Subject: [WanderlodgeForum] Re: Tire Pressure-Altitude v/s Temperature??
All this tech talk while sipping cocktails is what makes this forum
the best .. where else can you get this kind of intellectual
stimulation????
Roger Webb
91WBWL
Cedar Rapids, IA
--- In
WanderlodgeForum@yahoogroups.com, David Brady
wrote:
>
> 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
>
>
>
>
>
>
>
>
> [Non-text portions of this message have been removed]
>
[Non-text portions of this message have been removed]