Quiz for pilot and college edu-macated types…

[Creamo]

Im too lazy to look this up. Can someone that scored 130+ in the IQ test tell me/explain this answer.

A plane leaves Chicago with tire pressure of 200psi, and its say 70 degrees F there. It then lands in Denver at a much higher altitude, and it’s still 70 degrees F there. Forget the brakes could heat up the tire. Same temp, but different, higher altitude.

Would the tire pressure psi go up, or down, and by how much relatively?

[Modas]

Reaching WAY back to my chemistry class..

PV =nRT?

where P = pressure
where V = volume
where n = ??
Where R = gas constant?
where T = temperature

If I remember correctly assuming temp remains the same which you stated, the only way the pressure could change would be the volume would have to change.  Since we know the volume of the a/c tire doesn't change, the tire pressure is still 200 psi regardless of the altitude.

Course, I have been known to make mistakes from time to time

[nuchpatrick]

I could totally be wrong here but the tire pressure never changed and is still 200 psi.

A question for you is the actuall tire temp is 70* or the ambient air temp is 70*

[Saurdaukar]

I dont think the actual PSI would change at all - but the tire itself would expand because there is less atmospheric pressure on the tire at that altitude... I think.  (???)  

[capt. apathy]

you should have asked for info from divers (Boyles Law you know).

yes the pressure would increase.  not sure how much. (if you where taking it below the surface of water I could tell you how much the pressure would decrease) as I don't have a formula for that direction.

your gauge doesn't read the amount of air in your tire (that shouldn't change). it reads the difference between pressure inside the tire and the pressure outside the tire.  as altitude increases, air pressure outside decreases. making the difference in pressure greater.

[Creamo]

I honestly think they should show higher as well, but can’t explain it. The question I hoped would show how much. The consensus here though, so far says they will be the same, so I just don’t know.

Maybe dripsnore will run out of images to link in the offtopic and run some macro’s and do some multitasking on it before he gets his management incentive check.

The FAA is hard on this, and I don’t have a viable answer. I’m trying to be prepared.

[AKIron]

Originally posted by Saurdaukar
I dont think the actual PSI would change at all - but the tire itself would expand because there is less atmospheric pressure on the tire at that altitude... I think.  (???)  

Sounds logical to me.

[Preon1]

The pressure in the tire remains the same.  The only difference would be the pressure gradient between that in the tire and that of the atmosphere.  That's why in Denver the tires may look like they have more air in them (there's actually more total outward force on the rubber (or to be more accurate, less force inward)).

[capt. apathy]

if you wanted to calculate the difference you could probably come up with it.

you'd have to know how many feet in our atmosphere, and how many feet difference between your 2 locations. then calculate the percentage.  whatever the percentage of our atmosphere the difference is should be the percentage your tire pressure increases.

can't be exactly sure though.  as I said I'm used to working with pressure going the other way so this is kinda like working in negative numbers which can throw things off

[CyranoAH]

The pressure would be the same, since the temperature does not vary.

Having said that, the tires would appear more inflated, since the outside air does not "push" the tire as much as before, as the air inside has, comparatively, but not absolutely, more pressure.

Daniel

[midnight Target]

Well consider this..

If you are at a higher altitude and the rest stays the same, it might be possible for the volume of the tires to change due to the reduced external air pressure.

THEN WHAT???

[midnight Target]

awww man, Daniel beat me to it.

[capt. apathy]

The pressure in the tire remains the same. /
there's actually more total outward force on the rubber (or to be more accurate, less force inward
____________________
the tire pressure cant remain the same and have the force increase, as 'tire pressure' is a measurement of that force.

the amount of air in the tire would be the same.


as an example-  if you took a tire with 200psi (at sea level) down to 33' in water you would add 1 atmosphere of pressure (or double the existing pressure)  your gauge would then read 100 psi.

try taking a basketball down 5 or 10 feet in a pool.  it will look very flat, won't have enough pressure even to keep it round. but the amount of air inside never changes.

[Wlfgng]

I think the gauge would still read 200 psi...
twice the outside force on the ball...
same inside force...

it takes twice as much air to fill the same volume but that'd be a different scenario

[myelo]

As someone said, a tire pressure of 200 psi means 200 psi relative to the atmosphere. Because the atmospheric pressure is less at higher altitude, the reading on the gauge will increase provided everything else is constant.

Chicago is about 200 m above sea level, which according to this chart has an atmospheric pressure of 14.36 psi (743 tor).  

Denver is about 1600 m above sea level, corresponding to an atmospheric pressure of 12.15 psi (628 tor).

So, rounding off: Xpsi + 12 psi (Denver) = 200psi + 14psi (Chicago)

Solving for X, the new gauge reading is 202 psi.