Aces High Bulletin Board
General Forums => The O' Club => Topic started by: Creamo on February 06, 2003, 09:24:12 AM
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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?
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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 :D
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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*
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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. (???) :confused:
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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.
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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.
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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. (???) :confused:
Sounds logical to me.
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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)).
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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
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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
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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???
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awww man, Daniel beat me to it. :)
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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
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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.
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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
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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 (http://www.virtual-sub.org/rferdiez/en/dekopro/altitude.html) 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.
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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.
Well, this would depend on the material of the container of the gas.
Scuba tanks contain the same pressure at all depths because the volume in the container doesn't change. In tires, the volume would decrease and the pressure go up.
So in the case of tyres on land, the situation is the same. The volume in the tires increases because of less outside pressure, and the pressure in the tire decreases. The volume of the gas will increase, but the density decrease. This is assuming that there's a difference between inside and outside pressure; otherwise the tyre would expand until the pressure is equal (if there's BO resistance in the tyre compound) or until the pressure exerted by the gas is equal to the outside pressure plus the resistance by the rubber. Or something.
Now I might be talking outta my arse, as I just woke up.
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Well, this would depend on the material of the container of the gas.
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obviously if you use a container that protects the contents from the effects of pressure then there would be no effect. tires are not such a container.
btw- I just remembered something that happened last year that might illustrate this point. I took my kids up on mt Hood. (not sure of the elevation we reached but we where as high as the road can take you) as we neared the end of the road a bag of chips my son had brought opend itself with a loud pop. as outside pressure decreased inside pressure increased ripping the bag open
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Pressure can't increase... the volume of the tyre increases, but the air quantity remains the same. I think pressure will decrease or remain the same, but never increase.
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I think many of you are confusing 'internal pressure' with 'amount of air in container' not at all the same thing.
pressure (psi) is how many pound of force (per square inch) is being put on the container. this is not at all the same thing as the number of air molecules inside the container.
pressure will try to equalize (much the same as water seeking it's own level). the greater the difference between the internal and external pressure of a vessel the greater force trying to escape. you can't change the pressure on one side (inside or outside) without it effecting the gauge pressure
trust me on this- I build boilers and other pressure vessels for a living.
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capt, in a scuba container, the presure is the same, independent of what depth you're on. Of course at 10 metres you'll be in two atmospheres pressure, so the tank will only last ya half as long, since you breathe the gas at ambient pressure. This is because the outside pressure is really negligent and doesn't have to be considered due to the properties of the steel in the tank - totally rigid.
Tank shows 212 bars at surface, and shows (212 bars - consumption) at 40 metres.
But in an exandable rubber thing, it's different. If you reduce the volume of the container to half its original size without removing any pressurized gas in it, you'll double the pressure.
Since a rubber thingy isn't rigid and is affected more by the outside pressure I am lead to believe that at lower outside pressure, the volume of the tyre will increase due to the force exerted by the inside gas, and therefore the pressure will be lower.
Take a scuba tank with 2400 litres of compressed air. Put it in a 2.4m^3 closed container and open the valve.You'll have the same amount of gas, but at a different pressure (1 atm), because of the increase in the volume of the container.
Bolye's Law states:
P1V1 = P2V2
where the variables with the 1 subscript mean initial values before the manipulation and the variables with the 2 subscript mean final values after the manipulation. P is pressure, V is volume.
So if we assume that the tire has a volume of say (just for easy math) 10 litres. The pressure at the start Creamo said is 200 psi. We can discount other stuff like temperature etc for this, so Boyle's simple law applies. I believe this was Creamo's intention - disregard temperature etc and just keep it basic.
200 psi = 200/14.7 atmospheres, or 13.6 atmospheres.
That means P1V1 = 13.6*10
We wanna know p2. Let's assume that the tyre expands by 1 litre to keep it simple.
13.6*10 = x * 11 which is simply converted to
(13.6*10)/11 which is rougly 12.4 atmospheres. Converting it back to psi gives us 12.4*14.7 = 181psi.
Maybe I am talking outta my arse. Was a LONG time ago I had physics in school.
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That's the question, the pressure will try to equalize... so if the tyre can expand enough, at the end internal and external pressures will be the same, no? (I'm talking about a theoretic (sp?) tyre, able to expand without limit). If the tyre is more or less rigid (wich is the case we are talking about, a real tyre) then pressure will increase... So the end internal pressure will depend on the tyre ability to expand under pressure. Did I understood it right now? Or is pressure, in this specific case, independent of volume changes?
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in this case pressure is independent of volume.
the tire will only expand to what the inside pressure foreces it to in relation to the outside pressure..
the inside pressure stays the same assuiming temperature is the same.
btw Capt apathy I know what you mean. I regularly travel between Snowmass and Denver ( aprox 1 mile alt change) and I've learned through experience that when I leave Denver to come home, I squezze extra air out of flexible bottles.. like sunburn cream or bottles of water.. if I don't, when I get home and open them they squirt their contents out all over.
naturally the inverese happens without leakage.
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Mmmmm what happens inside a sub, when it goes to say 500 meters under the sea surface... pressure remains the same, correct? So internal pressure is independent of external pressure... Is Santa right, then?
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in the case of the sub the outer hull flexes but the inner hull is rigid, meaning the inside pressure doesn't change when they dive.
if the vehicle were flexible, say like a basketball, then they'd have to increase inner pressure to keep the 'hull' from colapsing.
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the sub is a rigid form. protecting the inside atmosphere (and crew/ contents) from the effects of pressure. but pressure is still being inflicted on the structure. if you had a gauge outside the sub that read the internal pressure. it would show the internal pressure decreasing as the sub went deeper. just as the gauges on the inside would show the outside pressure increasing.
psi is not a measurement of 'how much is in here'. it's a measurement of 'whats the difference between inside and outside'
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in this case pressure is independent of volume.
the tire will only expand to what the inside pressure foreces it to in relation to the outside pressure..
the inside pressure stays the same assuiming temperature is the same.
Ok, following the pV/T=nR formula, if the gas mass don't change, pressure is volume and temperature dependant. So if the tyre is flexible enough to expand a little due to the increased internal/external pressure gradient, increasing the internal volume, the gas pressure inside the tyre will decrease, correct? If the tyre volume remains the same, pressure will not change...
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cap't, if the structure is truly rigid, there is no change.
the rigid hull takes up all the pressure diff, and not the inside air.
only if the volume changes (or temp) will there be a pressure change. IMHO that is
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correct, there will be no actuall change in pressure inside the sub. so you wouldn't feel any pressure changes if you where in there. or if you had another container in the sub with you it's gauge readings would be uneffected.
but the pressure differential between the sub and the ocean would change. and would show up on an external guage that was reading the subs internal pressure.
so back to the tire.
if you had some sort of internal presure sensitive gauge inside the tire (that read the pressure inflicted on the guage) then as your altitude increased the reading on this gauge would show a drop in internal pressure(as much as tire expansion would allow)
but we messure tire pressure with gauges that are in the outside atmosphere and read the difference between inside and outside, this tells us the amount of stress inflicted on the inside of the tire. (this stress is the relevant info your looking for when checking tire pressure anyway, not how much air is in there). this kind of guage will show an increase in internal pressure.
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A common pressure gauge measures pressure vs weight (gravity), so I bet it would read the same. However as pointed out, pressure outside at more altitude is less, meaning that the tyre could appear more inflated. Well, they're so stiff inflated anyway, always rock hard...
Now, with an unchanged inner pressure, outside pressure could be decreased (with alt) untill the tyre will explode. Of course, aircraft tyres are quite strong and would not explode, even in space, but imagine what happens to all them weather ballons..
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Yikes.
see: altitude and tire pressure (http://members.aol.com/carleyware/library/tirepres.htm)
Altitude will also affect tire pressure. For every 1,000 feet in elevation above sea level, atmospheric pressure decreases about a half a pound. As a result, tire pressure goes up an equal amount.
My answer is correct.
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Ask again using scientifc units please, then you'll get an answer, without "i think", "i guess", "i might be wrong", "maybe", "probably", "if i rember correctly"...
:p
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Myelo is right.
The absolute pressure (psi) inside the tire does not change.
But the gauge pressure (absolute pressure minus pressure of outside air, psig) will increase as you go higher.
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Does the planes weight have an affect on it as well?
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no the same weight was on them when you filled them at the lower alt. however if you could somehow check it you would have lower pressure off the ground than you would on the rubber (assuming alt and all other variables where the same)
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Too many bad math class flashbacks in this thread. Thanks for the nightmares I'm going to have tonight :(
Charon