Author Topic: Quiz for pilot and college edu-macated types… (Read 809 times)

StSanta · « **Reply #15 on:** February 06, 2003, 11:17:09 AM »

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.

capt. apathy · « **Reply #16 on:** February 06, 2003, 11:24:25 AM »

Well, this would depend on the material of the container of the gas.
_______
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

DA98 · « **Reply #17 on:** February 06, 2003, 12:01:21 PM »

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.

capt. apathy · « **Reply #18 on:** February 06, 2003, 12:22:31 PM »

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.

StSanta · « **Reply #19 on:** February 06, 2003, 12:43:45 PM »

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.

DA98 · « **Reply #20 on:** February 06, 2003, 12:49:27 PM »

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?

Wlfgng · « **Reply #21 on:** February 06, 2003, 12:53:48 PM »

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.

DA98 · « **Reply #22 on:** February 06, 2003, 12:54:47 PM »

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?

Wlfgng · « **Reply #23 on:** February 06, 2003, 12:58:30 PM »

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.

capt. apathy · « **Reply #24 on:** February 06, 2003, 01:02:55 PM »

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'

DA98 · « **Reply #25 on:** February 06, 2003, 01:04:05 PM »

Quote

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...

Wlfgng · « **Reply #26 on:** February 06, 2003, 01:06:46 PM »

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

capt. apathy · « **Reply #27 on:** February 06, 2003, 01:16:34 PM »

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.

Angus · « **Reply #28 on:** February 06, 2003, 01:22:56 PM »

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..

myelo · « **Reply #29 on:** February 06, 2003, 01:39:55 PM »

Yikes.

see: altitude and tire pressure

Quote

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.