# Pressure at altitude

1. Aug 4, 2009

### seilerbird

If you inflate an RV tire to 100 psi at sea level and then drive to 14,000 feet, discounting temperature, will the tire pressure be the same, lower or higher?

2. Aug 4, 2009

### mgb_phys

It depends if this is a real tire or a 'homework' tire
In theory the absolute pressure doesn't depend on the altitude. If you put 100psi inside a container, that is 115psi absolute pressure inside and 15psi outside. The 100psi is called gauge pressure because it is what you would read on a gauge.

however in a real tire at altitude the outside pressure will be slightly lower and so there will be 115psi inside and perhaps 10psi outside. Although th epressure inside the tire hasn't changed a pressure gauge would read 105 (115-10) psi.
But in a real tire this change in the pressure difference would cause the tire to expand slightly (it's made of rubber) and the pressure inside would drop to slightly less than 100psi over the outside pressure, ie just less than 110psi absolute,.

Last edited: Aug 4, 2009
3. Aug 4, 2009

### seilerbird

Thanks for the reply. This is a real tire I am asking about, not a homework tire.

4. Aug 4, 2009

### seilerbird

But wait, if the tire would expand wouldn't that be because the volume increases inside the tire causing the pressure inside the tire to decrease?

5. Aug 4, 2009

### mgb_phys

Sorry, typo - should have said "and the pressure inside would drop to slightly less than 100psi over outside"

6. Aug 4, 2009

### rcgldr

Tires that run at 100 psi are pretty stiff and don't expand much. The absolute pressure would decrease slightly below 100 psi, but the relative pressure to ambient at 14,000 feet would be well above 100 psi, and there would be more strain on the tire.

7. Aug 4, 2009

### mgb_phys

Yes - wouldn't be much effect in the real world.

There is one telescope on Mauna Kea who insists on venting all the LN2 tanks on the basis that the safety valve is set for sea level pressure not 14,500ft. Don't know where their safety guy studied physics but I think it was a school that advertises on matchboxes.

8. Aug 4, 2009

### seilerbird

Yes, I realize there would not be much effect in the real world. My question was based on a discussion on an RV forum. Some people were claiming there would be absolutely no change in the pressure of the tire since it is a closed system. I argued that the outside pressure would effect the internal pressure of the tire since rubber is plyable. I wanted to hear some opinions from some physics experts to see if the tire pressure would change or remain constant. The fact that the change would be small does not matter, only that the change would be there.

9. Aug 4, 2009

### mgb_phys

Then yes the tire absolute pressure will slightly decrease while the gauge pressure will read the same.