Post: #1

I was on a flight recently. I had an empty plastic water bottle. Before the airplane descended, I sealed the water bottle. I then observed it develop a vacuum, where it began to progressively shrink as we got closer to landing.

After landing, I opened the bottle slowly and heard air rush into the bottle, suggesting that the seal was air tight. Why did this vacuum develop?

Post: #2

? Seems like your science knowledge has been eradicated by too much sex with hot women.

You sealed air with low pressure in the bottle, as the plain descended the atmospheric pressure outside the bottle rose to normal levels again and compressed the bottle to match the inside pressure with the outside pressure.

p_high_altitude_is_low * V_normal = p_low_altitude_is_high_ * V_smaller

http://en.wikipedia.org/wiki/Air_pressur..._variation

Post: #3

(10-13-2013 07:09 AM)Roosh Wrote:  I was on a flight recently. I had an empty plastic water bottle. Before the airplane descended, I sealed the water bottle. I then observed it develop a vacuum, where it began to progressively shrink as we got closer to landing.

After landing, I opened the bottle slowly and heard air rush into the bottle, suggesting that the seal was air tight. Why did this vacuum develop?

As the aircraft climbs in altitude after take-off, the decreasing cabin air pressure causes gases to expand. Similarly, as the aircraft descends in altitude before landing, the increasing pressure in the cabin causes gases to contract. -- This is with reference to the air pressure within the cabin.

Gases occupy space like any other matter.
When you sealed the bottle, you isolated that system from the surrounding air pressure. As the plane descended, the air inside the bottle contracted. Air occupies space like any other matter. When it contracted, suddenly a part of the bottle's space was empty, thereby creating a vacuum. The Higher pressure outside pushed the walls of the bottle -- and since the air pressure inside was lower, the walls of the bottle got pushed inside.

Air flows from high pressure (or temperature or height) to low pressure. At this point, there was lower air pressure in the bottle -- vacuum is actually considered to have negative atmospheric pressure-- so when you opened the seal, the relatively high pressure outside then caused the gases in the surroundings to flow into the lower pressure area of the bottle --- till the pressure in the bottle and surrounding was at equilibrium. As the gases flew in, they occupied space thereby making the bottle return to somewhat its original shape.

Post: #4

There's more air pressure on the ground than at 10,000 ft above. The volume of air in the bottle when you closed it was lower than if you did it on the ground. Same reason it's harder to breathe when you're at the top of a mountain. There's less physical air per cubic foot of space.

As the plane descends the relative air pressure around you squeezes the bottle because of the difference in air volume inside the bottle and outside until the air pressure is equal on both sides.

The airplane is supposed to regulate the cabin pressure so it feels like you're on the ground but it's never acurate. That's also why your ears pop during take off and landing.

Post: #5

I understand now, but I thought that airplanes maintain constant air pressure in their cabins.

Quote: Seems like your science knowledge has been eradicated by too much sex

Yeah it really bothered me that I didn't know the answer. I had to find out.

Post: #6

The gas pressure at 20,000 feet is only half of what it is at sea level. So you can imagine what it would be at 30k

That means less air pressure on your bottle of water at that height. As you descend the bottle was subjected to more air pressure than previous so that's why it shrank.

Same effect would be seen if you closed a bottle at sea level and dove 1,000 feet.

Post: #7

(10-13-2013 07:59 AM)Roosh Wrote:  I understand now, but I thought that airplanes maintain constant air pressure in their cabins.

Quote: Seems like your science knowledge has been eradicated by too much sex

Yeah it really bothered me that I didn't know the answer. I had to find out.

Have a squiz through the Feynmann lectures, you'll feel 166x smarter

Post: #8

Airplanes do maintian constant pressure but when descending it takes time to adjust the pressure in the cabin so that's why your ears pop and the bottle deflates.

You go from 3 inches of atmospheric pressure at 30,000 to 23 inches at 9k feet. Such an increase at only 20k feet.

And when your plane has lost it's function to regulate air pressure inside? It will drop out of the sky and at that speed you will pass out.

Post: #9

Does anybody get a headache when the plane is descending? I get terrible headaches on about 1/5 flights during descent. I think it is because the change in pressure varies on different flights, and also part of the reason may be my sinus congestion problems. I tried Sudafed before take off last time I flew, but it didn't help much.

Post: #10

^^^^again it's due to air pressure.

Your sinus cavalties are just empty spaces in your head. Think of a balloon. If the plan descends to fast then your cavaty won't have time to adjust. That leads to irritation. On top of that airplane do not all fly at the same altitude, speed, etc. Next time you fly try to find out what aircraft you flew in. Make a log and see if there is a correlation.

I don't think Sudafed will help since there is no biological function that's messing with your nasal membranes. More of a physical force.

Post: #11

The airlines are saving money by putting less and less air into the planes at altitude. You aren't getting sea level pressure in the plane.

Post: #12

(10-13-2013 07:59 AM)Roosh Wrote:  I understand now, but I thought that airplanes maintain constant air pressure in their cabins.

They maintain a set minimum of air pressure in the cabins. Otherwise it is allowed to fluctuate.

Post: #13

As other people mentioned it's due to the air pressure, and pressurized aircraft generally don't pressurize up to sea level pressure.

The rule for unpressureized aircraft is 30 mins between 10k and 14k ft, don't go above 14k ft.

Because it's still safe at that altitude, (10k ft = ~10psi, as I recal they pressurize up to a little below 10k ft equivilent) the benifit of having higher air pressure in the cabin is so marginal, that it doesn't make sense given the extra strain on the aircraft. Just to give you an idea, that 4psi difference in pressure would equate to ball park aircraft profile 15 ft * 12 in high x 100 ft * 12 in long = 216,000 sq in. * 4psi = an additional 432 tons of force trying to rip the airplane appart. It's literally like an aeresol can.

On the other end, sometimes maybe you notice bags of nuts or chips are over pressureized when they give them to you. Opposite thing happening there. They're packaged at sea level, then the gas expands but has no where to go.

If you're particualrly keen, you could take baloons on your next flight to illustrate the point very nicely.

Post: #14

(10-13-2013 10:13 AM)CThunder86 Wrote:  ^^^^again it's due to air pressure.

Your sinus cavalties are just empty spaces in your head. Think of a balloon. If the plan descends to fast then your cavaty won't have time to adjust. That leads to irritation. On top of that airplane do not all fly at the same altitude, speed, etc. Next time you fly try to find out what aircraft you flew in. Make a log and see if there is a correlation.

I don't think Sudafed will help since there is no biological function that's messing with your nasal membranes. More of a physical force.

I asked a number of people, and they don't seem to get headaches on flights or react to pressure change, but I couldn't even dive more than a few feet - got a horrible headache and had to come back up. I'm not sure why this is happening. Talked to a doctor a few years ago, he said it was probably congestion, but that didn't help me much.

Post: #15

I was scuba diving once and took a bag of chips with me down to about 30 meters, it became hard as a rock, pretty cool.

Post: #16

Boyles law applies to sinus cavalties too.

Brodiga: Sounds like your cavaties aren't adjusting when diving maybe due to a mucus buildup?
Do you find yourself blowing your nose a lot after diving?

Post: #17

(10-14-2013 09:29 PM)CThunder86 Wrote:  Boyles law applies to sinus cavalties too.

Brodiga: Sounds like your cavaties aren't adjusting when diving maybe due to a mucus buildup?
Do you find yourself blowing your nose a lot after diving?

I only tried diving once and stopped because of the headache. It was a few years ago, can't remember if I had to blow my nose.

Post: #18

(10-13-2013 07:59 AM)Roosh Wrote:  I understand now, but I thought that airplanes maintain constant air pressure in their cabins.

They pressurize the plane but it is not sea-level pressure. From what I understand, the pressure is equivalent to about 10,000ft when you are cruise altitude(no need to pressurize to sea-level, it's unnecessary). So when you sealed the bottle, it was like sealing it at the top of a mountain and then driving down to sea-level and opening it.

Post: #19

(10-13-2013 08:14 AM)CThunder86 Wrote:  And when your plane has lost it's function to regulate air pressure inside? It will drop out of the sky and at that speed you will pass out.

It won't drop out of the sky if it's on autopilot, but you will pass out and die.

Google Payne Stewart once.