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Wednesday, September 16, 2015

How to crush something without touching it

What if I told you I can crush a water bottle without touching it? You probably wouldn't believe me, would you? Of course you wouldn't. How can something get crushed without having something doing the crushing?

But I can. I have proof:

Crushed it!
I did not touch this water bottle for at least 30 minutes before I took this picture, and when I put it in that pouch, it was a normal-looking, quite round bottle of water.

So what happened? And how did I crush it without touching it? Well, to be technically correct, I personally didn't crush it; I just set up the conditions for something else to do the actual crushing.)

The thing that did the actual squishing was atmospheric pressure: that invisible ocean of atmosphere that surrounds us. You can't see it, you can't hold it in your hand, and you don't even notice that it is pushing down on you with the weight of an entire bowling ball on every square inch of your body. That thing.

You don't usually notice it at all, since our bodies basically push out against it with the same 14.7 pounds per square inch (psi) of pressure that it pushes in on us with. We're usually in equilibrium with it, so we take no notice of it. Kind of like in-laws who live on the other side of the country: they're there, but we don't notice them.

If the bottle was perfectly fine half an hour ago, why isn't it fine in this picture? Because of something we did in the time in between: we descended from our cruising altitude and landed.

When we were up at 24,000 feet, the pressure inside the cabin was roughly the same as it would be if we were only at 8,000 feet. Think of climbing a few thousand feet up a mountain outside Denver: that's about how thin the air inside the cabin would have been.

By the time we landed, the pressure inside the cabin was a mere 313 feet above sea level, which is the elevation of Washington-Dulles International Airport. So in the meantime, the effective change in air pressure was the same as climbing 7,700 feet down a mountain. There's a lot more air down here. That means a lot more air to push on the bottle.

At 8,000 feet, the atmosphere was pushing on the bottle with a weight of 10.9 pounds per square inch. Because of that, when I drank the last of the water and put the cap back on, the air pressure inside the bottle was 10.9 psi, the bottle was perfectly round, and everyone was happy.

However, by the time we landed, the air pressure was up to 14.6 psi. More air = more pressure on the bottle. However, since I didn't touch the bottle, the air pressure inside it was still only 10.9 psi. Since there was more pressure outside than inside, the bottle collapsed. By opening the cap and letting air in, the bottle would "inflate" back to its round shape, since the pressure inside the bottle and outside would both be 14.6 psi.

If things get crushed as you descend, why wouldn't the bottle start bulging as we're climbing up? After all, the pressure inside the bottle would be 14.7 psi if it were filled at sea level, and back to 10.9 at 8,000 feet. It should do the opposite in the climb as it did in the descent, right?

Right. It does just that, actually. However, the bottle tends to be filled with water and not air on the way up, making less room for the more compressible air. Also, the bottle stretching outward is much less outwardly visible than its collapsing inward. Nonetheless, it is bulging. It's a lot easier to see in the seal on this Pringles can, which was bulging in cruise just before I opened it:

That seal is just bursting with flavor at a cabin altitude of about 6,000 feet.
This expansion and contraction cycle is why your ears pop twice during a flight. On the way up, the air on the inside of your eardrum is trying to push it outward. On the way back down, the air on the outside of it pushes it inward.

Imagine if we could hook up some gears and a meter to the outside of the water bottle. The gears could drive some sort of indicator that shows us how much air pressure is around us based on how much the bottle bulges or gets crushed. That would be a neat way to indirectly measure how high we are.

Don't rush to the patent office just yet, as someone already came up with this idea a long time ago. (Long before airplanes, even. Mark Twain mentions a device using a similar concept in his book A Tramp Abroad, which was written way back in 1880, and the idea wasn't new even then.) There already is something in the airplane that uses this expansion and contraction of trapped air inside a container and puts it to good use. A very vital thing, actually. It's called the altimeter.

Figure 7-2 from the Pilot's Handbook of Aeronautical Knowledge. You can download the entire book for free from the FAA's website. Your tax dollars at work.
That thing labeled "Aneroid wafers" that looks like a stack of discs or an accordion's bellows operates on the same general principle as the water bottle. As the wafers bulge outward, they're connected to some gears that are connected to the dial on the altimeter's face and make the hands indicate the altitude winding upward. As they shrink, the hands start going in the opposite direction. If the aneroid wafers are not bulging or shrinking, you're in level flight.

In the altimeter's case, it's connected to an air pressure source outside the airplane (the static port) so it isn't measuring the air pressure inside the cabin like the water bottle was, but the principle is exactly the same.

When you're learning to fly, everything in aviation seems to run off of two sources of energy: money and magic. While it has always been the case that flying isn't cheap, I think it's an excellent investment because of the magic it brings along with it. Your instruments, however, don't run on magic. They actually run on simple principles of physics that are harnessed in a way that sometimes looks like magic. And it can turn you into a magician who is able to crush things just by looking at them.

See you next Wednesday!

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The author is an airline pilot, flight instructor, and adjunct college professor teaching aviation ground schools. He holds an ATP certificate with a DHC-8 type rating, as well as CFI, CFII, MEI, AGI, and IGI certificates, and is a FAASafety Team representative and Master-level participant in the FAA's WINGS program. He is on Facebook as Larry the Flying Guy, has a Larry the Flying Guy YouTube channel, and is on Twitter as @Lairspeed.

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