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How an Altimeter Works (gif)


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This is probably common sense for most people on here, but for whatever reason this picture was really helpful in understanding how an altimeter works. I know most of us do a lot of studying and possibly overlook the small details as to the mechanics of why and how things work.


This "wafer" expands and contracts depending on the outside ambient pressure, which in turn causes the gears to move turning the dials. Think of it like a balloon (or weather balloon) as it gets higher the particles of air within the balloon are still at the same pressure, whereas the surrounding particles are at a much lower pressure; causing the outside of the balloon to stretch out and expand.





Consequently, if you immediately take all of the ambient air pressure out of a container, the surrounding atmospheric pressure will crush it:
























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Cool info there. Never actually seen a detailed picture. Just what my instructor has drawn on the board in the form of chicken scratch.


My thoughts exactly haha.


Here is a video showing how sensitive the gearing really is. Just looking at a diagram it seems like the gearing could spin the dials half a turn before running out of teeth.


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The picture with the tanker in it, what pressure did the tank have inside of it? Higher or lower than the ambient pressure? Your wording is confusing.


Forget I used the word ambient, as that is probably somewhat misleading.


The atmosphere is constantly exerting a pressure on us at all times. This is measured in many ways, with inches of mercury being the standard and the one that is most common to pilots. On a standard day the pressure of the atmosphere is 29.92" hg at sea level, which equates to 14.7 lbs/in^2.


The train car video is just an exaggerated example of what happens when you drink from a plastic water bottle. If you start drinking the water bottle with your lips sealed around the top, you begin to lower the pressure inside the bottle causing the outside pressure of ~14.7 lbs/in^2 to overcome the integrity of the pastic and begin to crush it. If you do some drinking and then crack the side of your lips, you'll hear air rushing in and the bottle returning to its normal shape (equalizing).


I did a little reading about the train video and there is some debate as to how it was done. The most common answer I found was that they pumped hot steam into it and as it cooled, returning to its liquid state, the pressure inside the container dropped. Since the container is completely sealed (with no release valves) there is a very high pressure outside (the atmosphere) and a very low pressure inside the container. Since the container is built of metal the structure retained its shape long enough until the weight of the atmosphere simply proved to be too much and crushed it.


This video is somewhat different, but still cool to see the power of pressure differentials:




And finally a quick look at a picture to help visualize the particles in a container as the volume increases and decreases under weight.



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