Figure: Early Altimeters, from "Evolution of the Modern Altimeter".

Eddie Sez:

There is a lot of magic going on in a modern cockpit. Some of that magic isn't very modern at all, but it works great:

What follows comes from the references shown below. Where I think it helpful, I've added my own comments in blue.


Evolution of the Modern Altimeter

["Evolution of the Modern Altimeter"]


How does it work?

Figure Sensitive Altimeter Components, from Instrument Flying Handbook, Figure 3-3.

[Instrument Flying Handbook, pg. 3-3] The sensitive element in a sensitive altimeter is a stack of evacuated, corrugated bronze aneroid capsules like those shown in figure 3-3. The air pressure acting on these aneroids tries to compress them against their natural springiness, which tries to expand them. The result is that their thickness changes as the air pressure changes. Stacking several aneroids increases the dimension change as the pressure varies over the usable range of the instrument.

Have you ever wondered why a bag of potato chips is filled with air? Well chances are the "air" is nitrogen to keep them fresh and the bag appears to be mostly plump to keep the chips from being crushed when the bag is handled. The nitrogen is under a little pressure so the bag stays inflated. But I digress . . . Take that bag of chips with you on a flight and notice that as you climb the cabin altitude also climbs. As that happens the cabin pressure decreases. But even as the pressure around the bag decreases the pressure inside the bag stays the same. That means the bag appears to get even plumper. So what does this have to do with altimeters?

The aneroid inside the altimeter is composed of a stack of sealed chambers, just like that bag of chips. As the pressure around the aneroid decreases, the aneroid itself expands, moving the gears and levers that eventually move the needle on your altimeter. Flying a glass cockpit? Well it is the same principle, but the gears and levers are connected to electronic gizmos that feed the computers with the same information.


References

du Feu, A. N., "Evolution of the Modern Altimeter", Flight International, 26 Dec 1968, pg. 1066.

FAA-H-8083-15, Instrument Flying Handbook, U.S. Department of Transportation, Flight Standards Service, 2001.