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Altimeters work via an aneroid barometer to sense the atmospheric pressure. If I climb from 2000ft to 4000ft, the altimeter senses that the pressure has reduced by, let's say, 70hpa, and is calibrated to know that 70hpa = 2000ft.

But the pressure gradient (or rate of change of pressure) reduces with altitude. A climb from FL240 to FL260 might result in a pressure reduction of only 30hpa. Yet it still knows that the aircraft climbed 2000ft. How does the altimeter do this?

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    $\begingroup$ As long as everyone’s altimeter uses the same 'pressure to reading' calibration, then, to some extent, the reading doesn't need to represent the actual difference in altitude. (See, for example, law.cornell.edu/cfr/text/14/29.1325, Part (e).) $\endgroup$ – copper.hat Mar 6 '15 at 17:08
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    $\begingroup$ Except that obstacle heights are not based on pressure altimeters. You still need altimeter accuracy when overflying an obstacle or high terrain. $\endgroup$ – rbp Mar 6 '15 at 17:54
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It uses a model called The International Standard Atmosphere (ISA) - so may be inaccurate if you don't have one of those to fly through :-)

enter image description here

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Indeed: climbing from FL240 to FL260 represents a smaller pressure drop than climbing from, say, 2000' to 4000'. The instrument is calibrated for this (it is mechanically built in such a way to represent this pressure drop in a non-linear way) and can properly represent the measured pressure into a ISA calibrated altitude.

Just bear in mind that since altimeters have a fixed error margin in hPa, that hPA error will signify a greater error margin in feet as you climb. Which is a good reason why most GA altimeters are certified only up to 20.000 feet or so depending on how much you wanna spend on them. It is also the reason why there is an increased vertical separation (2000' instead of 1000') for flights above FL280.

eg. Regulations for TSO'd altimeters allow up to 20 feet of error at altitudes up to 1000', but allow for 140 feet or so at FL250

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