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Wrist watches usually use the barometric pressure sensors they have to calculate the altitude. I have also confirmed this from the manual of my watch, which states that:

The watch displays altitude values based on air pressure readings taken by a built-in pressure sensor.

Fine that's understood and makes sense and has always worked for me wherever I go.

Question

But then how does it know altitude during flight? The cabin of all commercial airliners are pressurized and hence the air pressure is not changing inside the cabin with the change in altitude (as happens for descent or ascent on ground). I know that even the aircraft itself uses pressure sensors for many calculations but those sensors are mostly usually on the Pitot tube and they sense the real outside pressure and not inside an pressurized cabin

I would also like to mention that yes the altitude reading it takes during a flight is not always close to what the real value is but still it does track the altitude difference when pressure inside the cabin remains same, how so?

Please note

That I didn't include the exact details of my watch because neither am I promoting any special model or a brand nor am I asking specifics of how a watch works, focus is on air pressure inside the cabin, not on watch. If needed I can provide the model and manual reference links.

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But you should be more specific regarding what value it shows. –  Jan Hudec Aug 24 at 10:12
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The pressure inside the cabin does NOT remain the same. That's why you need to equalise the pressure in your sinsuses, usually during descent. Your watch will be showing you the cabin altitude, probably somewhere around 2500 metres. Pitot tubes are not used to measure static pressure. They are used to measure dynamic pressure and calculate airspeed. –  Simon Aug 24 at 10:27
    
Thanks @Simon, Yes it does remain around there –  Iceman Aug 24 at 13:41

1 Answer 1

up vote 15 down vote accepted

The pressure in the cabin does not remain the same. It decreases, but much more slowly, in such a manner that when the aircraft is at 37000 feet the pressure in the cockpit is the equivalent of 7000 or 8000 feet. Keeping the cabin pressure at sea level would pose incredible stress on the fuselage, due to huge difference in pressure between the inside and the outside.

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Thanks Emil, makes sense. –  Iceman Aug 24 at 11:55
    
This note by Boeing can substantiate this answer: Today's airplanes are pressurized to a typical cabin altitude of 6,500 to 7,000 feet (1,981 – 2,133 m), with a maximum certification altitude of 8,000 feet (2,438 m). boeing.com/paris2013/pdf/BCA/bck-787%20interior.pdf –  Iceman Aug 24 at 11:58
    
"incredible stress on the fuselage" -- to be clear, do common planes have a hypothetical failure mode in which the plane fails to release pressure during ascent and the fuselage ruptures? By "hypothetical" I suppose I mean it's realistic, it is what would happen if pressure weren't released, but some kind of fail-safe doohickey no doubt would prevent that by blowing out first even if the normal systems failed to let any air out. –  Steve Jessop Aug 24 at 15:38
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@SteveJessop: Yes, normally the system incorporates valves that will open if a certain pressure differential is reached (positive or negative). There should be at least two such valves and only half of them should be enough to keep the pressure difference between safe limits. –  Emil Aug 24 at 17:29
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Like most things, there is some fail safe redundancy built in just in case the valves all fail. On some types, there are "frangible" panels or burst discs which will blow out to equalise the pressure. –  Simon Aug 24 at 21:05

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