How does an altimeter exactly work?

[autoset 0]

I have searched for ages looking for a link that provides an exact explanation. I have tried "How stuff work" with no avail.

I want to know exactly how the hell this little device we carry in our wrist is able to transform air pressure into mechanical movement to make the marking needle(sp?) move to indicate us different altitudes.

[sdctlc 0]

http://www.thealtimeterstore.com/howtheywork.html

"He who Hesitates Shall Inherit the Earth!"

[JohnRich 4]

Quote

http://www.thealtimeterstore.com/howtheywork.html

I can understand how an analog altimeter measures air pressure and converts that to mechanical movement.

For extra credit, tell us how a digital altimeter works.

And for that matter, also those digital compasses in our cars and GPS units...

[BIGUN 1,069]

Select the PDF file about a third of the way down the page for a white paper on how digital altimeters work.

http://www.qsl.net/ok2xdx/Altimeter/altimeter.html

Nobody has time to listen; because they're desperately chasing the need of being heard.

[Mark4 0]

Try this
http://en.wikipedia.org/wiki/Image:Faa_altimeter.JPG

[billvon 2,426]

>For extra credit, tell us how a digital altimeter works.

Pressure is sensed by a solid-state absolute pressure sensor. These are now commonly available, and operate by sensing the pressure differential between a reference cell (usually vacuum) and the exterior temperature. The stress on a silicon membrane between these two generates a specific voltage level at the output.

An A/D converter reads the voltage from the pressure sensor and converts it to a digital signal. This is then fed to a processor which performs functions such as zeroing, altitude alerting, pressure/temperature compensation and logging.

[JohnRich 4]

Quote

>For extra credit, tell us how a digital altimeter works.

Pressure is sensed by a solid-state absolute pressure sensor. These are now commonly available, and operate by sensing the pressure differential between a reference cell (usually vacuum) and the exterior temperature. The stress on a silicon membrane between these two generates a specific voltage level at the output.

An A/D converter reads the voltage from the pressure sensor and converts it to a digital signal. This is then fed to a processor which performs functions such as zeroing, altitude alerting, pressure/temperature compensation and logging.

Dayum! Thanks for that explanation.

Now, how does a digital compass electronic chip know which way is north?

[yourmomma 0]

Same as whiskey compass. Measures flux lines via the Hall effect.

[mnskydiver688 0]

I guess that trumps my little green men theory.

Sky Canyon Wingsuiters

[kelpdiver 2]

Is it correct that the mechanical altimeter won't be able to have the same accuracy at all altitude ranges because the pressure change is not a constant? End result, they're optimized to be most accurate near the ground where it matters.

Digitals, otoh, can map pressure to altitude more closely, though with the disadvantage of the dependency on batteries.

[headoverheels 291]

Since the pressure is being sensed, and the air density varies with temperature, the altitude reading will only be correct on a "standard temperature" day. ( Well, it will be correct on the ground at any temperature, if you zero it. ) On hot days, the true altitude will be higher than the reading. This error could be mostly compensated by measuring the outside temperature, but it generally is not.

[billvon 2,426]

> Is it correct that the mechanical altimeter won't be able to have the
>same accuracy at all altitude ranges because the pressure change is not a
>constant?

It is correct that pressure change is not linear - but good mechanical altimeters compensate for that through gearing and nonlinearity of the bellows. Skydiving altimeters do not do such a good job of that, and will show significant errors above about 20K feet. Depends on the make of altimeter. On World Team we saw 2000 to 3000 foot differences between altimeters at 26,000 feet - but they worked well near breakoff altitudes (7500 feet.)