Altimeters and air pressure variation

Sriram Narayan wrote:

It doesn't look that linear to me. I found a website with a similar
graph
and it appears that at sea level and at 10000ft the slope of the
curve is at
least 2x different. Your curve is quite a bit more linear (maybe 20%
increase in slope at 10k). There must some sort of mechanical
compensation
involved otherwise altimeters would be off quite a bit even at 10k
(even
with your curve). Isn't it something like 75ft accuracy requirement
for
altimeters?

http://www.atmosphere.mpg.de/enid/16h.html

His graph has more curvature than mine does because his covers a much
greater vertical distance (11000 meters is around 36000 feet). Mine
only goes to 18000 feet. But I agree that a linear assumption will
result in a worst-case error of several hundred feet. Not good enough,
so the nonlinearity must be built into altimeters even if the're only
good to 20000 ft.

Jim Rosinski

Do altimeters for higher altitudes have some kind of clever
mechanism to deal with the non-linearity of pressure at higher
altitudes.

NACA (precursor to NASA) from about 1917 on, did a lot of the early
research on this kind of thing. Here's a link with a great explanation
of how they came up with a clever altimeter mechanism:
http://naca.larc.nasa.gov/reports/1930/naca-report-310/

"jim rosinski" wrote in message
ups.com...
Sriram Narayan wrote:

It doesn't look that linear to me. I found a website with a similar
graph
and it appears that at sea level and at 10000ft the slope of the
curve is at
least 2x different. Your curve is quite a bit more linear (maybe 20%
increase in slope at 10k). There must some sort of mechanical
compensation
involved otherwise altimeters would be off quite a bit even at 10k
(even
with your curve). Isn't it something like 75ft accuracy requirement
for
altimeters?

http://www.atmosphere.mpg.de/enid/16h.html

His graph has more curvature than mine does because his covers a much
greater vertical distance (11000 meters is around 36000 feet). Mine
only goes to 18000 feet. But I agree that a linear assumption will
result in a worst-case error of several hundred feet. Not good enough,
so the nonlinearity must be built into altimeters even if the're only
good to 20000 ft.


Oops, thanks for pointing that out. Didn't check the Y-scale carefully.