Altimeters and air pressure variation

"jharper aaatttt cisco dddooottt com" "jharper aaatttt cisco dddooottt
com" wrote in message news:1105391055.635118@sj-nntpcache-3...
At sea level, the change in atmospheric pressure with altitude is
close to 1"Hg/1000'. Logically, this would mean that the air
pressure would drop to zero somewhere not much above 30000'. It
doesn't, because as the density drops the variation with
altitude also changes.

Which brings to mind the question, how does an altimeter deal
with this? As far as I know, it's just a simple aneroid barometer
with a bunch of linkages and gears to turn its expansion into
pointer movement.

My altimeter is marked "accurate to 20000' ". Is this why? Do
altimeters for higher altitudes have some kind of clever
mechanism to deal with the non-linearity of pressure at higher
altitudes.

I asked my acro instructor (10K+ hrs, airforce instructor pilot,
ex U2 pilot so should know a thing or two about high altitudes).
He explained the non-linearity of pressure to me but was
stumped on how this translates to the altimeter mechanism.

Anyone know?

John

Visit this website and it will answer your questions about the relationship
between pressure, temperature and altitude... altimeters are designed to
take the non-linearity into account...

http://www.lerc.nasa.gov/WWW/K-12/airplane/atmosi.html

Dean Wilkinson wrote:


Visit this website and it will answer your questions about the relationship
between pressure, temperature and altitude... altimeters are designed to
take the non-linearity into account...

http://www.lerc.nasa.gov/WWW/K-12/airplane/atmosi.html


Nice site, thanks. But presumably there is some standard
atmospheric model that altimeters use? After all nobody actually
cares whether FL300 is really 30000' feet above MSL, as
long as everyone flying there is at the same altitude and,
more importantly, not at somebody else's FL290 or FL310.

Which implies that there must be some standard mechanical
way of making the translation? I'll ask next time I visit my
avionics shop, but considering what each visit costs I
quite hope this won't be for a while.

John

jharper aaatttt cisco dddooottt com wrote:

Dean Wilkinson wrote:

Visit this website and it will answer your questions about the
relationship
between pressure, temperature and altitude... altimeters are designed to
take the non-linearity into account...

http://www.lerc.nasa.gov/WWW/K-12/airplane/atmosi.html


Nice site, thanks. But presumably there is some standard
atmospheric model that altimeters use? After all nobody actually
cares whether FL300 is really 30000' feet above MSL, as
long as everyone flying there is at the same altitude and,
more importantly, not at somebody else's FL290 or FL310.

Yes, there is a standard model and if you click on the first
link on the cited page you get to:
http://www.lerc.nasa.gov/WWW/K-12/airplane/atmos.html
which gives the equations describing that standard model.

Which implies that there must be some standard mechanical
way of making the translation?

There's a mathematically defined correspondence
between altitude and pressure under the standard
atmosphere assumption. But I doubt if the specific
mechanical means of achieving that correspondence is
specified anywhere. As long as the manufacturer makes
an instrument that is shown to give the right correspondence
to within a specified accuracy why should it matter exactly
how they do it?

I'll ask next time I visit my
avionics shop, but considering what each visit costs I
quite hope this won't be for a while.

Didn't see that, thanks for pointing it out.
At least that explains the note on my altimeter
saying "certified to 20000' " which I hadn't
understood before.

I wonder how altimeters for airliners work, given
the change that happens at 36152' - or indeed the
U2 altimeter. Must be some interesting stuff inside.

John

Peter wrote:
jharper aaatttt cisco dddooottt com wrote:

Dean Wilkinson wrote:


Visit this website and it will answer your questions about the
relationship
between pressure, temperature and altitude... altimeters are
designed to
take the non-linearity into account...

http://www.lerc.nasa.gov/WWW/K-12/airplane/atmosi.html


Nice site, thanks. But presumably there is some standard
atmospheric model that altimeters use? After all nobody actually
cares whether FL300 is really 30000' feet above MSL, as
long as everyone flying there is at the same altitude and,
more importantly, not at somebody else's FL290 or FL310.


Yes, there is a standard model and if you click on the first
link on the cited page you get to:
http://www.lerc.nasa.gov/WWW/K-12/airplane/atmos.html
which gives the equations describing that standard model.


Which implies that there must be some standard mechanical
way of making the translation?


There's a mathematically defined correspondence
between altitude and pressure under the standard
atmosphere assumption. But I doubt if the specific
mechanical means of achieving that correspondence is
specified anywhere. As long as the manufacturer makes
an instrument that is shown to give the right correspondence
to within a specified accuracy why should it matter exactly
how they do it?

I'll ask next time I visit my
avionics shop, but considering what each visit costs I
quite hope this won't be for a while.

On Mon, 10 Jan 2005 18:09:48 -0800, "jharper aaatttt cisco dddooottt
com" "jharper aaatttt cisco dddooottt com" wrote:

snip

I wonder how altimeters for airliners work, given
the change that happens at 36152' - or indeed the
U2 altimeter. Must be some interesting stuff inside.

http://www.rockwellcollins.com/ecat/...html?smenu=109

http://www.cas.honeywell.com/ats/products/airdata.cfm

Typically the DADC's are corrected for known issues/errors in the
pitot/static system of the specific type aircraft it is installed in,
as well as the "change"s you've noted.

TC