Altimeters and air pressure variation

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.