Re: Garmin altitude problems

On Fri, 9 Aug 2024 23:42:01 -0400, Frank Krygowski
<frkrygow@sbcglobal.net> wrote:

I'll note that the instruments probably need to have temperature
compensation built in. I was once given a small portable electronic
altimeter/barometer. Today, because of these discussions, I played with
it a bit. I calibrated it at my home (where I know the altitude based on
USGS maps, which is confirmed by my car's GPS) and drove about 8 miles
away. There I left it in the parked car for a couple hours.
>
The sun was shining (despite this being NE Ohio!) and heated up the
car's interior and the altimeter. When I returned, the altimeter
displayed nearly 100 feet less altitude than when I first arrived.
>
I wonder how temperature compensation is or was handled for the old
mechanical aneroid barometers.

<https://en.wikipedia.org/wiki/Barometer>
"Temperature compensation of an aneroid barometer is accomplished by
including a bi-metal element in the mechanical linkages. Aneroid
barometers sold for domestic use typically have no compensation under
the assumption that they will be used within a controlled room
temperature range."

If this is what your marine aneroid barometer looked like, it MIGHT be
temperature compensated:
<https://www.google.com/search?q=temperature+compensated+marine+aneroid+barometer&tbm=isch>
Long ago, I bought a rather nice looking brass wall mounted barometer,
which I later discovered was intended to be only used indoors and was
therefore not temperature compensated. 

As I understand it, all aviation barometric altimeters are temperature
compensated.  I would think the aviation temperature compensation for
air density method is somewhat similar to a marine barometer:
<https://www.faa.gov/air_traffic/publications/atpubs/aip_html/part2_enr_section_1.8.html>
I couldn't find anything that shows how the various gears work and
where the bi-metallic temperature compensator is located (because of
numerous interruptions today).




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Jeff Liebermann                 jeffl@cruzio.com
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