[pct-l] how an altimeter works, take 2

[Brian Lewis]

Lenny wrote:
"The question remains though, how does the watch supposedly determine
which pressure change is due to elevation and which pressure change is
due to changes in weather??"


While the key answer (from me at least) is "I don't know", one of two
altimeter watches I've owned was a Nike that claimed a "zero drift"
technology.  It was supposed to have a "special algorithm that
determines whether a barometric pressure change is caused by altitude
change or a weather system".   Elsewhere I read the claim that it
"virtually eliminates barometric drift".

Catch is, I never could find any information on how it was supposed to
do that, and my personal experience was that it was no better at this
than the watch I use now (a Suunto).   There might well be situations
in which it did help --- maybe it tracks barometric patterns, assuming
the watch isn't suddenly moved (car, airplane, etc).   Dunno, but a
couple of reviews I read on line suggested that others also had
difficulty in detecting any advantage from this.   Few people will run
exhaustive tests, to include knowing the precise barometric pressure
at differing altitudes at different times and use this in objective
testing against other altimeters ...   Nice marketing bullet point
anyway!

Bottom line for me is that I assume that any pressure-based altimeter
is indeed impacted by weather-based changes in barometric pressure.
In situations where I use mine, I either calibrate periodically at
known elevation points, or I only pay attention to the elevation delta
rather than trust the actual reported elevation.

I'm certainly no expert in this area, perhaps someone knows more here
and can enlighten us all.


Brian Lewis / Gadget '08
http://postholer.com/brianle