On Mar 12, 2:00*pm, es330td wrote:
On Mar 12, 10:15*am, "Darkwing" theducksmail"AT"yahoo.com wrote:



"es330td" wrote in message

...

The other day I saw a watch advertised as a watch for pilots that
includes an altimeter. *While I have seen these before for mountain
climbers this is the first I have seen targeted at pilots. *My first
question in seeing it was "Where is the Kollsman window?" *I tried to
find more data online but I can't seem to find one of these that can
adjust. *I assume they are all set to 29.92 but I am trying to figure
out how useful this feature would be if it can't be adjusted for local
conditions.

Here is a GPS watch that has altimeter, speed and a lot more.

http://www.aim-sportline.com/mytach/index_eng.htm

Pilots can't use this. *GPS altitude (while theoretically correct) is
not the same as pressure altitude because pressure does not vary in a
linear fashion with altitude; if it did, flight levels above 29,920
feet would require liquid oxygen on board for the engines.

You're correct that GPS altitude will differ from pressure-based
altitude, but it's not because of the non-linearity. That's taken
into account when the altimeter converts from pressure to the
equivalent altitude. The difference is that the pressure-based
altimeter assumes the standard atmosphere model (and above the
transition altitude also assumes a fixed sea-level pressure of 29.93
"Hg). If the actual atmospheric conditions differ from the standard
model then any pressure-based altimeter will differ from the actual
value. Of course the GPS measurement also suffers from its own error
sources and generally has greater random fluctuations - but it's not
affected so directly by atmospheric variation from the standard model.

But pressure-based altimeter watches are commonly available. I used
to have a pretty cheap ($30) Casio with this function and I could
calibrate it either based on a currently known altitude or by setting
the sea-level pressure (i.e. the Kollsman window 29.92 setting when
appropriate). It updated the reading every 6 seconds and worked well
but I never used it for aviation other than for curiousity.