[pct-l] Watch and Altimeter

[byarcand at mtu.edu (Benjamin Y. Arcand)]

Here's a link to a chart for the standard atmosphere that relates
altitude, pressure and tempurature:

http://www.usatoday.com/weather/wstdatmo.htm

I have a Nike Ascent watch that seems to have similar accuracies in
measuring altitude as others discussed on the list (usually within 100ft,
hysteresis is evident after elevator rides).  The Ascent's claim to fame
is that it reads barometric pressure and altitude simultaneoulsy with its
'zero drift' technology.  How it does this, I don't know.  My guess is
that it has some kind of mathmatical algorithm that works out the two
since a pressure change due to going up/down in elevation or weather 
change would be indistinguishable to a pressure sensor.  Anyway, the
altitude reading has typical accuracies and the weather prediction is
usually right on for a 5-8hr forecast (it shows a little icon for it's
weather prediction then shows the graph of barometric trend).  I've seen
it on ebay for less than $100.  Don't bother with the temperature gage
though, you have to take the watch off for about ten min. so it's no
longer reading your body heat, the compass works good though.

Ben

PS If anyone has any info on how the 'zero drift' works on this watch I'd
be curious to know.


On Thu, 15 Apr 2004 adrian.borner@ch.abb.com wrote:

> 
> As has been mentioned, a barometer and an altimeter are basically the same.
> It just depends which of the variables you keep or at least assume as
> constant. When using it as a barometer one must not change the altitude,
> i.e. one is stationary while when using it as altimeter one assumes the
> barometric pressure at a given heights remains the same (weather does not
> change). As a result altitude readings are affected by the weather. However
> the sensitivity to change in altitude is much bigger than change in
> pressure due to weather conditions (I do not know the formula off hand).
> Obviously when you run into a storm front then you will be completely off
> with your altitude reading.
> Temperature is another variable which affects the readings. As a result the
> meters have a temperature compensation which is more or less accurate.
> As said earlier, frequent recalibration is required. GPS with additional
> barometric altimeter generally do this automatically. The barometric
> altimeter is more accurate short term, while the GPS ensures long term
> accuracy.
> Hope I did not confuse the issue
> Adrian
> 
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