[pct-l] how altimeters work

[CHUCK CHELIN]

Good morning, Lenny,



To retain reasonable accuracy a barometric altimeter must be recalibrated
regularly.  The usual recommendation is to recalibrate every 12 hours, or
every 50 miles if driving.  I used an analog altimeter for many years and my
practice was to recalibrate during the day whenever I encountered what I
thought was a reasonably accurate point or feature based upon a topographic
map or a vertical control point such as a brass-cap benchmark.  Then, every
night before I slept, I wrote down the elevation of my camp.  When I awoke
in the morning I compared the morning’s elevation with what I noted the last
night.   Overnight the barometric pressure often changed a bit so I
corrected to last night’s elevation.  If some time passed and I couldn’t
find any reliable elevation I just set the barometer to standard pressure
which is 29.92 in. Hg. and used the resulting elevation.



I don’t carry an altimeter much anymore, I carry a GPS.  It’s not the GPS
model that has a separate built-in altimeter, I only have the
satellite-based altitude reading which is less accurate, but it’s usually OK
for my purpose.  I’m more interested in the X and Y coordinates rather than
the Z altitude.



Steel-Eye

Hiking the Pct since before it was the PCT -- 1965

http://www.trailjournals.com/steel-eye

On Tue, Mar 3, 2009 at 3:30 PM, Lenny Leum <littleleum at yahoo.com> wrote:

> I don't recall any discussion of altimeters in the last few months, just
> GPS / journal gizmos and other silly electronics, so let me break the ice
> here.  I went down to Outdoor World and asked if they had any altimeter
> watches and picked up a Timex gizmo.
>
> My primary conundrum concerns how this thing works!  As I have found, all
> altimeters also have barometers incorporated into the same gizmo.
> Aparently, both programs use air pressure measurements to operate.  If you
> know your correct altitude and set the watch, then the barometer program
> will have correct readings.  Similarily, if you set the barometer, then the
> altimeter will also be set correctly.
>
> My question is...... how does the watch determine which pressure change is
> due to elevation and which pressure change is due to changes in weather?????
>
> I have done some loose testing of the altimeter, including taking it to the
> beach and setting the altimeter to zero.  It seems to work fairly well, when
> I climb up a hill it goes up, down goes down, but there have been some
> serious disconnects (right now it says I am 70 feet bellow sea level and I
> am definitely not).  I was thinking that the problems I encountered with the
> altimeter may be related to the changing weather, and I was hoping that a
> better understanding of how these work might lead to improved performance.
> Thanks for any imput or ideas you have.
> Lenny
> _______________________________________________
> Pct-l mailing list
> Pct-l at backcountry.net
> http://mailman.backcountry.net/mailman/listinfo/pct-l
>