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

[CHUCK CHELIN]

Good morning, All,



Barometric altimeters can’t be “drift-proof”, they all rely upon a changing
air pressure, and they all have some kind of lag.  As you change altitude
the diaphragm in your device that senses pressure change must communicate
that change mechanically.  In the case of a digital altimeter usually a
linear transducer changes the electrical signal to the logics.  In the case
of an analog altimeter a gear/lever arrangement changes the position of a
needle.  Depending upon the design sophistication and manufacturing quality
of the components the altimeter will translate pressure changes in steps:  Very
little steps for high-quality components and much larger steps for marginal
components.  To use a California analogy, sometimes the continental plates
move in a series of very small –almost unnoticeable – slips and sometimes
huge forces accumulate and the plates jump a considerable distance all at
once to make an earthquake.  In instrumentation, that slip-stick tendency is
called hysteresis.



A digital altimeter only has two signal inputs: pressure from the diaphragm
and time from the watch.  I don’t know the design philosophy of the digital
altimeter manufactures but it would be possible to incorporate a number of
different “if/then” algorithms to make things look better.  One could be as
simple as an algorithm in the logics which says, “Don’t display big jumps in
a short period of time.  Instead, display a series of small jumps in the
same period.”  The reason to do so would probably be to make the altimeter
look more stable when using unsophisticated components by masking their
klutzy jumps.  Conversely, the logics could avoid displaying small display
changes that look like “drift”, and displaying only larger changes.



I’m low-tech:  I use analog altimeters which have some small amount of
friction in the gears, levers, and needle pivot bearing.  What I do every
time I look at the altimeter is to gently tap the side of its case a few
times to rattle its insides slightly and neutralize any accumulated
hysteresis.  My best altimeter is graduated in 20-foot altitude increments,
and after tapping I can usually read the scale to half an increment, i.e. 10
feet.  However, while I can read a change in altitude to within 10 feet, the
absolute altitude still depends upon the accuracy of its periodic
recalibration.



Steel-Eye

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

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


On Tue, Mar 3, 2009 at 8:59 PM, Lenny Leum <littleleum at yahoo.com> 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?  I can see that
> this contradiction is what makes it inaccurate, but, does the device
> generally assume a steady barometer?  I'm just wondering if these devices
> are "smart" and how that works.
> Lenny
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