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

[Phil Newhouse]

That was from a "LOW Tech" guy...

;-)

phil

On Wed, Mar 4, 2009 at 6:51 AM, CHUCK CHELIN <steeleye at wildblue.net> wrote:

> 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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