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Date: Wed, 07 Oct 1998 09:48:25 -0400
From: Tom Winsor <altom@provide.net>
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Subject: Re: Koltsman Window
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Chris B wrote:

> I have a question regarding the pressure window in an altimeter. I dont
> understand how you can put the current pressure in the window when you
> recieve it in flight. I thought that the pressure decreased with
> altitude, so wouldn't you need a pressure for the altitude you are at?
> Does the pressure setting in the window move as you climb or descend?
>
> Thanks
>
> Chris B.

 This is a common misconception among students.  The pressure you set in
the altimeter is actually sea-level pressure corrected for field
elevation.  This means that if your airport is 1000' MSL and the altimeter
setting is 29.92,  the actual pressure outside the airplane is 28.92.  The
altimeter senses this difference and indicates 1000' eleviation.  As you
climb, pressure decreases and the altimeter displays the difference between
the altimter setting and the outside pressure.

Mind you, this is only a rough explanation of how an altimter works.
Obviously, if pressure truly decreased 1"/1000 feet you'd run out of oxygen
at 29,920 feet.  Lapse rates are always "average" numbers.

--
Thomas C. Winsor


