[simpits-tech] Zero point on a two needle altimeter

[Craig Rochester]

The device is not for an altimeter, it's for a G-suit regulator.  The
pressure range is 0-5 psi as configured.  I found a table that lists the
barometric pressure vs. altitude:
http://www.usatoday.com/weather/wstdatmo.htm#us It's about 30-3.4 inHg
0-50,000'.  If you convert that to vacuum at sea level, 50,000' would
require about 27 inHg vacuum.  That's a pretty deep vacuum.  Maybe the
"zero" setting of the instruments could be offset to read, say 50,000' at
ambient pressure and the pressure rig could maintain a positive pressure
above ambient to achieve the same thing.

In PSI this is in the 1.6-14.7 PSI range.  That's pretty easy to do with
stuff off eBay.  I don't know what the "velocity pressure" is on the pitot
tube, I'd guess it's in this same range.

Craig R.

-----Original Message-----
From:	simpits-tech-admin@simpits.org [mailto:simpits-tech-admin@simpits.org]
On Behalf Of Matt Bailey
Sent:	Saturday, October 26, 2002 8:56 PM
To:	simpits-tech@simpits.org
Subject:	Re: [simpits-tech] Zero point on a two needle altimeter

Excellent idea! Actuating the bellows is sorta what I was thinking early
but I wasn't sure if it would allow enough precision (I was thinking of
gearing the servo or something). What is the altitude range on your
altimeter?

	-Matt Bailey

Craig Rochester wrote:
>
> I've used a servo with a cam to get very precise position output.  Here's
> the concept:
>
> In practical application I epoxy the servo's splined hub off center in a
> bearing.   Over 180 deg rotation the deflection is the distance from the
> rotation axis and the bearing center.  It works well for the 1/8" travel
to
> operate a pressure regulator:
>
> IIUC, the altimeter work on the expansion of a sealed bellows as the
> atmospheric pressure decreases.  The bellows must drive some precision
gear
> system.  Maybe the servo could do the work.
>
> Craig R.
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