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

[Matt Bailey]

Craig Rochester wrote:
> 
> 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.

	Yes that would probably be much easier. I *think* this would require
modifying the altimeter some if you wanted to be able to use the
Colesman window, if you just had to have this feature. I think it would
be "cool" but not really necessary.

> 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.

	Well the airspeed indicator compares static air pressure with air
pressure at the pitot tube, if I'm not mistaken. If you did use an
unmodified ASI you would need to either calibrate your sim output for
the current barometric pressure in your sim room, or seal the static
port on the ASI so that it's always at the same pressure. If you wanted
to get fancy I guess you could have a single air chamber that is
pressurized to the atmospheric pressure your sim is indicating (like the
static port on a real plane), and hook all the static instruments to
this. Then make another chamber that is pressurized to the pressure at
the sim plane's pitot tube (just use the sim's indicated airspeed).
Cool, but seems unecessarily complex. I think the onyl other static air
instrument is the VSI, correct? I have no idea what's inside these
things but I guess if you did have this functional simulated static
system you could just hook the VSI up directly to it.

	-Matt Bailey