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  1. #1
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    Need help with uncompensated altimeter extraction

    Hi guys. I need some help figuring out how to extract an altimeter indication which is not compensated for barometric pressure. My altitude gauge has a mechanical adjustment for barometric pressure built into the gauge. Iím using a servo to drive the gauge, and the servo mounting bracket is configured to rotate when the adjustment knob is turned (When used in the original aircraft, the pressure bellows rotated with the knob).

  2. #2
    500+ This must be a daytime job Jim NZ's Avatar
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    Re: Need help with uncompensated altimeter extraction

    Steve this rang bells in my head ,,, I remember when I was sorting out the bulk extractions and in the Simconnect Extractions list I kept running into ,,

    PRESSURE ALTITUDE

    It says in 'metres' but it also extracts in 'Feet'.

    Some background info ,,,,
    http://en.wikipedia.org/wiki/Pressure_altitude

    It may or may not be what your looking for ????

    Good luck ,,, Jim
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  3. #3
    150+ Forum Groupie BuzziBi's Avatar
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    Re: Need help with uncompensated altimeter extraction

    It's possible I'm wrong ... but ..
    The values ​​you get from FSX, the altimeter altitude, is not compensated for the pressure differences.
    You have to adjust this youreself with the QNH.
    Is it possible to mount a rotaryencoder on the pressure adjustment knob? (to adjust the QNH)
    You must still have a scale that shows the current QNH, and the new QNH.
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    Re: Need help with uncompensated altimeter extraction

    Jim – I don’t see Pressure Altitude as a simulation variable in Microsoft’s ESP. I know it can be calculated from static pressure, but static pressure is also not an available simulation variable. What I do see listed in the ESP is three possible extractions:
    PLANE ALT ABOVE GROUND
    PLANE ALTITUDE
    INDICATED ALTITUDE
    I have no idea what the differences are between these three extractions. I assume the Indicated Altitude is what you are reading with the command (D).
    .
    Buzzi – I’m with you on this, but my intent is to handle the QNH locally in the cockpit rather than transmitting it to FSX. My altimeter gauge has a mechanical knob which adjusts the indicated altitude for pilot initiated changes to QNH completely within the gauge. As designed, the gauge senses static pressure and uses a brass bellows to convert the static pressure to needle movement. By turning the adjustment knob, the bellows assembly (and the indicator needle attached to it) rotates against a fixed altitude scale, allowing the mechanically indicated altitude to be adjusted.

    One possible option is see is to use the void setup to transmit a Kohlsman setting of 1,013.25hPa to fsx, in which case the indicated altitude will be the uncorrected altitude. My concern is that with this method I don’t have any way to freeze the QNH once I start the simulator.


    Another possible option would be to extract both the indicated altitude and the Kohlsman setting from fsx and recalculate an uncorrected setting within the Arduino. Messy, but it’s possible to do if I’m willing to accept the scan times to be slowed down by floating point computations. My preferred solution is to find an extraction which doesn’t use the QNH adjustment.

  5. #5
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    Re: Need help with uncompensated altimeter extraction

    Jim, I just found the Pressure Altitude you referred to buried way down on page 51 of the FSUIPC4 IPC Status list. Wow, this is exactly what I was looking for. Thanks very much.
    Steve

    "34B0 Pressure Altitude (meters), double float. This is the indicated altitude when the altimeter Kollsman setting is 1013.2 hPa"