Standard "Pressure" in Software params

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Gustavo, Nov 15, 2004.

  1. Gustavo

    Gustavo Writers Guild

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    I've found a discrepancy in the "standard" value for the Pressure, mainly, that affects VERY MUCH the calculatiosn for Long Range shooting.

    As someone out there may be aware of some programs use as a default value 29.53 inches of mercury, (correspondig to the Army Standard Metro ) and others 29.92 inches of mercury, which corresponds to the ICAO standard metro.

    In short, any clue of which one is the "right one"????

    tks!
     
  2. Jeff In TX

    Jeff In TX Well-Known Member

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    Gustavo,

    The answer to your question is YES. Either will work fine. If your truly interested in precision long distance shooting and you want the help of a ballistic program, using standard default sea level values is going to be very inaccurate. That is of course unless your shooting at sea level and that is the current BP.

    You need a weather meter capable of reading the correct BP at your current location. I use the Krestel 4000 hand held weather station. It gives me Altitude, BP and temp as well as many other readings I'll never use. Inputting all of the correct info into your program will get you much closure at longer ranges. 29.53 or 29.92 is splitting hairs and will have very little change at 1000 yards + or – two inches or so.

    Hope it helps

    [ 11-16-2004: Message edited by: Jeff In TX ]
     
  3. Gustavo

    Gustavo Writers Guild

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    I agree on using defaults values. Of course any decent software will correct for actual conditions (temp, pressure, humidit and altitude)

    What I tried to point out, still remains in terms that the necessary equations need a "standard" value. Of course, this is needed beond, say 800 yards.

    tks!
     
  4. 700

    700 Well-Known Member

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    Gustavo

    Army Metro Standard Conditions are

    Temp: 59 Deg Farht.
    Humidity: 78%
    Altitude: 0 Ft.

    Using the Army Metro model this give Pressure of 29.53 Inches of Hg.


    ICAO Standard Conditions are

    Temp: 59 Deg Farht.
    Humidity: 0%
    Altitude: 0 Ft.

    Using the ICAO model this gives Pressure of 29.92 Inches of Hg.

    The Army Metro and ICAO models give almost the same results, diverging only at very high altitude. At standard conditions the only difference is humidity. If you use the Army Metro model corrected to 0% Humidity, you will get resulting pressure of 29.92 In Hg, and if you use the ICAO model corrected to 78% Humidity, you will get 29.53 In. Hg.

    The two models are inter-changeable in this way.

    Regards

    700