Altitude vs. Barometric pressure

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Michael Eichele, Jun 19, 2005.

  1. Litehiker

    Litehiker Well-Known Member

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    Gene,
    As I mentioned above, the answer to providing a correct shot is not just a ballistics program but a program combined with a weather station that has sensors for station pressure, wind, coriolis effect, angle, etc.

    The instrument that does all this is the Kestrel 5700 weather meter with the AB ballistics program installed. It uses the ballistics program with your SPECIFIC load and muzzle velocity along with all its sensors and internal program to give you a final firing solution.

    True, the Kestrel 5700 can't give you downrange wind but the wind at your FFP is the most important reading. Determining hold-off for downrange wind is up to you.

    Eric B.
     
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  2. wildcat westerner

    wildcat westerner Well-Known Member

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    Lite hiker,

    Thanks for a specific factual answer. The move from Seattle to retirement in the Rockies has cut me off from the vast amount of technology which was easinly accessible and also, much more important, people familiar with these subjects. I am absolutely sure that I would be the only owner of one of these kestrels in this entire county.\

    Gene S.
     
  3. johnlittletree

    johnlittletree Well-Known Member

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    Not only is that a problem but what counts as a standard day at Sea Level is not at all universal each field has it's own version of a "standard day" in Aviation : Standard day parameters. For Pilots: At sea level, Altimeter:29.92 in/Hg at 15 °C (59 °F) The "standard day" model of the atmosphere is defined at sea level, with certain present conditions such as temperature and pressure.

    Even in music various countries use different tuning for A in the USA A=440 is the modern norm but that is not at all universal. Likewise we normally think of the speed of light as a constant but if you dig deep enough you soon find out that the speed of light has had many different speed's attributed to it. Anytime anyone thinks anything is simple and universal I laugh because that is either a sign of ignorance or stupidity since almost nothing is universally agreed upon the deeper you dig.

    For the most part a large chunk of the worlds problems stem from people wanting nice neat well defined problems, causes and answer's that can have a neat little package with a ribbon and nice labels.

    Most people do not want the truth and do not want to be educated. They want simple and absolutes.
     
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  4. Dog Rocket

    Dog Rocket Well-Known Member

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    This thread has some long legs...it's 13+ years from the OP. This isn't aimed at any poster, just general info.

    The difference between Station Pressure and Barometric Pressure needs to be understood to do this correctly.

    At any given location, station pressure doesn't change enough from day to day... much less throughout a single day... to make a difference at distances from 1000 yards and closer.

    The higher you are in altitude, the less difference it makes anyway, because your bullet is flying with ever increasing efficiency as altitude increases.

    You could literally look at a topo map, put in the standard pressure for that altitude and be perfectly fine. Or, you could just wear a G-Shock watch that gives station pressure without spending $600 on an Applied Ballistics Kestrel.

    If you don't know how temperature effects your powder burn rate for your cartridge then a AB Kestrel won't help you there either because it will be guessing for you. But, 0.7 fps per 1 degree ambient temp is a pretty good average. A simple keychain thermometer will get you more than close enough to actual temp for these purposes.

    You can literally plug in all this basic stuff into JBM Ballistics for free before you leave the house, write it down, and be perfectly fine. It doesn't need to be complicated.
     
  5. Litehiker

    Litehiker Well-Known Member

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    Dog Rocket, You are correct about temperature affecting burn rate and therefore velocity. BUT, there are now powders like H4350 that are fairly temperature tolerant so that factor will be greatly mitigated when compared to an old powder like WW II era 4895.
    As far as I know even an excellent ballistic calculator/weather station like the Kestrel 5700 Elite will not correct for this factor.

    Eric B.
     
  6. Dog Rocket

    Dog Rocket Well-Known Member

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    I don't know about the Kestrel 5700. I don't have one. But both Applied Ballistics app and Strelok Pro have a place to put in average change for powder burn rate. It is a user input, so garbage in / garbage out.

    As far as H4350 being non-temp sensitive, that is a can of worms.....every powder is temp sensitive above a certain temperature. Some light better at cold temps than others, allowing the manufacturers to claim less overall sensitivity. But at 80 degrees and hotter, they all succumb to chemistry. They are all made of nitroglycerin.
     
    Last edited: Nov 14, 2018
  7. gpo1956

    gpo1956 Well-Known Member

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    So........in Strelok Pro, do I use temperature, humidity and pressure from "Get fro internal barometer, or do I use Density Altitude button?
     
  8. Dog Rocket

    Dog Rocket Well-Known Member

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    Put in actual temp, actual station pressure and 50% for humidity. You will be golden. Using Density Altitude is a poor substitute for these individual measurements.
     
  9. gpo1956

    gpo1956 Well-Known Member

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    Great. Thanks very much.
     
  10. Dog Rocket

    Dog Rocket Well-Known Member

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    I should have mentioned earlier, if you don't have a way to get station pressure in the field, then you can just put in standard station pressure for that altitude.

    If you don't know it, it is easy to figure. It is 29.92 inHg at sea level and is very very close to a 1 inHg change per 1,000 ft elevation change. So 1,000 ft would be 28.92 inHg, 3,000 ft would be 26.92 inHg etc...

    At any given location, the station pressure very rarely varies by more than 0.5 inHg from standard pressure from day to day, much less throughout a single day.
     
  11. Hand Skills

    Hand Skills Well-Known Member

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    How is it any different from using DA at that point? ... other than you need to plug in 3 values instead of one.

    I find it a lot easier to think in Density Altitude to adjust for changing conditions through the day. I know I'm not unique in that respect.
     
  12. Dog Rocket

    Dog Rocket Well-Known Member

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    The way that heat affects the density altitude equation doesn't really apply to a ballistic trajectory in the same way it applies to a fuel burning machine trying to predict power production.

    A bullet only cares about the resistance of the medium it is moving through. That is represented most accurately by station pressure.

    Heat affects cartridges in a different way, altering powder burn rate. So this needs to have it's own individual input anyway.
     
  13. Hand Skills

    Hand Skills Well-Known Member

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    False.

    True, and this is exactly what Density Altitude describes.
     
  14. Dog Rocket

    Dog Rocket Well-Known Member

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    Then you go right ahead.