Altitude vs. Barometric pressure

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b.After calculating my holds based on standard pressure and altitude, I run the equation again, only this time I uncheck the "Calculate Standard Pressure" box, and input the current station pressure obtained from my Kestrel 4000, and then run the program again.


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If youre using altitude along with station pressure, youre hosed.

If youre using station pressure, you should have your altitude set on 0' regardless of your actual altitude.
 
Looks like Tubb/S&B has generated interest in use of Density Altitude as a single air density input in the field. Might work ok, I'll have to try it.
It is easy to figure and graph for one bullet. More difficult for the rest of the world.
Here we go:

Air Density(Kg/m3) from DA =
((3280.84*44.3308-DA)/(3280.84*42.2665))^(1/0.234969)
Air Density ratio(Ro) to ICAO conditions would equal the air density above divided by 1.225
Local BC would be ICAO BC/Ro

Now Std Metro is less pretty because it's meaningless to the rest of the world.
Std Metro BCs COULD be(and really should be) normalized to ICAO BCs by [StdMetro BC*.982].
Or the density altitude formula above could be biased to account for a zero under StdMetro conditions by adding 625 to DA: ((3280.84*44.3308-(DA+625))/(3280.84*42.2665))^(1/0.234969), adjusting air density.
Then figure Ro = [StdMetro Air Density/1.20275]
And Local BC = [StdMetro BC/Ro]

Some seek a 'rule of thumb' for density altitude and drop table adjustments. But any rule of thumb would be specific to your BC as adjusted above and as washed through ballistic software for affects in the field.
Only then could these affects could be defined with a 'rule' or named something like 'DTAC'.
 
I don't know what program you're using to get there. I don't know of any rules of thumb for DA, as it doesn't directly correlate to single variable changes. And back to the beginning of this thread; Altitude is still never needed. It's still about pressure, temp, humidity..

For example: given 59deg, 78%rh, and 24.9"Hg(5,000ft pressure altitude), DA = 6,345

DA would equal 5,000ft above if only temp changed to 39.7deg
DA would equal 5,000ft above if only pressure changed to 25.9"hg
DA would equal 5,000ft above if temp was 40deg & Rh was 62%
Or if 59deg, 78%, and altitude was 3908ft(giving a standard pressure at altitude of 25.9")

I'm not a weatherman, I don't fly, and don't even know the history of density altitude right now(I have CRS, ADD, and a new one; self imposed misery -SIM). I researched it in the past, and wrote it all into a spreadsheet I use to calc bullet BC.
If I knew how to post a small spreadsheet here, I would make one just for it.
 
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I'm not a weatherman, I don't fly, and don't even know the history of density altitude right now(I have CRS, ADD, and a new one; self imposed misery -SIM).

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LOL!

I can relate. Sometimes these software issues can cause more headaches than they are worth. Someone once said to me, give a man a program and you will give him a headache. Teach a man to program and he will become an alchoholic.

Just trying to learn here. I came up with an example of what I THINK means I understande DA.

Example: shooting a given load in standard conditions and the bullet dropping 383.01" @ 1K. Bump only the temerature up to 110 degrees F. it drops only 353.90" @ 1K. By selecting DA and defaulting temp to 59 and humidity to 78% and BP to 29.92, and using only altitude, if I enter 2433' I come up with 353.90". Is this a correct way of understanding and putting to use DA?

I do understand its all about pressure and temp and that altitude is only valid by way of its effect on pressure which still goes back to the local pressure being the only one of the two that matters, but there are still other ways of arriving to one accurate bullet drop.
 
You seem to have 'Density Altitude' and 'Pressure Altitude' mixed up. Different animals all together.

This may APPEAR to work under standard conditions at altitude(in your software), but a Krestel will not likely give you a DA equal to PA. Also, BP won't be 29.92 @ 2433ft(more like 27.4"), and 78% is not in the ICAO std, but instead the Std Metro standard: [59degF/29.53"/78%].

Above, your input of 110deg,78%,29.92"Hg will cause an air density of 1.087Kg/m3 and a DA of 4034Ft
Using 4034'DA with the formulas above I get AD=1.087

On a sidenote, You would have a PA=2433 AND DA=2433 AND Altitude=2433, at standard conditions for that altitude of 50degf & 27.38"Hg & 0%Rh. But air density here is 1.140 and it should have effected BC and your drops.
 
Fellas'

After reading all of this twice, I am still confused as to what to enter into my software. I am using Exbal and a Kestrel 4000. Could somebody clarify in lamens terms? Please...


Murf
 
Murf,

This is what I do when running my version of Exbal <font color="blue">(Tools, About Exbal, a screen shows saying: Exbal Version 5.6) </font> which works for different elevations at long distances:

Run Exbal click on File, New --- it might ask you wheather or not you want to save the changes for what you were running. Answer by clicking "Yes" or "NO". In the text field on top, type in a name for the load, click the icon to save, give it a name and click "OK".
Now we're ready to start inputting information.
Click on top Button that says "Bullet Data", input all needed information (I zero at 100 yards) and click "Update". It goes back to opening screen.

<font color="green">From Perry (Exbal Programmer):
The second logical form is the "Field Conditions" form, which has input fields to specify muzzle velocity and current atmospheric conditions at the target location. These include altitude, temperature, pressure (not corrected to sea level), and relative humidity. A check box is used to indicate that system should calculate pressure based on altitude.
</font>


Now, I click the next button down "Field Conditions" fill in the Altitude, (NOTE: <font color="red">Unless you don't you the target pressure, all the information on this screen has to be for the actual altitude or elevation that you are zeroing your rifle for. Other wise it would defeat the purpose when you go elk hunting at 10,000 ft elevation </font>.) Temperature, uncheck "Calculate Standard Pressure" unless that's what you intend to use to zero your rifle in. Get a reading from your Kestrel 4000 in in/Hg and fill it in as well as the "relative Humidity". Click Update.

At this point all your "zero in" information has been entered. We're ready to shoot the rifle. But first let's save "sight in conditions", so when you go to 10,000 ft elevation, the program knows at what conditions the rifle was sighted in and compensates for our new conditions.

Click on "Program Options" button, click on "Save Sight-in Conditions" if "Use Field Conditions" is Checked, it already broght in this screen the info. we input. Let's Uncheck "Use Field Condition" so we keep our zero conditons.
When you do that, "Calculate Standard Pressure" gets checked automatically, uncheck it! <font color="blue">The reason you uncheck it is because your Kestrel 4000 can give you the target pressure. If you want the program to calculate the pressure based on the altitude then leave it checked. </font> Click Update and we're done.

At this point I'd click on the save icon.

Let's say now (at the same location) you want to shoot at a rock at 700 yards, here's what I do:

On the opening page, click on "Target Engagement" fill in all the info. in that screen.

NOTE: For wind directions remember you're always in the center of the clock, your target is 12:00 O'clock. If the wind is hitting you 90º to your right then type in Wind From "3". If it's 90º left then type in "9". If it's just passed between 9 and 12:00 you may want to call it "11" and so on.
NOTE: If you're using the "Cosine Indicator" to messure the vertical angle, check the circle next to cosine and if the indicator reads 87 make sure you enter 0.87; if you messure the angle in degrees, then make sure the (deg) is checked first and then enter 30º.

Once all is entered click "Update". Takes you back to the opening screen. Click "Calculate" and dial it in.

Now you want to shoot at 850 yards, in the same screen just type 850 click on "Recalculate" and dial it in.

Now you move to 10,000 ft elevation.

Go to the opening screen, click on "Field Conditions" fill in all info as before, click "Update" click "Target Engagement" fill info. as before, click "Update" click "Calculate" and you're ready to go. If the wind changes, click "OK" click "Target Engagement" make the change, click "Update" click "Calculate" and you're back in bussiness.

You don't have to rezero the rifle, the program remembers your zero conditions and you'll be impressed how now at 700 yards you will dial in less than before.

/ubbthreads/images/graemlins/blush.gif I know this is probably way more than what you were asking but it may help someone else.

Good Luck!
 
Eaglet,

Thanks for the informative reply. I am still confused when it comes to entering altitude, pressure and "calculate standard pressure". I have been entering my elevations (gps) and entering the actual pressure (kestrel 4000) and un-checking the "calculate standard pressure". This is what I used last weekend to sight in my rifle, is this correct? Now when I go to colorado later this fall am I going to enter the new conditions in the same manner as my sight in conditions.

I forgot to mention, I am running Exbal on a Palm if it makes any difference.


Murf
 
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