Range Report Density Altitude how-to, I got this right?

[RECKON SO]

I have been searching/reading about making drop cards with DA, and I have made myself a "Barney style" checklist to work some up. I need to know if I an missing something before I get started. I dont have any sort of PDA, but I do have a kestrel, thats why I am interested in making these drop cards.

1. Get info from my specific load on how it acts at different temperatures and form a sort of temp vs. muzzle velocity ladder to have good #'s to put into the ballistics program.

2. Work up some different weather scenerios for my part of the world using temp,hum.,station psi

3. Plug these scenerios into the DA calculator that Lindy provided in one of his articles to get DA values for each one.

4. Use the same scenerios entered in my ballastics program to get drop values.

5. Make drop cards with DA values referenced to the drop values.

6. Use the kestrel in the field to get a DA reading and refer to the cards.

I read in Lindy's article that you have to be sure that the measuring device you are using is calculating DA the same way that the DA calculator is. Does anyone know if the kestrel 4000NV calculates DA correctly?

MUCH OBLIGED

 

[Victor N TN]

Re: Density Altitude how-to, I got this right?

I can't answer many of your questions. But I do know the SKYMASTER unit I have doesn't have the Density Altitude system. The next one I get will be a Kestrel 4000 or newer.

I have a notebook that goes to the range with me. I record all the data and when I get home I figure the DA on the computer. But this doesn't help when you're in the field. I'm sure Lindy will probably chime in. He knows more about actually using it. In my case it's mostly theory.

 

[RECKON SO]

Re: Density Altitude how-to, I got this right?

I write all that stuff down too, but I have never had the presence of mind to go back and compare real time drops while firing to the conditions at the time. I spec you could make some sort of chart with all this in there if you had enough recorded info to cover several different weather conditions. Range time for me thus far has been hit and miss but Im hoping to get stuff squared away soon so I can go on a regular basis and make them count each trip. Thats why Im trying to get a leg up on as much as I can before hand.

 

[DaveV]

Re: Density Altitude how-to, I got this right?

The beauty of using DA is that it combines multiple atmospheric data into one usable number. Unless you are going from one extreme location to another,however, the values don't change that much up to 1000 yards. Past that, the numbers become very significant. Your thoughts on developing a DA ballistic chart are correct. JBM has a function for this, but I found that creating my own on Excel is a little more user friendly. The important thing is to go out in the field to confirm your chart data. You can alter muzzle velocity or B.C. in order for your chart data and field data to match.

 

[Lindy]

Re: Density Altitude how-to, I got this right?

The Kestrels do calculate DA correctly. They include humidity in their calculation, which is ignored in the ICAO Standard Atmosphere.

The easy way to do a DA elevation table is to use the JBM Trajectory Card for DA. It's here:

http://www.jbmballistics.com/cgi-bin/jbmcard-5.0.cgi

You can put your change of muzzle velocity with temperature in, too.

See the notes on that on my web page, which I recently added some information to about the JBM page:

http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/DA/

 

[RECKON SO]

Re: Density Altitude how-to, I got this right?

Lindy, I went to the JBM page that you listed and put in all my numbers and I got some really strange outputs in the trajectory card. So I went back and read all of the small "?"'s beside each input and made sure I had put the right numbers in, and to the best of my knowledge I did. I set the ranges for 100,500,1000 just to get a feel for the program, and it gave two values for each yard line, one in gray and one in white. What I am having trouble with is at 100 it gave me 34.4 in gray and -0.0 in white, and at 1000 it gave me 7.7 in gray and 0.6 in white. I dont understand how to take these numbers, I was thinking that the 1000 yard values should be higher, I know I am missing something simple but I cant figure out what it is. I set the output to .25 MOA, what am I missing?

AND MUCH OBLIGED FOR ALL THE RESPONSE AND HELP

 

[30cal user]

Re: Density Altitude how-to, I got this right?

Is Density altitude as simple as:

Low DA (-1500 ft) = more drag/friction in the air

High DA (7500 ft) = less drag/friction in the air

???

by the way, this morning in south Louisiana it was 27 F and a -2345 DA reading according to my Kestrel.

 

[skinnypitt]

Re: Density Altitude how-to, I got this right?

The density altitude is the calculated altitude based on the things that effect your bullet drop- temp, and humidity. It is different from actual physical altitude because it is actually a pressure altitude adjusted for pressure; so as the humidity and temperature goes up the density of the air goes down but the pressure goes up (High DA) and thus less drag on your bullet.....

 

[sosicmcise]

Re: Density Altitude how-to, I got this right?

I made my DA chart the same way as outlined in the original post, the DA calculator I used included humidity along with the other fields. It's worked very well all the way out to 1000 yards with a .308 and a 175smk @ 2700 fps. Ive used it from about -1800 to +3000 DA and it's tracked right along with the Horus Atrag software that I bring to the range with me. My elevation is usually within +/- 1 MOA from my intended point of impact.

 

[howdydoit]

Re: Density Altitude how-to, I got this right?

tag for future referance.

 

[Lindy]

Re: Density Altitude how-to, I got this right?

Just to clarify this a bit, density altitude is a single-number representation for air density, which is calculated using temperature, atmospheric pressure, and humidity.

The higher the density altitude, the lower the air density.

As temperature goes up, air density goes down, which by itself will cause density altitude to <span style="font-style: italic">increase</span>.

As atmospheric pressure goes up, air density goes up, which by itself will cause density altitude to <span style="font-style: italic">decrease</span>.

As humidity goes up, air density goes down, which by itself will cause density altitude to <span style="font-style: italic">increase</span>.

For those familiar with mathematical terminology, density altitude is directly proportional to temperature and humidity, and inversely proportional to atmospheric pressure.

 

[Wade]

Re: Density Altitude how-to, I got this right?

Tag. Good Stuff!

 

[coues7]

Re: Density Altitude how-to, I got this right?

For additional information you could do a search for coues7 and Lindy has given additional info on DA on a post I started some time ago. Great topic.

 

[MrButterpants]

Re: Density Altitude how-to, I got this right?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lindy</div><div class="ubbcode-body">
For those familiar with mathematical terminology, density altitude is directly proportional to temperature and humidity, and inversely proportional to atmospheric pressure.
</div></div>

You're dead sexy when talking math.....Lucky I'm a married man.

 

[Lindy]

Re: Density Altitude how-to, I got this right?

Lucky for both of us...

 

[AKAce]

Re: Density Altitude how-to, I got this right?

OK, So in Dumbass terms when my Kestrel gets here (2500) what do I need to set it at ?

 

[Lindy]

Re: Density Altitude how-to, I got this right?

The Kestrel 2500 does not do a Density Altitude calculation.

 

[DaveV]

Re: Density Altitude how-to, I got this right?

No worries, AKAce. Get on Lindy's site and follow his instructions for calculating DA. Not as handy as the Kestrel quick read, but same math.

 

[30cal user]

Re: Density Altitude how-to, I got this right?

this morning in South LA:

21F

30.18 BP

98.17% Humidity

-3123 DA

2.3 MPH Wind

Doesnt get to be 21F around here very much....Im sure know one cares...