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Range Report Making Ballistic Cards Using Density Altitude

Lindy

Lindy

Gunny Sergeant
Full Member
Minuteman
Mar 26, 2003
4,002
14
Rifles Only
www.arcanamavens.com
There have been several recent discussions of density altitude. There is at least one commercial firm which will make ballistic cards which use density altitude for you, if you give them your load data - for a fee.

But what if you are cheap, like me, want to change your load, or want cards for another load?

In one of those discussions, I described how to make your own. Upon reflection, however, I realized the instructions for that were not very clear.

So, I built a web page with instructions on how to make your own density altitude ballistic cards, which I hope are more clear.

Here's the link:

Making Ballistic Cards Using Density Altitude

You will still, for the moment, be dependent on a Kestrel or similar device to calculate the density altitude for you, but there is a reasonably accurate manual method for calculating density altitude, and I will add that method to the page later, or make a separate page for that and link it to this one.

I hope you will find it useful.

 
Re: Making Ballistic Cards Using Density Altitude

Nice! Thanks. I was wondering how to apply what Jacob was talking about.
cool.gif
 
Re: Making Ballistic Cards Using Density Altitude

Always a wealth of knowledge. Thanks!
 
Re: Making Ballistic Cards Using Density Altitude

Nicely done. I'm curious, why don't you just use a ICAO Standard Atmosphere table?

ICAOStdAtmosphere-1.jpg
 
Re: Making Ballistic Cards Using Density Altitude

You could. However, it might be confusing for some people who don't know what it means, and some people might want increments other than even 1000 foot increments, so I wanted them to know the method.

For example, some people might want altitude increments in meters rather than feet. The DA Calculator link I posted also has versions which work in metric units.


 
Re: Making Ballistic Cards Using Density Altitude

Good points. The term, "density altitude" was invented for aviators, but can be confusing as hell for the rest of the world.
 
Re: Making Ballistic Cards Using Density Altitude

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">...mounds of knowledge between those ears...</div></div>

My wife has claimed from time to time that there is something else there.
laugh.gif


Thanks, though!
 
Re: Making Ballistic Cards Using Density Altitude

I don't know where that chart came from, but it's not correct.

The ICAO Standard Atmosphere for 6500 feet has a pressure of about 23.53. The temperature is correct.

The pressure parameter listed with the temperature of 35.8 calculates to a density altitude of 5350 feet.
 
Re: Making Ballistic Cards Using Density Altitude

cant we just use temp and actual altitude above sea level or am i all kinda confused here?
 
Re: Making Ballistic Cards Using Density Altitude

Use it to do what?

The purpose of the page I wrote was to explain a couple of methods of coming up with Density Altitude if you don't have a Kestrel, for use with a Density Altitude dope table, which the earlier article explains how to compute.

You may <span style="font-weight: bold">need</span> to use the temperature and the altitude in doing that calculation, as the page explains.
 
Re: Making Ballistic Cards Using Density Altitude

sorry lindy think your over my head with this at the moment
 
Re: Making Ballistic Cards Using Density Altitude

marduk185: This table will get you close for most purposes.

DAChart.jpg


If you substitute your actual altitude for pressure altitude in this table, you will essentially be assuming standard pressure (a sea level adjusted pressure of 29.92" Hg) at your location. The table also assumes 50% relative humidity and this is good enough except when both humidity and temperature are particularly high.

To use the table, start with your temperature on the bottom, follow that up to the angled blue line that represents your altitude, then read DA horizontally to the left.

 
Re: Making Ballistic Cards Using Density Altitude

Thanks, I really like this chart.

I was kind of GUESSING if it was 50-degrees at 10,000' I'd probably want to use my 12,000' DA ballistic card. It's nice to verify.

Nice to have an idea ahead of time which DA's I'm likely to encounter.

Mike
 
Re: Making Ballistic Cards Using Density Altitude

I have a new page up for calculation of density altitude without a Kestrel.

It has two additions. One is a table which shows pressure altitude as a function of station pressure.

The other is the chart rmfield posted above, which he has kindly granted me permission to include.

See:
Manual Calculation of Density Altitude
 
Re: Making Ballistic Cards Using Density Altitude

Lindy, Thanks for the well written article. I'm still learning all this and trying to get my head wrapped around it, so please excuse my ignorance.....

BUT.....I still don't understand why density altitude is needed if you have the station pressure, as the only thing affecting bullet flight is the density of the air, the bullet could care less how high you are or how cold/hot it is, only how hard it is to push through the air.....

What am I missing? Is the force of gravity enough greater at lower altitudes to pull downward harder on the bullet? I would think this would be a negligible factor except at extremes.

Again, what am I missing?

Thanks, Brad
 
Re: Making Ballistic Cards Using Density Altitude

Brad:

The three things which affect air density are temperature, pressure, and moisture content.

Density altitude is simply a single-number representation of air density.

If you have dope from shooting your rifle at every possible combination of those three things, you would need neither a ballistic program running on a computer, nor a density altitude dope table.

But that's obviously impossible.

Since a shooter may need to make a shot in an environment that he's never been in before, we need some way to predict ballistic performance in that environment.

One way to do that is to put those three variables into a computer program, whether that be a P.C. or a PDA in the field.

Another way is to have previously computed a DA table, and use either a Kestrel or similiar weather meter - which calculates DA - or use the other methods of obtaining DA which I explained in the second article.

It's just two ways of achieving the goal of being able to predict ballistic performance in a range of environments.
 
Re: Making Ballistic Cards Using Density Altitude

The light just came on, I was getting air pressure and air density confused....

If the temp drops but the station pressure stays the same, the air becomes more dense...

I had it in my mind that as long as station pressure remained the same, the density was the same.

Thank you sir, for turning on the light switch!

Brad
 
Re: Making Ballistic Cards Using Density Altitude

You got it!

As temperature increases, air density decreases.

As air pressure increases, air density increases.

As moisture content increases, air density <span style="font-style: italic">decreases</span>, although that's counter-intuitive, as it seems like more humid air is heavier - but it's not.

Glad to help, Brad!
-- Lindy
 
Re: Making Ballistic Cards Using Density Altitude

Brad this is the same system I use for my drops on both my .308's you've seen. Make sure you remind me during the next match.
 
Re: Making Ballistic Cards Using Density Altitude

Oustanding! So for JBM you just change the pressure and temp's like the chart on your page?

"If you are using JBM Ballistics, in the section on atmosphere, put in the same temperature, pressure, and humidity you put in the DA Calculator, leave the altitude set to zero, and uncheck both the box which says "Std. Atmosphere at Altitude", and the one which says, "Pressure is Corrected"."

I suck at spreadsheets, how can I generate a nice table like you have in the article Lindy?

Thanks!

 
Re: Making Ballistic Cards Using Density Altitude

JLM:

Yep, that's how to use JBM.

There's nothing complicated about the table. I just put the data in an Excel spreadsheet, and adjusted the column size to get the spacing I wanted.

For that matter, you could put the data in a word processing program for printing.

 
Re: Making Ballistic Cards Using Density Altitude

Thanks a bunch Lindy!

 
Re: Making Ballistic Cards Using Density Altitude

Interesting, just did a 'test run' if you will, and my results with JBM don't match my DTAC cards for my load. Wonder why that might be. Did 4000k DA 175SMK at 2650 FPS 1.65" HOB on the tube.

DTAC card shows about 5MOA more drop at 1k.

It looks like JBM rounds the pressure to two decimals could that make a difference?

 
Re: Making Ballistic Cards Using Density Altitude

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: JLM</div><div class="ubbcode-body">Interesting, just did a 'test run' if you will, and my results with JBM don't match my DTAC cards for my load. Wonder why that might be. Did 4000k DA 175SMK at 2650 FPS 1.65" HOB on the tube.

DTAC card shows about 5MOA more drop at 1k.

It looks like JBM rounds the pressure to two decimals could that make a difference?</div></div>

I ran JBM against the online DA calculator Lindy references a while back and found its output and that from JBM didn't match, reported it, and it was fixed - this was quite a while ago BTW. All subsequent runs matched my DTAC cards virtually exactly across the board.

BTW, for those who don't know, DTAC cards come with a handy card to manually determine DA if you don't have a Kestrel. And of course it matches up nicely with my Kestrel as you'd expect from Mr. Cole's quality of work.

Anyway, I built a spreadsheet by running JBM repeatedly to fill the DA data points over range and then ran a linear regression on the data discovering the change for any given absolute elevation is linear with DA, however the slope is different for each elevation.

From this it was possible to generate a simple BDC that places a number above each distance numeral indicating how many clicks per 1000ft of DA change from 0K ft to make or, recently as shown in another thread in this forum, how many feet of change in DA it takes before a single click (in this case a tenth of a mil) is required to regain idealized zero. The form is that of the PR Heritage 22mil ST turret. I think the second method would be a lot easier to use. And the linearity is good enough that interpolation is plenty accurate for minute-of-man use, <span style="font-style: italic">I think</span>. Changes in MV can be linearly related to a change in DA but a low ES load would obviously be prefered.

A number of people got copies of the original spreadsheet and it has calculations for meters and yards but is set for a 300m zero. I recently modified the tables for a 100yd/m zero to create a more generally useful, to me, BDC. And is the spreadsheet used to gen the Heritage BDC graphic.

For those who use a databook to enter shooting results and conditions I figured it'd be easier to build a table for each ballistic combination (308 175 2650 for example) having a row for each range in 100m increments (in my case) and a column for each DA from -2K to +9K, again in my case. The intersecting boxes provide a place to enter a +/- click correction as data is collected over time for each point in the table. Using pencil to fill in the table allows for correction as the barrel wears, loads change, etc.

This way I can use a RITR field notebook and cover reducing shooting bag space considerably. And a MilDot Master fits in the pocket like it's a custom fit. Some more work with a ruler and barrel logs are easily added, too. Zak Smith made a reference to this RITR product, glad I was paying attention. The pages are perfed so if you make a mistake, rip it out and start over. Being bound there're no rings to loosen, no pages to lose, etc. Not the easiest to write in if you use both sides of each sheet but it'll do.

BTW, all the calculations for idealized come-ups assume a low MV tempco, as Lindy (and others) has often pointed out as important, and I see no simple way to add this degree of freedom to the method I'm currently working on so it comes down to powder choice. That and the relative importance of real-world error from the calculations compared to error sources not modeled by JBM or created by my drag on the trigger, cant variation, recoil response, parallax error, etc. For really long range there's the ballistic computers so my databook table format doesn't extend beyond 1400m and that's questionable ime for the 300WinMag data. Data doesn't extend beyond 900m for 308 and 500yds for 25-06, for example.

Oh, and thanks Lindy for a great, concise article - definitely sticky worthy IMHO.

FWIW,
Pete
 
Re: Making Ballistic Cards Using Density Altitude

Thanks, Pete.

Before you go to build the DA table, you must first ensure that the output of whatever ballistic program you use matches your real-world shooting data.

That may require manipulating B.C.s, muzzle velocity, or both, and is preferably done using long-range data, because that's where the accuracy is most important. I use 1000 yard data to do that.

Once that's done, then your DA table <span style="font-weight: bold">will</span> be accurate.
 
Re: Making Ballistic Cards Using Density Altitude

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lindy</div><div class="ubbcode-body">Thanks, Pete.

Before you go to build the DA table, you must first ensure that the output of whatever ballistic program you use matches your real-world shooting data.

That may require manipulating B.C.s, muzzle velocity, or both, and is preferably done using long-range data, because that's where the accuracy is most important. I use 1000 yard data to do that.

Once that's done, then your DA table <span style="font-weight: bold">will</span> be accurate.
</div></div>

Absolutely. It's very difficult to normalize shooting conditions of each session across sets of data though, error stackup and all. Might start out with an idealized BDC and analyze the correction table over time to create a gen 2 BDC turret. Could be more time efficient. Or aggravating, flip ya' for it.

The idealized BDC isn't much different from carrying DTAC or DEMIGOD cards I figure except the derived linear regression characteristic allows for all the cards' data to be compressed right into the extra BDC engraving. Pretty good lossless compression algorithm I think. Anything to lighten the load and simplify the process. And that's what spotters are for anyway, right?
whistle.gif


Tnx,
Pete
 
Re: Making Ballistic Cards Using Density Altitude

My first round hits went up a lot using the jbm DA cards - great quick data source as long as you know the DA. I have 3 columns for temp correction (for muzzle velocity changes w/temp that is) and this helped alot too.
 
Re: Making Ballistic Cards Using Density Altitude

Lindy,

Entering the data and receiving the output as seen below I have a few questions.

jbmcard01.png



jbmcard02.png



I legitimately have no muzzle velocity change over the temperatures entered (20F - 80F). If temperature is only entered so jbm can plot our results based on our own muzzle-velocity-change-as-a-function-of-temperature curve, then why is jbm calling for more correction for warmer temperatures? My MV isn't changing and the DA hasn't changed as the temperature went up (since it isn't using temp to calculate DA). Aside from that, the correction would go the opposite way if it was using temp to calculate DA. I'm thoroughly baffled at these results. I'm trying to get some cards made up for a shoot on Thursday and I'm at a stand still. Any help you can offer would be appreciated.
 
Re: Making Ballistic Cards Using Density Altitude

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">why is jbm calling for more correction for warmer temperatures?</div></div>

One thing that comes to mind is temperatures affect on the speed of sound. In warmer air, the speed of sound is faster, so the bullet would encounter 'transonic' effects sooner, thereby causing more drop at the higher temperature. The glitch in this reasoning is that your solutions begin diverging at 600 yards. It seems like the bullet would be far enough away from sonic at that range.

I suggest contacting Brad directly and asking him what's up. I don't use the DA card program so I'm not familiar with it.

-Bryan
 
Re: Making Ballistic Cards Using Density Altitude

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Bryan Litz</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">why is jbm calling for more correction for warmer temperatures?</div></div>

One thing that comes to mind is temperatures affect on the speed of sound. In warmer air, the speed of sound is faster, so the bullet would encounter 'transonic' effects sooner, thereby causing more drop at the higher temperature. The glitch in this reasoning is that your solutions begin diverging at 600 yards. It seems like the bullet would be far enough away from sonic at that range.

I suggest contacting Brad directly and asking him what's up. I don't use the DA card program so I'm not familiar with it.

-Bryan</div></div>

Bryan,

Thanks for taking a look at this. I did send Brad a "bug report", though I doubt it's actually a bug. I'm sure he's been through this thing backwards and forwards. Hopefully he responds soon enough that I can get these made up properly for my shoot.
smile.gif
 
Re: Making Ballistic Cards Using Density Altitude

my point of wiev is you have forced some values in a not correct way.
If you use 2900 with 20 F, you cannot have 2900 with 80 F... so you create may be a bug in the program.

tell us more when you get answer from JBM

thanks
 
Re: Making Ballistic Cards Using Density Altitude

I don't have time to experiment with JBM right now, as we are starting Phase 1 of the Fall Shooter's Batch today, but if it's a bug in JBM, it's a recent one. Look at my web page at the output sheet of JBM, and the dope goes down as the temperature goes from 50 degrees to 80 degrees, as it should.
 
Re: Making Ballistic Cards Using Density Altitude

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: davide</div><div class="ubbcode-body">my point of wiev is you have forced some values in a not correct way.
If you use 2900 with 20 F, you cannot have 2900 with 80 F... so you create may be a bug in the program.

tell us more when you get answer from JBM

thanks
</div></div>

I can and do have 2900 at both temperatures. Varget is very temperature insensitive and through testing I've concluded that over that spread I have no measurable change in velocity. If you wish to loan me a Oehler 35P I'll do a little more checking, but until then my CED will have to do the job.

I've tried going with a 5 or 10 degree fluctuation and it did not remedy the situation.
 
Re: Making Ballistic Cards Using Density Altitude

Thanks for sharing this info, this is an area that a lot of long range shooters are not up to speed on, and this should help a whole lot!

Best of Luck,
M Richardson
 
Re: Making Ballistic Cards Using Density Altitude

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: brasscow</div><div class="ubbcode-body">
davide said:
my point of wiev is you have forced some values in a not correct way.
If you use 2900 with 20 F, you cannot have 2900 with 80 F... so you create may be a bug in the program.

tell us more when you get answer from JBM

thanks
</div></div>

I've only a PVM-08.... but I'm thinking about your problem and for me if you want to use density altitude you have to use only this value and disregard temperature... so if you put for example 59 F in all the fields and for zero altitude 0....like a standard day, and then your value from 600 y to 1200 and density altitude value from -3000 to + 2000 you can see the correct dope...positive density altitude lesser dope, negative density altitude higher dope.... it works for me.
I'll use temperature only if my powder is temperature sensitive, and in this case I use it, I've seen that with my load I've 0,4 m/s every 1 C.
Click to expand...
 
Re: Making Ballistic Cards Using Density Altitude

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Bryan Litz</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">why is jbm calling for more correction for warmer temperatures?</div></div>

One thing that comes to mind is temperatures affect on the speed of sound. In warmer air, the speed of sound is faster, so the bullet would encounter 'transonic' effects sooner, thereby causing more drop at the higher temperature. The glitch in this reasoning is that your solutions begin diverging at 600 yards. It seems like the bullet would be far enough away from sonic at that range.

I suggest contacting Brad directly and asking him what's up. I don't use the DA card program so I'm not familiar with it.

-Bryan </div></div>

Sorry, I've been pretty busy with work lately.

Brian nailed it. The speed of sound changes with temperature and my calculators account for that. You're just seeing it a little more clearly because you have multiple trajectories on one card.

The speed of sound is proportional to temperature (square root of temperature to first order). As temperature goes up, speed of sound goes up and mach number of the bullet falls (mach number = velocity / speed of sound). This puts you on a different part of the drag curve which has a little higher drag and you see a little bit more drop at the higher temperature (for the SAME density altitude which is the same air density). It's really pronounced here because the calculator uses the altitude for density so you have the same density for both temperatures. One can argue if that's realistic or not, but it is what it is.

Thanks!

Brad
 
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