Density Altitude

brian47374

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I found out about Density Altitude about 6 months ago by a friend. I have a couple of questions. First of all, when you zero say in 30-degree weather, do you simply use the density altitude chart or do you have to re-zero in current weather? Logically, I would think that DA is “generic” and can be used regardless of zeroing weather conditions. Second, I have seen where people say do not use DA in your ballistics programs. Why is that? Isn’t that how I get my DOPE? And, should I only worry about DOPE on every 1k of DA or should I go every 250, 500, 750 as well?

In short, I am very curious about all of this using DA. If not through a ballistics app, is there a better way to calculate DA?
 

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Density Altitude Combines

Temperature
Barometric Pressure
Humdity

It's the only time we consider humidity when talking shooting

DA is where the bullet "thinks" it is flying

temperature is a factor built-in but also temperature has a separate component

If you use DA you use DA, so in zeroing it would be the DA at that time. Why we zero at 100 yards is because the external conditions do not have a noticeable factor, it's too short of a distance to have time to work on the bullet.

You gather your dope as normal recording the DA, then you use that particular DOPE when you experience the same values.

Data on Previous Engagement or DOPE

You do everything normal, you just match the DOPE to the DA, then when you see that same DA again you use it.

Only every 1000ft matters, you can round up or down, 5500= 6000ft DA etc
 

brian47374

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Ok.. so I zeroed in 1k DA. Then I plugged in all the different DA's in the screenshot into Strelok using my DOPE. I guess I am still at a loss as to why people say do not use it in ballistics calculators. How else would you do it without doing a whole lot of math? I downloaded all the different DA and copied and pasted them into my DOPE sheet. Am I wrong in doing this because it worked when I was out shooting 1k+.
 

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I have no idea what you are referencing as people not to use it

DA for ballistic calculators was a bit of an afterthought they were originally written to use the raw data, but today it;s not much different to use the DA. In extreme examples, there are variations when you combine extreme temperature swings, but for 99% out there, it's a non-issue.

Not saying the data you posted is correct, but let's assume it is, you created the correct way to use DA, the software manages it and then you can print a hardcopy to reference absent software.

The difference is probably those who want to use software and electronics vs those who do not and want hardcopy data. You are probably hearing this debate.

There is no real rule of thumb with DA, so you have to use software to establish the values minus shooting under those conditions
 
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brian47374

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In the Strelok FB forum, the creator of Strelok was one of the people who said not to use DA in the software. It made no sense to me because it is in the software. I've just seen a few posts on other forums where people say not to use DA in ballistics apps with no real explanation. I just wanted to make sure that I am not missing anything.

I have a pretty extensive printed DOPE chart with DA and wind drifts because I don't want to lean on technology other than my Kestrel for acquiring the DA.
 

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He wants the raw data so apparently he is one of the programs that did not seamlessly incorporate it correctly

My guess's how his software is written, he wants the separate values because his software is not properly breaking it down.

The numbers should be the same, but if he is saying no, people must see a variation. He is probably obsessing over the temperature component variations that can happen.

As an example, and with my limited understanding of the issue, you can have a DA of 3000ft at 30 degrees and 3000ft at -30 degrees and the values are different, but in our normal operating ranges, it's not a real issue. On the edges, DA and Temp can butt heads.
 

Skookum

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The main issue I see with DA is the method of collection.

If all the necessary components are collected at your actual location, it should come out the same. But if your device is getting data remotely from some station somewhere else, it could be an issue.

When using the raw data, it is easier to gather data using more robust devices and methods, and plug in numbers as they change.

For instance, you might have a paper DA dope chart as a back up, but if your Kestrel shits the bed how will you recalculate any changes? Do you have a back up paper DA calculator chart?

With raw data, leave humidity at 50%. You can probably guess the temp to within 5 degrees, and the station pressure isn't likely to fluctuate more than 1/2 inhg up or down from standard for your given altitude. If you want actual numbers, it is easy enough to get a cheap keychain thermometer and a watch that gives station pressure.

The thing that neither DA nor raw atmospheric data will correct for you is the change in powder burn rate due to temperature. This is a separate factor that needs to be considered and is often overlooked.
 
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brian47374

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I have paper DA DOPE chart and I did not use "remote" info for my DA. I used my Kestrel to give me the DA. Then, I reference my paper DA DOPE sheet. I do not have a Kestrel with a firing solution built into it.

Good call on the powder burn rate! How do I go about obtaining that information? I think that a friend of mine who introduced me to DA said something about 25-30fps = 1k DA change... Does that sound right?
 

tsu45

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Buddy you are kind of overthinking this. You are already setup and have a better understanding than 99% of the general shooting population, just forget about the “don’t use DA” aspect.

On powder temp sensitivity, either use a more temp stable powder and forget about it or do some velocity testing at different ambient temps (very far apart ambient temps, don’t get too granular).

The numbers are you are chasing on vertical accuracy at this point are fractions of horizontal wind calling that comes from experience, so get out there and have gun shooting.
 

Skookum

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I have paper DA DOPE chart and I did not use "remote" info for my DA. I used my Kestrel to give me the DA. Then, I reference my paper DA DOPE sheet. I do not have a Kestrel with a firing solution built into it.

Good call on the powder burn rate! How do I go about obtaining that information? I think that a friend of mine who introduced me to DA said something about 25-30fps = 1k DA change... Does that sound right?
Powder sensitivity is heavily tied to case capacity.

With cartridges in the 308 class, you can expect about 0.5 fps change per degree of ambient temp on average.

30-06 class using 60 grains of powder or so, you can expect about 0.7 fps per degree.

Magnum cartridges like 300 winmag often show 1fps per degree or more.

These are averages, some powders are better, some worse. But no powder is impervious once the temp goes above 80 degrees. They all change, and they all change faster than they did from 40 degrees to 80 degrees.

I use 40 degree increments. I want to know the actual change from 20 to 60 to 100 degrees. I can extrapolate between those. All my data is set up with a baseline of 60 degrees @ 2000 ft elevation.

This is just my method and philosophy, I'm not saying it is the only way.
 
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brian47374

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I would like to thank you all very much. It is nice being in a forum where you don't get beat up for asking questions!
 
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Clocked92

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You can buy or just print out density altitude calculators off the web. Very common for General Aviation (recreational) pilots to calculate take-offs and landings considering the aircraft's weight and performance at different airfields.

I like and use JBM because it's fairly good and it's free.

I shot a bunch of matches last year using the JBM DA charts. Occasionally throughout the day I'd check the DA on someone's Kestrel to make sure nothing crazy had changed. It definitely works well once you get them lined up with what you're actually seeing in the field.
 
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AIAW

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Just as a follow-up: There were app versions in the past that only accepted station pressure, temperature and humidity. FFS prefers these today (but it uses the same field for powder temp offset, that’s why). Most all apps today will take a DA number directly, or you can also input raw atmospherics.

I could see where some could be confused when density altitude isn‘t referenced as such.
 

TacticalDillhole

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Using DA is fine in the field, but accounting for temperature changes which affects your velocity which affects your drops regardless of DA is difficult. It’s best to use temp, pressure and humidity (although setting at 50% and never changing it is fine).
 
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TheGerman

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Remember with DA adjustments that temperature gets you twice. You need to look at the change in DA (if its significant) as well as the change in temperature from your zero temperature.

Temperature is both a componant of DA as well as again, a secondary componant to your ammunition velocity. This is why you need to track and record your velocities for a specific bullet/load in your gun at different temperatures to see what your FPS changes are over the temperature differences.

Random made up example:

- You zero'd at 8000 DA when it was 80 degrees out

- You go somewhere that the DA is 2000 and is now 45 degrees out

If you recorded your FPS change and what it is at 45, you'll know your velocity will be down a bit and will make you add .2MIL to get your 'current' zero.

Then you compare your DA chart from 8000DA (your zero on the gun currently, from earlier) and see you have to adjust up yet another .3MIL to make up from the changes in DA.

This would mean that in order for your gun to hit your zero, on the scope you zero'd at the 8000DA, you'd have to add .5MIL.

These numbers are pulled out of the air, but the premise is the same and it works equally while using an app (if you understand what affects what) or using manual data and a DA card.
 
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