DA calculation from barometric pressure?

[jeffersonv]

Guys.

According to JBM, it seems that differences in pressure has a pretty big impact on bullet drop at longer ranges.

Is this barometric pressure?

If so, is there a formula to calculate density altitude from a barometric pressure reading?

I don't carry a smart phone because I have made the decision to go through my days keeping my head up rather than committing to a little hand held device. This being the case I am looking for a formula I can work out without using a smart phone or having a computer on hand.

Is this doable?

Or, is the relationship between temperature and altitude so consistent I don't need to worry about pressure readings and can just use a FDAC or similar device?

Thanks.

 

[Jo.]

You can use something like the Density Altitude Estimation Table which is printed on the back of the DTAC.

You also find some good info in this thread.
http://www.snipershide.com/shooting/showthread.php?t=112124




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[proneshooter]

As an FAA certificated airman, I can tell you that barometric pressure is not an input in the density altitude calculation. All that matters are altitude above MSL (mean sea level) and air temperature.

 

[jeffersonv]

That makes it simple enough. Keeping it simple is good, especially for me!

I still gotta wonder what that value for 'Pressure' is about in JBM though. Changing the values for this parameter really makes a difference in bullet drop.

I got this from Wikipedia -

"Inches of mercury, (inHg and "Hg) is a unit of measurement for pressure. It is still widely used for barometric pressure in weather reports, refrigeration and aviation in the United States, but is seldom used elsewhere."

How can this not be an important factor in calculating bullet drop?

Thanks.

 

[alfson]

Proneshooter, you got it backwards. Your density altitude is purely based on absolute barometric pressure and temperature. Which is why depending on the day the density altitude at Pueblo, Colorado can vary from 4000 to 14000 feet above Msl.

 

[proneshooter]

Proneshooter, you got it backwards. Your density altitude is purely based on absolute barometric pressure and temperature. Which is why depending on the day the density altitude at Pueblo, Colorado can vary from 4000 to 14000 feet above Msl.

The FAA says you're wrong. Cut and past this in your browser if you want to learn what DA really is: "https://www.faasafety.gov/files/gslac/library/documents/2011/Aug/56396/FAA%20P-8740-02%20DensityAltitude[hi-res]%20branded.pdf"

I've flown into and out of Front Range airports in summer many times. I know what I am talking about, as it is not only something pilots are tested during almost every flight review, it is also a lot more than an academic subject for us. It can mean the difference between living and dying.

 

[kis2]

jeffersonv, I think of baro pressure as the weight of the air. temp and alt can affect air density (and measuring those together gives you density altitude as previously stated), but so can weather patterns such as a storm being present, moving in or moving out. baro pressure will let you consider the weight of the air aside from its density (high and low air pressure like in a weather forecast).

the two things that affect the baro reading are weather and altitude. you remove altitude from the equation by pretending you are at sea level. for instance, on a kestrel, you set a reference altitude so it can display a baro pressure without regards to your altitude. if you are at 2,000ft, you set the reference altitude for 2,000ft and it displays an altitude-ignored baro pressure considering only changes in weather/air pressure. if you are at 2,000ft and you set the reference altitude 0ft, it will show 'station pressure' or a baro pressure that considers both change in altitude and change in weather/air pressure.

what does all that mean to JBM? if you know the baro pressure for your current conditions (and if you do, it is probably 'corrected', so make sure you have the 'corrected' box checked and not the standard box), you can input that pressure and get some refined results. small potatos though if you consider that a pretty significant shift in pressure is going to change a good 308 loads drop by about 1" at 1kyds. and I think it tends to get eaten up into temp anyway, since warmer days (makes your bullet more effecient) are often followed by higher pressures (makes your bullet less effecient). so meh.

make sure you are using corrected, realistic (somewhere between 29.92 and 30.15 is a good start) baro's in JBM and you shouldn't see much of a difference.

ETA: the only thing I'm really using baro for right now, is so that my kestrel can tell me approximately what altitude I'm at, based on changes in pressure. that way I have temp and alt and can run the density chart sans smart phone. having some idea of what the pressure is likely works good 'nuff for JBM.

That makes it simple enough. Keeping it simple is good, especially for me!

I still gotta wonder what that value for 'Pressure' is about in JBM though. Changing the values for this parameter really makes a difference in bullet drop.

I got this from Wikipedia -

"Inches of mercury, (inHg and "Hg) is a unit of measurement for pressure. It is still widely used for barometric pressure in weather reports, refrigeration and aviation in the United States, but is seldom used elsewhere."

How can this not be an important factor in calculating bullet drop?

Thanks.

 

[jeffersonv]

Yep, I'm still on this one.

Thanks for your reply kis2. I followed what you were saying but what was news to me was your 'small potatoes' comment. I think I was ignorant of how little the barometric pressure actually changes in the real world. I see the record low was 25.69 in the western pacific in 1979 for a typhoon there.

That's a huge difference from punching in random numbers into JBM and getting radically different bullet drops.

And if you are correct that a significant change will result in a 1" difference at 1K yards with a .308, I think I have little to worry about.

Temp and altitude should get me what I need to know I recon.

Thanks again.

 

[Lowlight]

When using software, it is best to use Station Pressure or some programs call it Absolute Pressure.

When you use this, you ignore altitude in terms of the software. The Barometric Pressure tells the software where you are. If you are using corrected pressure like the news gives you, then you have to include Altitude which adjusts it.

DA is a product of Barometric Pressure, Temperature and Humidity which is the only place we need to worry about Humidity.

In most programs when you use Absolute pressure it will gray out altitude so you don't include it. If you have the ability to put in both, Absolute Pressure and Altitude and you do this, you are essentially doubling where the computer thinks you are. So if you are in CO and you put 24.92 for Barometric Pressure, and then put in 5280 for Altitude the computer will think you are at 10,000 feet...

if you are using software, it is smarter to use the raw values and not DA... they were designed to use RAW data, not the DA. It is best to use DA like a Databook, in the field in place of using the complete set of variables. For the Databook Pages I use that were created for me by Impact Databooks, I removed the Humidity block and put DA in... you can set humidity to 50% in any software on its own and it will be fine. But DA Cards are great field expedient ways to adjust for changes in location and conditions.

The chart Jo posted is a great way to figure our your conditions if you dont have a kestrel, or weather meter. You use that, and then record the DA because you dont have access to 100% of the conditions... then if you are in the same situation you can use your DA again, or just like with the FDAC. They are battery-less replacements for a PDA / Kestrel Combo in the field.

With your Kestrel for software and shooting you need to leave it at factory settings. The reference altitude needs to be set to 0 (zero) and the reference pressure needs to be set to 29.92 otherwise you are screwing up the calculations and you have to use "corrected pressure" in the computer. It's best to leave them alone, or reset them to factory settings. This is where it is accurate for shooting.

 

[kis2]

Jeffersonv: I recommend avoiding shooting in tycoons ....when I type that record baro you listed into my calculator, the difference in drop from my corrected pressure today (didn't change temp or anything) was .7mils at 1,000yds. that is using corrected pressure methods, not the absolutes Lowlight listed, just fyi.

With your Kestrel for software and shooting you need to leave it at factory settings. The reference altitude needs to be set to 0 (zero) and the reference pressure needs to be set to 29.92 otherwise you are screwing up the calculations and you have to use "corrected pressure" in the computer. It's best to leave them alone, or reset them to factory settings. This is where it is accurate for shooting.

Hey sir please educate me: What is the danger of using altitude and a corrected pressure over absolute baro? As far as I can tell, it doesn't seem to hurt the calculations at all, but more importantly for a GPS-less fella like me, it allows my Kestrel (which is only a 2500 model) to tell me about what altitude I'm at, allowing me (field software-less like jeffersonv) to pick the right card. Thanks for the help!

 

[Lowlight]

The bullet doesn't care where you are standing, it cares what the air density is that it is flying through.

The reason DA works is, it gives you one number that tells the bullet what it will be flying through. You maybe at 1000ft above sea level but the bullet is gonna take into consideration the other factors like temperature and think it is flying at 3000 ft on that day. In the winter it might feel like 500 ft to the bullet. If you have corrected it to altitude you take that away. Unless you put it in software that way. It's why for years people thought, and the Army manuals incorrectly stated the effects of Humidity. People felt the air was thicker because to humans it felt oppressive. But humid air is less dense to the bullet. High humidity is the bullets friend. Not so much people.

Again, it is why when using Absolute pressure you don't use altitude. We want air density not corrected altitude. Standing at 5000 feet is not the same as shooting at 5000 ft when it is 85 degrees. The bullet feels 8000 ft or there about.

With a DA card you use the DA which like above you can use the chart. You don't need DA down to the foot but really only every 1000 feet, 500 at the most. Any change less doesn't matter.

If you are software-less and not plugging the numbers in a computer it really doesn't matter what you do. But corrected pressure is not as telling as Absolute and you are adding an unnecessary step, altitude to the process. But how you work it is up to you. If it works then don't listen to me. I don't have to shoot with your data.

Altitude to the bullet is merely a function of barometric pressure.


But really without a ballistic computer none of this matters for you, you're just using the low model kestrel to tell you temp as most know the Altitude to within a 1000 ft of where they are standing. So how you use it works fine for that... I stated very clearly, "with software".

 

[kis2]

Copy all, thanks for the explanation. In that case, sans computer and sans GPS, the best way (*as far as I know*) to calculate my DA is to have my Kestrel tell me physical altitude and temp, then reference a chart like Jo's. I can certainly accomplish those within the parameters of 500-1k feet. A couple of verified charts at every 1kft of DA with humidity set around 50% and a computer free guy should be close. Obviously not as precise as real time raw data, but not un-scientific.

thanks again

 

[jeffersonv]

That's the conclusion I am coming to as well.

I am trying to get away with not having a computer and if the lowest pressure reading (yes, during a typhoon) only throws the bullet off .7 mils at 1K yards I'm pretty sure there plenty of other ways I can mess things up more than that on my own. It's also well within an easy correction for a 2nd shot hit.

Anyone know the formula a computer uses for station pressure and temperature? I'm thinking a solar powered calculator may come in handy.

Even so, for battery free calculations I am realistically looking at the FDAC. Just considering my options.

 

[Lowlight]

Jeff, .7 mils at 1k is 25.2 inches.... That is a miss.

If you shoot in the same place all the time the Barometric pressure only changes a bit, if you travel to shoot you have to figure it changes at least 1" of pressure for every 1000 ft of elevation you change. If I was still in CT my dope would be set for 29.92 but here in Denver my pressure is 24.92 that is 5" of pressure that changed. This is significant if I decide to shoot K&M in FLA when compared to my Denver dope.

Shooting in Pike national forest here 1 hour from home the DA in Fall can be 12,000, yet in Byers also 1 hour away it might be 8,000 ft. I have shot in CO with a DA as high as 15,000 and as low as 5000. Run that through your FDAC.

 

[DarkGhost]

Jeff, .7 mils at 1k is 25.2 inches.... That is a miss.

If you shoot in the same place all the time the Barometric pressure only changes a bit, if you travel to shoot you have to figure it changes at least 1" of pressure for every 1000 ft of elevation you change. If I was still in CT my dope would be set for 29.92 but here in Denver my pressure is 24.92 that is 5" of pressure that changed. This is significant if I decide to shoot K&M in FLA when compared to my Denver dope.

Shooting in Pike national forest here 1 hour from home the DA in Fall can be 12,000, yet in Byers also 1 hour away it might be 8,000 ft. I have shot in CO with a DA as high as 15,000 and as low as 5000. Run that through your FDAC.

Like a boss! The more I read on this site, the more I learn. Outstanding!

Sent via UHF communicator doohickey

 

[jeffersonv]

Yes it's still a miss, just not as much of one as I was imagining when I started this thread.

Thanks LL. It looks like you have a fan in NC!

I'm not thinking an FDAC will be as optimum a tool as an I-phone or other hand held device and I am actually looking for a formula I can use to come up with an accurate DA by using the current station pressure and the temperature with a solar calculator.

Anyone have that one?

Other than that, I can use a hand held app or FDAC I know.

Thanks.

 

[kis2]

per some aviation dudes:

part 1: pressure altitude = (standard pressure - your current pressure setting) x 1,000 + field elevation

part 2: density altitude = pressure altitude + [120 x (OAT - ISA Temp)]

they define all the variables and have a good explanation here:
Calculating Density Altitude with a Pencil | Flying Magazine

 

[jeffersonv]

Thanks kis2. That's exactly what I was looking for.

I actually just found it here too -

Density altitude - Wikipedia, the free encyclopedia

Now to work it out a few times to get it down.

 

[kis2]

good luck! I work with a few weather guys, I'll see if they don't have an easier route to DA.

 

[Papa Zero Three]

Here is a nice write up and a dowloadeable cheat sheet.
8541 Tactical - Manual Calculation of Density Altitude

 

[Pinecone]

As an FAA certificated airman, I can tell you that barometric pressure is not an input in the density altitude calculation. All that matters are altitude above MSL (mean sea level) and air temperature.

I hope neither I nor any of my family ever fly with you from a high density altitude airport.

 

[Pinecone]

For aviation, it is easy. To determine pressure altitude, you set the Kollsman window on your sensitive altimeter to 29.92 (standard sea level pressure), and read the altimeter. The altimeter will read the pressure altitude.

Most of the simple hiking altimeters or watch altimeters are "non-sensitive" in that they only read pressure altitude, as there is no way to set the local altimeter setting to correct for weather pressure differences.

 

[RonboF117]

Jeff, .7 mils at 1k is 25.2 inches.... That is a miss.

If you shoot in the same place all the time the Barometric pressure only changes a bit, if you travel to shoot you have to figure it changes at least 1" of pressure for every 1000 ft of elevation you change. If I was still in CT my dope would be set for 29.92 but here in Denver my pressure is 24.92 that is 5" of pressure that changed.

Frank for those that may still be trying to understand the principals here, the pressure change you noted above would equate to a five thousand (5000') foot change in altitude. When changing pressure values, the following is correct. Pressure is delineated by four single-digit placeholders; such as AB.CD. The A place is for 10,000, B is for thousands, C is for hundredths, and D for tens of feet. In your example if you started with an altimeter of 29.92 and ended with 24.92, the difference btwn those two would be 5000'.

You can see this in action with the Kestral 4500. On the altitude page where you input the station pressure, you can see it. if you've shot many times in the same spot, as you input the correct station pressure, you will see the correct altitude above sea level now be displayed with an error of +/ 10 feet or less. All the other caveats apply with making sure you are inputting the correct data on the correct page using altitude of zero and no sync for those points, on that page.


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