Station vs absolute pressure

Tempest 455

Tempest 455

Sergeant
Full Member
Minuteman
Mar 4, 2012
1,218
1,214
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Middle TN
#1
Getting ready to head out west in quest of hopefully some 2,000 yard hits with my .338. Plugging data into JMB, I've noticed a significant difference in MOA between these two when shooting at higher altitude.

Example: Checking pressure corrected (station pressure) I need approx 74 MOA for a 2,000 yard shot at 3,200ft. Checking box (absolute pressure) shows I need approx 81.25 MOA for same shot.

I always enter my barometric pressure and elevation via GPS into my mobile app and JBP. My question is, for JBM, do I check or un-check this when entering both station and elevation from GPS?

Thanks!
 
#3
Corrected station pressure is what you see from the news weather man or on a weather station. It is always around 29.9x. It has been "corrected" for altitude. Here in Colorado, the uncorrected pressure or "station pressure" is more like 24.xx. If you have a kestrel, you can give JBM the current station pressure from your kestrel. If you don't have that, use the "stand atmosphere at altitude" box. This will get you close to what your dope will be at elevation. Since you are shooting to 2k, you may need exact data to input, but this will get you close. You may need to calibrate your kestrel at a local airport to get an exact station pressure.
 
#4
Tempest 455 said:
Getting ready to head out west in quest of hopefully some 2,000 yard hits with my .338. Plugging data into JMB, I've noticed a significant difference in MOA between these two when shooting at higher altitude.

Example: Checking pressure corrected (station pressure) I need approx 74 MOA for a 2,000 yard shot at 3,200ft. Checking box (absolute pressure) shows I need approx 81.25 MOA for same shot.

I always enter my barometric pressure and elevation via GPS into my mobile app and JBP. My question is, for JBM, do I check or un-check this when entering both station and elevation from GPS?

Thanks!
Click to expand...

Station and absolute are the same thing. Barometric pressure is station pressure corrected for mean sea level (this is what airplanes use and the local weatherman). Don't use barometric pressure. As delfuego was saying, you can use standard pressures and temps to get you in the ballpark for your location.

standard temp at sea level is 59f at sea level and the standard lapse rate is 5f per 1000 ft increase in altitude.

Standard pressure at at sea level is 29.92 in Hg at sea level and lapse rate is 1" per 1000ft (although it's actually more because there is a temp element to this but this is close enough).

The reason u are seeing such a discrepancy is you cannot use the same number for absolute and corrected. They are not the same thing (unless you are at exactly ISA).

also look here https://www.snipershide.com/shootin...manship-questions/6375549-air-density-factors
 
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#5
If you have an iphone 6 or higher, download a free app called "Barometer". Once you open it once and look at it, all the above discussed terms will make sense. Especially if you live (or open the app) a decent height above sea level.
 
#6
eicas said:
If you have an iphone 6 or higher, download a free app called "Barometer". Once you open it once and look at it, all the above discussed terms will make sense. Especially if you live (or open the app) a decent height above sea level.
Click to expand...

Great app.
Pretty close to the kestrel.
 
#7
I posted this in a related thread elsewhere...

I had a question on whether the sensor was truly internal or if the app was cheating and grabbing the nearest weather station data. I tested by a few methods. I put the phone into "Airplane Mode" and took some readings. Accurate. Although it WILL NOT give you local temp since it DOES appear it snoops the web for that.

i also took it to work and tried it on the airplane. It was accurately reading ambient cabin altitude (in the upper 7000's) compared to the onboard sensor in the cockpit. The airplane only reads to 10' accuracy while the app appears to be to the foot. They were never off more than 20' in the 5 hours I periodically checked them against eachother. Again, no temperature was available as it showed the last location it had cell service.

I'd say thats pretty damn good. Get yourself a digital thermometer, and a gps for altitude and you got a poor man's Kestrel thats just as accurate.
 
#8
eicas said:
I posted this in a related thread elsewhere...

I had a question on whether the sensor was truly internal or if the app was cheating and grabbing the nearest weather station data. I tested by a few methods. I put the phone into "Airplane Mode" and took some readings. Accurate. Although it WILL NOT give you local temp since it DOES appear it snoops the web for that.

i also took it to work and tried it on the airplane. It was accurately reading ambient cabin altitude (in the upper 7000's) compared to the onboard sensor in the cockpit. The airplane only reads to 10' accuracy while the app appears to be to the foot. They were never off more than 20' in the 5 hours I periodically checked them against eachother. Again, no temperature was available as it showed the last location it had cell service.

I'd say thats pretty damn good. Get yourself a digital thermometer, and a gps for altitude and you got a poor man's Kestrel thats just as accurate.
Click to expand...

I was gonna ask about your username. Who you fly for?
 
#11
Lol. Small world. Live in upper midwest. Based in LAX with a crashpad. Beer in Manhattan Beach on me if we link up.

Former TZ. Thats where the EICAS comes from.
 
#13
Why not just use DA? This is why a kestrel is important. Many of the smart phone apps will pull the atmospherics from the closest cell tower which could be different than your current location. I like using density altitude as it's a combination of everything and o don't have to plug a bunch of different atmospherics into my app.


Sent from my iPhone using Tapatalk
 
#14
Darkside-Six said:
Why not just use DA? This is why a kestrel is important. Many of the smart phone apps will pull the atmospherics from the closest cell tower which could be different than your current location. I like using density altitude as it's a combination of everything and o don't have to plug a bunch of different atmospherics into my app.


Sent from my iPhone using Tapatalk
Click to expand...

I use DA for my quick reference DOPe sheets but if I'm doing a shot where I am actually taking the time to use a calculator, I put in everything.
 
#18
Darkside-Six said:
Why not just use DA? This is why a kestrel is important. Many of the smart phone apps will pull the atmospherics from the closest cell tower which could be different than your current location. I like using density altitude as it's a combination of everything and o don't have to plug a bunch of different atmospherics into my app.


Sent from my iPhone using Tapatalk
Click to expand...


The one I mentioned does not. You cant pull in a cell tower at FL370 doing 600kts. However it do what it do, it appears it do it accurately. As far as DA goes, its a value. A combined value, but still a value. The Kestrel is pulling all the same info, youre just not seeing it. I figure the phone is usually on me for the AE app and to order pizza, so might as well put it to use. I have drop cards based on DA I keep zipped in the stock pack for when I want to play rustic. Its all good. However anyone wants to get from "A" to "B" is all part of the fun. Certainly nothing wrong with DA. Its what I blame my crappy landings in Vegas on all the time...
 
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#19
vh20 said:
Is there a difference? I've always reckoned DA as the most accurate predictor of bullet performance, just as we do with our aircraft...
Click to expand...

No not at all, but one column on a spreadsheet is a lot easier than 3. I can set up cards for easy reference at say 500ft increments and get really reallly close. If I need to be super accurate I'll take my time with a full blown calculation, like trying to hit a beer can full of tannerite at a long distance.
 
#20
I think DA is the best predicter. One problem with it is the nomenclature. The non-aviation people are relating things in density. They need to be understanding it as altitude. Low air density altitude is low altitude. Whereas, low air density is high altitude.

I'm all about carrying your atmospheric instrument with you and not getting a station reading from nearby. You do need to give it some time to adjust to the environment you are in. Pull it out of your pocket and it will give you near body temp. Let it adjust to OAT. Don't rely on pres. or temp inside a vehicle with the windows rolled up.
 
#21
sandwarrior said:
I think DA is the best predicter. One problem with it is the nomenclature. The non-aviation people are relating things in density. They need to be understanding it as altitude. Low air density altitude is low altitude. Whereas, low air density is high altitude.

I'm all about carrying your atmospheric instrument with you and not getting a station reading from nearby. You do need to give it some time to adjust to the environment you are in. Pull it out of your pocket and it will give you near body temp. Let it adjust to OAT. Don't rely on pres. or temp inside a vehicle with the windows rolled up.
Click to expand...

I'm not confused at all. I use AB both ways, but I have easy field sheets broken down by DA in 500ft increments for when I need a quick shot or the shot doesn't demand the razors edge of accuracy. But all DA is comprised of is temp, station pressure and humidity which is what I put into AB. It's just doing the DA calc in the background.
 
#22
TacticalDillhole said:
I'm not confused at all. I use AB both ways, but I have easy field sheets broken down by DA in 500ft increments for when I need a quick shot or the shot doesn't demand the razors edge of accuracy. But all DA is comprised of is temp, station pressure and humidity which is what I put into AB. It's just doing the DA calc in the background.
Click to expand...

As you describe, one system without any other inputs is the best way. As long as DA is already calculated correctly and people understand it.
 
#23
Just go with DA, all that matters is the density of the material/air that the bullet is flying through. since DA takes it all into account the actual data (temp, humidity etc) doesnt matter.
 
#24
DA here , sometimes just off a cheap Casio watch , much to the disbelief
of my buddies . I have a cheap hand held wind meter that reads within 5%
of a Kestrel AB : I can't shoot the difference most of the time and certainly
can't guess downrange wind in the hills to 5% at 2000 yards ..... If shits
getting rung it ain't wrong .
 
#25
Eventually you end up at a DA equivolent anyhow, because in the end those are the factors that effect the flight of the bullet. If you draw it out of an instrument that crunches it all in the background and spits out "DA", or if you take the component measurements individually and insert them it doesnt matter. You end up with the same number. You can even use a "chase-around chart" and manually use your precision index finger and get pretty damn close. Aviation folks deal with this stuff all the time, although we dont pay much attention to the process. Its just one of the inputs to the normal calculations that go into performance...even more "in the background" than it is in shooting. Folks often think DA is some kind of "magical voodoo value" that is somehow is the pinnacle ballistic preciseness... Although it may indeed be final value when it comes to measuring air density, there's nothing voodoo about it.

I think the "corrected pressure" value is where the wheels start coing off the bus for most folks. Just remember the main purpose of this is to give the TV weather chic with the big boobs something to spout off that makes her seem intelligent.

There is an old aviation adage: "Measure it with a micrometer. Mark it with a grease pencil. Cut it with an axe." In the end, a few tenths of barometric pressure, or an altitude thats off by a couple hundred feet won't make much difference. Other factors like instantaneous wind, not drinking decaf in the morning...or just plain crappy technique will all add up to more.
 
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