Would you be referring specifically to the FDAC card? Or if not, what are your thoughts on that type of system?
Over-thinking barometric pressure...
- Thread starter Dan Newberry
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Would you be referring specifically to the FDAC card? Or if not, what are your thoughts on that type of system?
No, because I don't use it so I cannot perform some reverse engineering testing (needed to understand how the AIR DENSITY is computed) but I think it's a viable option if the preset conditions are a close match to your system and you keep your shots inside 1000 yards or about.
I agree with Lowlight in the robustness of Patagonia's solution where there is no compromise at all, so it's a 100% flexible and solid solution vs. a predefined set like the FDAC cards.
In reference to your opening post; i'd like to add that if you happens to only know ALTITUDE, the use of the ICAO standard conditions (for a given Altitude) is a viable option that will work for medium ranges surprinsingly good. Something that is often overlooked by a vast majority.
As I calculated, the difference at Dan's range, out to 1000 yards from the highest to the lowest pressure was 0.1 mils with a .308.
So the difference from ICAO standard pressure is about 0.05 mils. Hard to dial that.
Run the numbers.
So the difference from ICAO standard pressure is about 0.05 mils. Hard to dial that.
Run the numbers.
Out to 1000.
User manual here:
http://actsvirginia.com/wp-content/uploads/2012/03/fdac_user_guide.pdf
The XR version of the FDAC (i think it's the "mil-spec" model) goes to 1500 meters. I picked that one up at a prize table, but later got rid of it. The machine of the FDAC is fantastic, I LOVE the basic mechanism. But the limited bullet, fps, wind and target speed, and zero options available ultimately led me to my own system that I mentioned above. As was pointed out, you can get custom drop charts made but they don't drop into the FDAC and despite my best excell-fu, I couldn't make one that worked. If not for those criticisms, i would still be using it.
The card submitted to me for review ended up being in meters. We're still in America, for the short time being. So I didn't end up testing it. There's a card in "American" on the way. Yeah, I know, flame away...
Yeah, I know, flame away... 
The card submitted to me for review ended up being in meters. We're still in America, for the short time being. So I didn't end up testing it. There's a card in "American" on the way.
What Lowlight said. Pretty much everything in the Army is meters for distances. The only time I can remember hearing yards being mentioned was in SOTIC/SFSC but you have the option of using which ever you feel more comfortable with. Needless to say, most stick with meters since everything else we do from Arty to mortars also uses meters. Everything else though is business as usual, we still use inches, lbs etc. Meters and "clicks" (kilometers) make a lot more sense from a ground pounders perspective than yards or miles, especially when dealing with other nations.
So I was just reading a bit more about atmospheric density and finally understood something that i knew to be true, but seemed backwards. Namely, why does air density go down (very slightly) when humidity goes up? Water in the air would make it thicker, right? Well putting aside a lesson on partial pressures and Avogadro's law, it comes down to molecular mass. The assumption that I had unconsciously was that water was more massive than air... not so. Water=18g/mol, dry air=29g/mol. Of course humidity has an extremely small influence for us but...
Anyway, thought that would go well with the "over-thinking".
Anyway, thought that would go well with the "over-thinking".
Water vapor and density is a topic many have trouble with. Humid air is less dense, but it doesn't really make too much difference on the whole picture.
Gi'day...
The bigger influence on humidity is temperature.
The warmer the air, the amount of water it can hold increases dramatically, so the RH for that volume of water sinks very quickly...
Air at 0°C (32°F) has 6,088 PPM (volume) of water in it at 100% RH, and at 20°C (68°F) that 6,088 PPM is only 26.1% RH.
100% RH at 20°C (68°F) is the same as 24.4% at 45°C (113°F)
Basically doubles for every 10°C (18°F)...
The bigger influence on humidity is temperature.
The warmer the air, the amount of water it can hold increases dramatically, so the RH for that volume of water sinks very quickly...
Air at 0°C (32°F) has 6,088 PPM (volume) of water in it at 100% RH, and at 20°C (68°F) that 6,088 PPM is only 26.1% RH.
100% RH at 20°C (68°F) is the same as 24.4% at 45°C (113°F)
Basically doubles for every 10°C (18°F)...
Its honestly almost the only thing I use the Kestrell for besides getting a wind reading at the rifle and comparing it to conditions along the way to the target.
I pretty much turn it on, leave it on a bit, check DA when it has settled, adjust against my 'base' dope DA and shoot. The data book plays a part AFTER you shoot an off-dope shot to where you can now reference your new data after the fact; I will now enter results in it for the adjusted dope to compare what happened against what should have happened according to the DA adjustment information.
The way it was always explained to me was the DA is a colmunation of all of the exterior enviromental variables except wind, slope/angle and spin drift in one number to use as a reference point and adjust via the change in only that reference point.
I pretty much turn it on, leave it on a bit, check DA when it has settled, adjust against my 'base' dope DA and shoot. The data book plays a part AFTER you shoot an off-dope shot to where you can now reference your new data after the fact; I will now enter results in it for the adjusted dope to compare what happened against what should have happened according to the DA adjustment information.
The way it was always explained to me was the DA is a colmunation of all of the exterior enviromental variables except wind, slope/angle and spin drift in one number to use as a reference point and adjust via the change in only that reference point.
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