Thread: Driving your ballistic app

Nope, pressure is absolute gets rid of altitude effects.

still need temp.
density altitude is the 3 in one

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Except for gravitational attraction which is inversely proportional to distance 😀

Interesting just tried it and yep makes a difference. I have always just guessed temp.
I was incorrectly thinking that if colder air is denser then it would be accounted for in pressure which is a measure of air density.

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V C is correct. Temp can affect pressure but only if the gas is not free to expand like inside a pressure cooker.

PV = nRT

P pressure
V volume
n number of "air molecules"
R = a conversion constant
T temp above absolute zero

The real atmosphere is free to expand so V changes.

At a given pressure (say 850 mbar is typical at 1500m ASL), if the temperature is higher then that pressure can be maintained with fewer air molecules per volume. Its the air molecules which slow your bullet down.


It seems that air pressure doesnt fall off in a simple way wih altitude. So there is a "standard atmosphere" ICAO model which is used to reference the "density altitude". If someone could tell me where to find a table for that, covering hunting altitudes (say up to 4000m) that would be interesting.

Buggered if I can remember off the top of my head but working out density altitude from known variables was an easy formula.

I would have to go back thru my metrology books from flight training.

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Originally Posted by R93

Buggered if I can remember off the top of my head but working out density altitude from known variables was an easy formula.

I would have to go back thru my metrology books from flight training.

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the flash kestrels do that for ya r93!
saves burning up those brain cells that are fast dissapearing

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Dunno if my one does. But it gives me accurate enough readings to shoot as far as I am prepared to.

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mine is the 2500 and does pressure and temp which effectively is all ya need

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Originally Posted by R93

Buggered if I can remember off the top of my head but working out density altitude from known variables was an easy formula.

I would have to go back thru my metrology books from flight training.

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https://wahiduddin.net/calc/calc_da.htm
I used to use this until I noticed how much DA changed though out the day. Sorting out my s4 was as far as I got to having accurate DA in the field but you still must know your real alt and your phone can't be warm from your pocket.

Originally Posted by Dead is better

https://wahiduddin.net/calc/calc_da.htm
I used to use this until I noticed how much DA changed though out the day. Sorting out my s4 was as far as I got to having accurate DA in the field but you still must know your real alt and your phone can't be warm from your pocket.

your right about making sure your phone not been warm.
I have checked against my kestrel and the 3 values are very close to what the kestrel tells me as in it doesn't change the app result in adjustment.

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Warm phone, same goes for my watch

Originally Posted by andyanimal31

mine is the 2500 and does pressure and temp which effectively is all ya need

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Same. It seems to be very accurate.

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speaking of warm, I found carrying the ammo in my top pocket has kept my ES on 7mmremmag down and lifted the FPS.