Station Pressure or Density Altitude?

[Shabazz]

Please forgive a possibly dumb question but I am totally new to ballistic calculators. I just received my kestrel 4500nv and I downloaded Applied Ballistics to my iPhone. I have the option of using either measurement and I'm wondering if I should be choosing one over the other. Will one give me more accurate or reliable results, or it doesn't matter?

 

[Snapshot]

Density altitude considers barometric pressure, temperature and humidity; station pressure is barometric pressure only.

 

[Shabazz]

Density altitude considers barometric pressure, temperature and humidity; station pressure is barometric pressure only.

I was under the impression that Station and Barometric pressure are different things, and that if I use station pressure the altitude is irrelevant anyway. Is that not correct?

 

[Juster]

ht tp://www.gun-blog.com/2011/04/two-kinds-of-pressure.html
There is "station" pressure - the actual air pressure in the measurement location, unadjusted for altitude. If you have a Kestrel Pocket Weather Meter, you would leave the altitude at zero.

"Barometric" pressure is air pressure adjusted for altitude by a fairly simple formula. This is what weather reports usually use. With the Kestrel Pocket Weather Meter, you would enter the actual altitude.

So if you have a number for pressure, is it Station, or adjusted to sea level? And what does your ballistics program use? If the program has an "Altitude" box, it is Barometric, adjusted to sea level.

If the program has no altitude box, it is Station pressure.
ht tp://www.instesre.org/Aerosols/pressure.htm

 

[kombayotch]

Borrowed from Zak Smith's site:

What is Density Altitude?

Environmental conditions are often described in terms of pressure, temperature, and humidity. Pressure is reported both as "station pressure", which is the actual barometer reading, or as a corrected number normalized to sea level based on the actual location altitude. With these three numbers, the shooter can cross-reference similar conditions in his log book, or he can use the parameters as input to a ballistic program to generate the drop values for his load.

Keeping track of three independent parameters can be confusing. For a log-book, the shooter can end up with a lot of different environmental conditions, which are hard to match. If he runs data from one of the ballistic programs, he will have to sweep three variables to generate data for all likely conditions. There is also an overlap in pressure and temperature mapping to air density. For example, a cold 20-degree day in Denver will have about the same air density as a hot 95-degree day in Dallas.

One solution to this is a figure called "density altitude." Density altitude is one number that represents the density of the air, which is all the bullet cares about anyway, and it effectively replaces the need for the three original parameters.

 

[Shabazz]

Thanks Juster and Kombayotch, I now understand the benefits of density altitude. One more question, If I want my kestrel to show an accurate DA I know that my reference for pressure should be 0, but what should my reference be for my altitude? 29.92 inHg?

 

[kombayotch]

From the Kestrel FAQ:

What should I set as my reference altitude if I use density altitude for my ballistics program?

The Kestrel will always display the correct density altitude no matter what is entered as the reference altitude. Density altitude is calculated from temperature, humidity, and station pressure. Density altitude data is NOT impacted by reference altitude.

 

[Shabazz]

Thanks again!

 

[J996]

For anyone who has an old basic Kestrel (like me) and happens to use the Sierra 175 match king, you can do it cheap and get the FDAC by adaptive consulting (40 bucks us).

All you need to know is your physical altitude, the air temp and your MV to use it.

 

[flashman]

You want to use station pressure if you can, it's far simpler. The result will be just the same and just as accurate. Ignore humidity, set it at 50% in AB and forget about it. Using DA, you're correcting station pressure to what it would be If you were actually at sea level.

If you set reference for pressure at 0' on the Kestrel, you'll get station pressure, not DA.

 

[flashman]

I was under the impression that Station and Barometric pressure are different things, and that if I use station pressure the altitude is irrelevant anyway. Is that not correct?

Correct, if you use station press, alt is irrelevant. You're just directly measuring the air pressure. Humidity affects things so little you can ignore it. That leaves altitude, which again if you use station pressure you can ignore; you only need to consider it if adjusting station pressure to DA.