Altitude vs. Barometric pressure

[meichele]

OK

The has lit up!!

Mike CR, I see what you are saying. If you cant measure pressure but can moniter the weather you need to be able to change the standard sea level pressure so you can just enter altitude and still get a accurate calc for your area with out using a pressure guage such as the Kestral 4000.

Concider it done!

Thank you for your input!! [img]/ubbthreads/images/graemlins/laugh.gif[/img]

[Mikecr]

Thats right.
Say your nearest weather report calls for sunny skies, 80deg, with a barometric pressure(not station) of 30.55, and the wooden Tikki outside the diner you just ate at had 4000ft carved on it.
Your internally calculated pressure alt for 4000' is 25.84 and you have an resultant RO of .830. But being really sunny & nice out, pressure is actually alittle higher.
30.55-29.92= 0.63"hg higher.

With this correction of .63 entered and added to std pressure at altitude you get 26.47 and apply it to reach actual air density as though it were absolute pressure and altitude had been set to zero. Result is Ro=.850
This higher Ro follows a new/adjusted pressure altitude of 3,353' if calculated in reverse. Again, due to the pressure being high today.

That's all internal though, as the hunter just entered 80deg, 4000ft, and .63" of pressure correction from std(if using altitude and barometric pressure -29.92).
Your program figured out std pressure at altitude and added a pressure correction as though at sea level. Then, using this new pressure continued it's air density calcs.

How this would be interfaced is totally beyond me. But it would prevent human error, as you have locked them out of setting pressure if using altitude, but still allowing a correction to it as appropriate.

[meichele]

Interfacing it isnt a problem. I can insert another "hidden" field that will appear ONLY after you deliberatly select a check box for "If the current sea level pressure is known". Then the most recent pressure (at 0') may be entered. At that point, when an altitude of 4000' is entered, it will be based on the current sea level pressure.

I think it would be easier to just use a Kestral and select the pressure field!! Of course if the pressure box is selected, ALL OTHER OPTIONS ARE NULL AND VOID!

[bailey1474]

If I'm understanding this correctly, one really needs a hand held weather station to get spot on results w/any ballistics program.

I ask this b/c I just got exbal for a palm pilot. It seems to me that in order for the program to be useful at all, it needs to be accompanied by the Kestrel or similar device.

[meichele]

Yes, Those are my thoughts as well. I am trying to make a program that sets it apart from the others. I have had one in the works for a while, but its just now becoming what it should have always been. Ive sold quite a few copies and havent recieved ANY feed back at all. I have no idea if I should flush it or keep it. It did what I wanted it to do (sort of) but now it does exactly what I want it to. Thanks to some of the replies from this topic it does some other things that other shooters might be interested in.

The finishing touches to the reloaders archive will be to incorporate the ballistic calculator into the reticle optimizer so users no longer have to enter their bullet drops manually for the reticle to show them where their bullets will hit. Enter Velocity, BC, BP, elevation, temp, bullet weight, sight hieght, zero, wind speed and direction and hit enter and the reticle will show you the rest.

The cool thing about the reticle optimizer is that if you are contemplating a new scope, and dont know which reticle to get, say the mil-dot or the NP-R2 or the boone and crocket...You can enter your data into all of them and see in picture form which one follows your rifle/load best.

[meichele]

Links to previews of the above topic can be found here:

http://longrangehunting.com/ubbthrea...p;page=0#75065

[JBM]

The real variable here is density. Temperature, pressure and humidity play into it. Temperature and humidity also change the speed of sound which changes the drag (which is a function of mach number, not velocity), but this is less noticable. The problem with using altitude is that you have to assume an atmospheric model an calculate density from it which can be way off. The reason we have these atmospheric models and why altitude plays into it is so that airplane altimeters can be altitude corrected consistently around the world.

As for scope height errors, yes it can affect it, but it should be small at close ranges. The kind of error it adds is an angular error so at farther ranges it gets worse.