Altitude vs. Barometric pressure

[Michael Eichele]

OK

The light bulb 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!! /ubbthreads/images/graemlins/laugh.gif

 

[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.

 

[Michael Eichele]

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.

 

[Michael Eichele]

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.

 

[Michael Eichele]

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

http://longrangehunting.com/ubbthreads/showflat.php?Cat=0&Number=75065&an=0&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.

 

[Michael Eichele]

You are correct in your statments.

This program at this time doesnt utilize humidity. It will in the future. It does however utilize pressure, altitude and temprature. Users may choose between actual pressure at location, which eliminates the need for an altitude input. If users select altitude, then they can enter their local pressure based on wheather reports. The pressure they find on these reports will be corrected from actual to sea level. Either way they choose, they will be getting very accurate ballistics. In the future they will get even better figures when humidity is added. Fortunatly, the differance between 0% and 100% is a mere few inches at 1000 yards with medium calibers.

 

[Ernie]

I appreciate all the input shared in this post.
Now let me make sure I have this straight.
When taking readings (Barometric Pressure) from my Kestral into Exbal and I put the actual reading (BP) from the Kestral in, I leave the altitude on exbal at sea level or zero even though I am actaully at 8,500 ft. Correct?

Next question, is that the only thing I would do different as far as putting in information?

Thanks! I just want to get this straight in my head.

 

[Charles A]

XP, yes. And yes. /ubbthreads/images/graemlins/smile.gif

 

[Ernie]

Thanks for the response Charles.

 

[Eaglet]

Gentlemen,
If I'm wrong I want to be corrected, but it's my understanding that Exbal... let me quote what Perry says about it:
[ QUOTE ]
Altitude (feet)
Used to determine speed of sound. Also may be used to calculate standard atmospheric pressure.


[/ QUOTE ]
Sorry... I did not mean to post it...read my next post.

 

[Eaglet]

Perry also says this:

[ QUOTE ]
A distinctive feature offered by Exbal is the ability to capture and remember sight in conditions. Lets suppose that you establish a great load for your brand new 300 Win Short Magnum with a 180 gr bullet chronographed at 3070 feet per second. Lets say you live in Corpus Christi, Texas which is 50 feet above sea level. Further lets assume you that you sight in your rifle during June when the temperature in early morning (when the wind is minimal) will be about 80 degrees Farenheit. Not only that, but you have established a sight in such that your bullet path is never above three inches high, or below three inches low all the way out to 275 yards. This concept is called point blank range. In this case you could aim at the middle of a six inch target and always strike it if it is within 275 yards.

Preparation: Lets assume you have put a lot of effort into establishing this load and practicing, and have used up two hundred rounds of ammo (and precious barrel life). Now you get the good news that you drew an Elk permit in Wyoming. You will be hunting at 10000 feet elevation and the temperature is typically 30 degrees in late October. Without a ballistic calculator you will have no choice but to try and sight in again when you get to Wyoming. This may not work out too well because you will probably not have a bench rest to shoot from and you will surely make a lot of noise doing it. Trying to sight in across the hood of a pickup truck leaves a lot to be desired.

Exbal offers an alternative to that scenario. If you have Exbal "remember the sight in conditions" you can establish field conditions (altitude, temperature, etc) that match the ones you anticipate in Wyoming. Then Exbal will determine the bullet path that will occur in Wyoming, even though you sighed in at sea level. You can use the Excel interface to create a reference card and print it out and laminate it to carry with you. Now you can take full advantage of all the effort you put into creating your ideal load and you will know exactly how it will perform on your hunting trip. Having one less thing to worry about is a good thing. Thus another reason for using Exbal is that it enables you to get ready in advance.

[/ QUOTE ]

Therefore it's important that when using Exbal you input the elevation you zero-in for including barometric pressure, temp. and humidity... you save this info. and when you move to different elevations you input your new information including elevation and Exbal will give you the right info. If you make the Elevation zero then you're defeating the purpose.
Remember, for barometric pressure you want to enter is not the corrected pressure... the absolute pressure. If you do not have that info. then check the box next to it and Exbal will calculate the Standard Pressure for that given Elevation... that's another reason you need to input the elevation you're shooting at.

I really know how to use the Explorer Ballistic Program, and to 1000 yards it gives exactly the same values with Exbal. I played with both programs and using different elevations and they both agree. In Ballistic Explorer you enter the sea level corrected Barometric pressure for what ever elevation you're shooting at, you also enter the Elevation you're shooting at and when you change Elevations the barometric pressure is changed for you automatically; this takes place inside the program to my understanding.
I hope all this helps... We just have to learn how each individual program works, how the programmer intended it to work.

Javier O Moncada.

 

[bailey1474]

Now I'm a bit confused /ubbthreads/images/graemlins/confused.gif.....

I just purchased a Kestrel, which gives me the station pressure. Our altitude here is around 1200 ft. I've noticed that the readings I get are a little less than what the weatherman shows. Is this station pressure vs. corrected pressure? If so, wouldn't me putting in the altitude and my pressure readings result in bad data?