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Altitude vs. Barometric pressure

 
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  #57  
Old 03-10-2007, 01:21 PM
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Re: Altitude vs. Barometric pressure

Thanks for the informative reply.
<font color="blue">You're welcome!</font> I am still confused when it comes to entering altitude, pressure and "calculate standard pressure". I have been entering my elevations (gps) <font color="blue">That works very good! </font> and entering the actual pressure (kestrel 4000) <font color="blue">Yup! just make sure you know the manual for your kestrel 4000 and enter the on-location pressure. </font> and un-checking the "calculate standard pressure". <font color="blue">Well done! the only time you don't uncheck it is when you do not know the target pressure and you want Exbal to calculate it based on the target elevation. </font> This is what I used last weekend to sight in my rifle, is this correct? <font color="blue">Yes! Are you being able to save you sight in conditions as explained before? </font> Now when I go to colorado later this fall am I going to enter the new conditions in the same manner as my sight in conditions. <font color="blue">Yes, but do not save them as I explained before. Good Job! </font>

I forgot to mention, I am running Exbal on a Palm if it makes any difference. <font color="blue">I know nothing about the Palm, but here is something said by Exbal's creator:

<font color="green">Exbal for Palm hand held devices uses the same rigorous ballistic motion equations that are used on the PC version. </font>

Good Luck to you! </font>
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  #58  
Old 03-10-2007, 06:26 PM
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Re: Altitude vs. Barometric pressure

Eaglet,

Thanks a bunch for clearing that up, completely makes sense now. Maybe it'll help someone else in the future. Hope you have a wonderful weekend.

Thanks again
Murf
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  #59  
Old 03-23-2007, 12:51 PM
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Re: Altitude vs. Barometric pressure

Lets amplify on two points, altitudes and pressures. The real nit difference in barometric pressures are -.880 inches of mercurey per 1,000 feet of increased altitude; rounded off that is 1" per 1,000 feet. That is the standard just as the temperature drops approximately 2 degrees per thousand feet of elevation increase. As to altitude, the Kestrel give us DENSITY ALTITUDE which is the real data to input into the Palm for the Pejsa program. Density altitude is the most accurate input as it give the actual conditions our bullet experiences at the shooting location. The drop difference between the local, corrected to Sea Level baraometer, as given by the Weather Channel, and the Density Altitude is small BUT cumulative small errors at long range can become a miss. If you can pack a Kestrel use it. Just to verify, I ran this by Proffesor Pejsa to confirm. Overbore [img]/ubbthreads/images/graemlins/cool.gif[/img]
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  #60  
Old 03-23-2007, 06:33 PM
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Re: Altitude vs. Barometric pressure

1kft pressure differences under 'standard atmosphere at altitude' conditions might be assumed -.88"hg as rounded. A median value. But this would actually only be correct from 6500'to 7500'.
Here's an example:
ICAO --&gt;
59deg
29.92126"hg

0 to +1000ft=
29.92" to 28.86" --&gt; -1.06"

5000ft to 6000ft=
24.90 to 23.98 --&gt; -.92"

+9000ft to 10,000ft=
21.39 to 20.58 --&gt; -.81"

However, these are just cumulative small errors..

The Krestel does give density altitude as calculated based on it's sensed absolute pressure, temp, and humidity. It calculates this rather than measuring it. Therefore, I submit that density altitude is not the most accurate air density parameter to use in ballistics.
Does pejsa's ballistic program allow density altitude inputs? I don't know..
According to his user manual, it doesn't appear so:
http://www.pejsa.com/manualPage2.htm

There is an error in his ascertion regarding air density on page3 that you might want to pass on. Specifically, the part about StdMetro air density being equal to conditions of 39.4deg &amp; 1000ft(This is equal to ICAO +78%rh, rather than StdMetro).
Small errors...
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  #61  
Old 04-16-2007, 07:19 AM
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Re: Altitude vs. Barometric pressure

If density altitude is not the most accurate input,what is your alternate input suggestion???? Cordially, Overbore [img]/ubbthreads/images/graemlins/confused.gif[/img]
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  #62  
Old 04-16-2007, 09:44 AM
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Re: Altitude vs. Barometric pressure

Density altitude IS an alternate..
The most accurate parameters are those that are measured, and that have been used all along:
Absolute Pressure
Temperature
Relative Humidity

These are all that's needed and a Krestel measures them.
Ballistic software uses them for adjustment in different ways. Normally air density is calculated within this software, but not always. Density altitude could only be used directly in software that would convert DA to air density.
This is not common.

It may be that use of DA works well for folks. It's a single number that <u>indirectly</u> relates to air density. But let's face it, it's use would also be easy to screw up. DA is not altitude, real or pressure. It has NOTHING to due with altitude. How many will miss that?
Thats why I'm skeptical about it really.
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  #63  
Old 04-17-2007, 01:29 PM
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Location: Atlanta, GA
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Re: Altitude vs. Barometric pressure

Per Sable Systems International, here is their chart of altitude above SL and the Barometer in Hg(mecury) ABSOLUTE:
500' =29.38"
1,000 =29.86"
1,500 =28.33"
2,000 =27.82"
2,500'=27.32"
3,000'=26.82"
3,500'=26.33"
4,000'=25.84"
4,500"=25.37"
5,000'=24.90"
6,000'=23.99"
7,000'=23.10"
8,000'=22.23"
9.000'=21.39" and
10,000=20.58"

These are the standard atsmophere values published by "a company founded by scientsts for scientists". Argue with their credentials and published findings at your own professional peril. [img]/ubbthreads/images/graemlins/shocked.gif[/img] Overbore
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