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# Altitude vs. Barometric pressure

#29
10-19-2005, 12:29 PM
 Silver Member Join Date: Nov 2001 Location: tennessee Posts: 481
Re: Altitude vs. Barometric pressure

[ QUOTE ]
If so, wouldn't me putting in the altitude and my pressure readings result in bad data?

[/ QUOTE ]

Yes.

Once you get the Kestrel calibrated to real time pressure(not corrected) put the BP in the program, not the altitude. If you put the altitude(its only the "standard" pressure at Alt. it might not be the real pressure) it will give you the wrong data.
#30
10-19-2005, 02:18 PM
 Platinum Member Join Date: Feb 2003 Location: Pueblo, CO Posts: 1,154
Re: Altitude vs. Barometric pressure

I have the Speedtech.com watch and it gives me BP in inHg units, but then when i go to calibrate it it gives me units in hPamb. Anybody got any idea how to calibrate this unit??

Also, one other ?-- where do i find the std. to calibrate the BP in these units? When i go to weather.com it gives me corrected BP (inHg)-- how do i recalculate station BP from that value??
#31
10-19-2005, 05:13 PM
 Platinum Member Join Date: Aug 2003 Location: NC, oceanfront Posts: 2,845
Re: Altitude vs. Barometric pressure

1 Hectopascal = 100 Pascals = 1 Millibar = 0.02952999 Inches of Mercury
So Mb*0.02952999="Hg
EX:
1013.25mb*0.02952999 = 29.921"Hg/0.02952999 = 1013.25Hpa

I believe the answer to your second Q was given within the thread. You can calibrate your unit with the nearest, most recent station pressure converted and entered, given your known altitude(if you want it). You can get your altitude from a friend's GPS unit, if not known.
Give it a shot.
#32
10-19-2005, 05:43 PM
 Platinum Member Join Date: Aug 2003 Location: NC, oceanfront Posts: 2,845
Re: Altitude vs. Barometric pressure

[ QUOTE ]
Therefore it's important that when using Exbal you input the elevation you zero-in for including barometric pressure, temp. and humidity...

[/ QUOTE ]

Altitude is never needed. Not even with Exbal's "Edit Sight-In conditions". Just uncheck "Calculate Standard Atmospheric Conditions", and enter pressure you zero'd the rifle at. A bug in Exbal(atleast mine) is that it generally will not take until I've calculated &amp; re-entered this pressure. I have brought it to Perry Systems attention.

Altitude has no effect on Mach#, as JBM stated. Just another oversight from PS.
#33
10-22-2005, 02:40 AM
 Platinum Member Join Date: Feb 2003 Location: Pueblo, CO Posts: 1,154
Re: Altitude vs. Barometric pressure

Mike, thks. for the calculation-- it worked. But it turns out when i set the watch, i could change back and forth between units-- oh well not in the directions.

So now, i just calibrated my watch, i think. The weather station reported a BP of 30.21" Hg. So i entered that into the watch, and when i calibrated the altitude (4700 ft.), i saw that the BP is now 25.26" Hg. Now, in a couple days when we try to shoot a pr. dog at looong range, i check my watch for the new BP and don't worry about any altitude changes, and enter only the new BP into Exbal, and leave altitude at 0. This has to be correct since i'm now dealing with station pressure calc'd from the watch since the altitude has been entered, and appears to have corrected the BP in the watch---- RIGHT???

What is the factor to change between station pressure and sea level pressure??
#34
10-24-2005, 10:15 AM
 Platinum Member Join Date: Aug 2003 Location: NC, oceanfront Posts: 2,845
Re: Altitude vs. Barometric pressure

You got it. Just like you describe.

<u>Sea Level Pressure</u> can be figured from <u>Station Pressure</u> and your <u>Altitude</u> :
SLP=-1*(.00106168*ALT-1*SP)

<u>Station Pressure</u> can be figured from the <u>absolute(or SL) pressure</u> and your <u>Altitude</u>:
SP=SLP+(0.00106168*ALT)
#35
10-26-2005, 07:19 PM
 Silver Member Join Date: Dec 2004 Location: BINGHAMTON, NY Posts: 166
Re: Altitude vs. Barometric pressure

Just a quick note, maybe it might help someone. When I spoke to Gerald Perry we had discussed Exbal and the Kestrel 4000 unit and at the time I had hung up with him this is what I gathered from our conversation.

First if using the Kestrel unit you need to set the BARO reference to "0 ft at sea level" then the unit will give you station pressure which you will put into the box on Exbal for PC in the PRESSURE @ ALTITUDE after you unchecked the box for "calculate std pressure"

Then you will also need to put in the Altitude as well to get the most accurate info and you will notice that after you put in Pressure @ Alt and your actual Alt at the top and click someplace else that the Pressure @ Sea-level will change to 29.92 or somewhere very close to this number depending on weather which is actual pressure at sea level. Rather than using the model # of 29.53 inHg which is preloaded in the software.

If you do this you will see slight differences in the ballistic tables that are generated.

I too use to leave the Alt blank and had to make slight changes to Velocity or Sight Height or to Ball Coef or a little of all to get chart to match actual shooting. I have found that after doing this those charts that are generated seem to be a little closer to everyday shooting at least for me here where I live. I have not since doing this change gone to anyplace with any type of great elevation change to see how predictable it would be by adding in the two new bits of info (Alt and Pressure @ Alt.)

This is what the man who made the program (EXBAL) advised me to do and it seems to work. Easy way to tell if it will work for you is to try it. Make one set of drop charts by leaving Alt out then make another set by including them in the equation and see which work best for you. [img]/ubbthreads/images/graemlins/laugh.gif[/img]

Good luck as this has been very confusing to me as well as so many people have different ways of doing it.

Ben

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