Altitude vs. Barometric pressure

[Charles A]

[ 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.

 

[sscoyote]

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??

 

[Mikecr]

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.

 

[Mikecr]

[ 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 & 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.

 

[sscoyote]

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??

 

[Mikecr]

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)

 

[Glock119]

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. /ubbthreads/images/graemlins/laugh.gif

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

Ben

 

[overbore]

THE BIG WHY QUESTION:
Th reson for the different pressure settigs stems from the requirements of flying, which at lower altitudes ( (below 18,000') has everyone flying at an altimeter setting corrected to sea level pressure as a standard value so that, for example, using the local (corrected) station pressure will have aircraft flying at, for example, 1,000 feet above ground level at that station, will be at the same altitude.
Climbing above 18,000 we all set our altimeters to 29.92" to keep a the same "flight level". This is the why. Overbore, Capt. Pan Am, Ret /ubbthreads/images/graemlins/cool.gif

 

[triggerfifty]

29.53 vs 29.92 In. Hg.

Hi,

I've been developing data tables for years for SOE (which is my own company) and I did it on active duty for .50 cal rifles when I used them. The difference between these two is that the 29.53 In. Hg. is issued as a ballistic standard and agreement by Aberdeen Proving Grounds Ballistic Research Lab (BRL) a few decades back. It was done to standardize (but not a NATO standard) certain values of measurement to be used in the science of external ballistics. APG BRL Standards for air conditions are:

59 deg. F air temp
29.53 In. Hg. or 1000 mb air pressure
70 degrees ammunition temperature
78% humidity (which is a non-factor in LOW trajectory fire
0 degrees slant angle gun to target

The standard of 29.92 is an FAA and aviation standard. One of my trainers is a former pilot and says the same as the poster above saying that air pressure values are dialed into the altimeter so that the wheels are on the ground when landing or taking off, the altimeter reads 0.

Someone above also said, correctly, altitude means nothing, only the measured Raw air pressure is valuable. True. I helped develop the CheyTac Advanced Ballistic Computer that uses Doppler radar measurements to measure downrange speeds, drift, yaw, pitch, stability and launch velocities. That was 2 years of research. I wrote and modified the formulas that were used to write that software. It is in use by several military units in the US and overseas. It's been in use in combat since Jan 2002.

Very interesting subject.

Sniping Operations Executive

 

[overbore]

Re: 29.53 vs 29.92 In. Hg.

Close but no Cuban Cigar. With QNH pressure set in an altimeter, the altimeter will read, allowing for mechanical tolerances, the actual field elevation above sea level. With QFE pressure, on the ramp the altimeter reads zero no matter if your in Denver or Miami ; American used this for many years. /ubbthreads/images/graemlins/cool.gif Overbore

 

[overbore]

Re: 29.53 vs 29.92 In. Hg.

Here is the absolute chart: &lt;www.usatoday.com/weather/wstatmo.htm&gt; it gives the surprising altimeter setting for mountain shooting.. My son
lives at 8,000' and used the Pesja 29.93 baro input and almost mised his elk; he shot 4" inches high--duuh. He should have used 22.22"; it do make a difference.. Overbore

 

[overbore]

Re: 29.53 vs 29.92 In. Hg.

If your computer will not link tothe chart, the numbers are dead simple: if Seal level has astandard pressure of 29.92" and the pressure decrease is one ince (1") for every 1,000 feet of shooting elevation, at 2,000' the standard pressure (QFE) is 28.92" and at 8,500 feet, subrtact 8.5" from the standard (29.92") or if you were wise enough to get the days wx before going out to shoot and they gave 30.02", 30.02"-8.5" or 22.52" uncorrected for air temp and humidity. The chart value is 22.22" ; close enough for Mr. Moose ; son just woke up and nailed his bull elk one shot 30-378. Overbore

 

[rotorhead]

I'm not sure it this will help ,but in the world of aviation we use a standard day. This is a datum for figuring weather and performance. The pressure average is 29.92Hg at sealevel and temp of 15C or 59F The pressure lapse rate is 1" per 1000ft. The temperature lapse rate is 2 per 1000ft but the humitidy issue eirlier stated has a great effect on aircraft and a bullet in a sense is a aircraft like a rocket so to speak.In the big skeem of things Density is the controlling factor. The density of the air determines the performance. Example: Your are at 5048ft above sea-level the temperature is 5C under these conditions you are at standard day. The bullet will perfome as if you are at 5048 feet.if the temperature changes to 30C the bullet will perform as if it was at 7855 ft. conversley a temperature change to -25C the bullet would perform as if you were at 1232 ft. I have no idea if this helps but it may give you a standard to go off of.


RH

 

[Guest]

I do not know if anyone has posted this yet, but here goes my two cents…
When using Night Force / Exbal ballistic targeting software my procedure is as follows:

1)I use IMR 4831 in my .300 wm loads. That powder on average increases or decreases 2 fps per degree. Because of this, I either increase or decrease the velocity from my baseline velocity and input that sum into the data entry point of the software. I also input the current temp.

2)I input the humidity into the data entry point of the software. However, unless there is a lot of humidity in the air, it has a minor effect on the projectile. How minor? 1/10th of 1% which equals about 1/100th of an inch at 1000 yards. If there is 80% humidity or more, since it is lighter than air, it will increase the velocity of the projectile. As it is part of the equation I input it.

3)Using altitude with standard barometric pressure compared to Station pressure is a two part process.

a.I run the first part utilizing standard pressure and the current altitude. Altitude is obtained from my GPS or TOPO map. GPS is better.

b.After calculating my holds based on standard pressure and altitude, I run the equation again, only this time I uncheck the "Calculate Standard Pressure" box, and input the current station pressure obtained from my Kestrel 4000, and then run the program again.

I have found that following Gerald Perry's instruction on this does make a difference. In any regard, Night Force / Exbal ballistic Targeting Software is a professional software package, and I recommend it highly.