Altitude vs. Barometric pressure

[Michael Eichele]

Example: RSI Ballistic calculator calculates that at sea level in 59 degree air and 29.53 BP a bullet with a .540 BC at 2700 FPS zeroed at 300 yards will drop 307.9" at 1000 yards.

The same program, same BP, bullet, velocity, zero, temprature, but change the altitude to 5000' and the bullet drop is calculated to be 315.8 with the same barometric pressure imput.

The differance is 7.9"

I always thought that the actual pressure and not altitude changed the bullets perfomance. I know pressure changes with higher/lower altitudes go hand in hand, but thought that altitude in and of itself had no effect on the bullet and just the pressure change with the altitude.

Any ideas?? Is the RSI program correct or is it a glitch?

 

[Mike in Texas]

What really effects bullet flight is station pressure,(absolute pressure) at your location.

As an example, a station pressure change from 28.85 to 28.80 means that at 1000 yards my 308 load will need 1 full moa less elevation to strike the same point.

Different programs try to back out corrected sea level based on temp and altitude. Some hit, most don't.

 

[Michael Eichele]

So if I understand correctly, the most accurate way to use RSI is to leave the altitude field at 0' and just use the pressure field to imput the current pressure at my location no matter what the altitude.

Right?

Thanks for the reply!!

 

[Jon A]

If absolute pressure, yes. The confusion comes from "corrected" baro readings for elevation. Say, when the weatherman in Denver says the baro is 29.9, it isn't really absolutely. He's correcting for the elevation, taking the lower reading (24.7 or so) and giving you an equivalent for what it "would be" at sealevel. They do this because the corrected number is more important and useful as far as weather predicting goes. Otherwise it would look like every mountain had its own "low pressure system" surrounding it.

So in short, if you obtain corrected baro numbers for your altitude (29 or something at 5000 ft) you should also enter the elevation into the program. If you obtain absolute baro numbers (much lower for higher altitudes) and use those, you can leave the altitude as zero in the program as the pressure takes care of it.

I hope that makes some sense. I'm not familiar with this particular program but just speaking in general. With any program you sort of have to figure out what input it's looking for so it corrects, but doesn't double correct.

 

[marc357]

Couple pretty good articles on the subject here: http://www.riflebarrels.com/articles/bullets_ballastics/density_bullet_performance.htm

And here: http://www.riflebarrels.com/articles/bullets_ballastics/ballistic_altitude_temperature_humidity.htm

Helped me get a handle on it.

marc

 

[Mikecr]

Your program is assuming pressure at altitude of 29.53, instead of 24.923(std conditions at altitude). Nothing wrong with this if you measured and appropriately entered 29.53 at 5000'. Higher pressure, more drop.
When you enter 29.53 @ 0', this is your pressure at entered altitude. If thats your measured absolute pressure, then your right, you should leave altitude at zero. Even if your standing 5000' up.

 

[Michael Eichele]

That all helps ALAOT!!! Thankx guys. I am actually finishing up a ballistic calculator incorporated into The Reloader's Archive program.

What I have done is inserted two check boxes. One for Altitude and one for pressure. It allows users to choose between which they want to use, but allows only one. When altitude is selected, the pressure field dissapears and is defaulted to 29.92. When pressure is selected, altitude dissapears and is defaulted to 0'. That way the calcs dont get confused. I needed to verify that this was correct before I finish it up and start distributing updated copies.

Thanks again! /ubbthreads/images/graemlins/smile.gif

 

[Dave King]

Lots of good info in this thread. I'll make it sticky for future shooters.

 

[Mikecr]

"When altitude is selected, the pressure field dissapears and is defaulted to 29.92"

This wouldn't quite work. I really like your direction on this this though. But if the altitude block is selected in your scenario, and the pressure field disappears, the program should default to STD PRESSURE AT ALTITUDE.
Else you lock in the apparent/potential problem as started for this thread. 29.92 would only be the std pressure at sea level, ICAO conditions. If you need a very small program or specific math for standard pressure at altitude, let me know. I know I had a difficult time finding it, back when I was looking.

 

[Jeff In TX]

Meichele,

Your topic is very good and one that got me a couple of years back when I started using my Krestel 4000 handheld weather meter with my RSI Ballistic Lab program.

I was plugging in absolute BP numbers from the krestel and still keeping the altitude field populated. My results were wacky. A quick call to Jim Ristow at RSI and he set me straight. I think this is something a lot of shooters do who use b-programs to help with long distance shooting.

It's everyone's understanding that they need to populate all fields in their b-programs with most accurate field conditions they can provide. Sometimes, that's not always the case as in your example. If your using a weather station or hand held weather station and you know your exact BP, then you need to leave the Altitude at zero.

Great topic and thanks for sharing this one.

 

[Michael Eichele]

Micke CR, I know where you are going with the standard pressure at altitude. It does calculate "standard pressure at altitude" but it NEEDS a starting point. That starting point is 29.92. From that point it calculates the differance from the altitude input. Some programs use 29.53. 29.92 works best in mine with the formulas used here. It is DEADLY accurate.

In other words, once the altitude box is selected and pressure dissapears, it has to default to a specified standard so as to cancel out any other input a user may have entered. Otherwise it calculates them together (thats bad!)

I hope that helps.

Jeff in TX thanks for the input also.

 

[Brent]

Michael,

I run some numbers for a guy there in Florida a couple years ago, as he was heading to Colorado hunting and I stumbled upon the same head scratching issue you did. I've Never called Jim to ask him about it simply because It was a 10,000 foot change and I didn't anticipate having to deal with that just yet, still it did not seem correct, still doesn't, but I'm sure Jim understands why there is a difference in output on the RSI program, or could find out.

I set the Kestrel 4000 to 0 feet referance to get station pressure and use it with a 0 altitude in the ballistic program, if anything other than zero is entered for altitude when you are already using station presure (pressure at altitude) the ballistic program will reduce the pressure once again, or two fold. If all one has access to is weather report data for daily pressure it must be reduced for your altitude by the program, thus an altitude needs to be entered along with this pressure at sea level in order to be accurate.

29.92 or 29.53.... to be quite honest, I'm still a little confused over this one here and have been for a while. Anyone care to shed a little light on the origin of the two?

 

[Mikecr]

Well let's look at it carefully.
Say altitude is checked. Pressure disappears. You enter actual altitude, and the program uses a std pressure for that altitude in it's calcs.
What if actual pressure is higher or lower than std at that altitude? How you gonna enter that deviation?
In order to calc air density correctly you'd have to determine and enter "pressure altitude" from a CALIBRATED Krestel. I guess GPS(for actual altitude) would be best for calibrating a Krestel while observing absolute pressure from a source known to be correct.
It just seems weird and confusing.

Anyone using actual altitude from GPS or maps would likely observe corrected pressure as reported locally that day. That pressure would often be near 29.92"hg regardless of altitude, and if different(say 28.7"), there needs to be a way for them to enter this. Maybe in the form of a correction like -1.22"hg. This, to be applied to Std pressure at altitude, for the day.
It's not something I would use, but that or pressure altitude are the only ways I can picture using altitude instead of absolute pressure.
Just thought I'd throw that out there.

 

[Michael Eichele]

Mike CR,

You are correct once again in that "if the pressure is differant on a given day."

Think of it this way. If a user is entering altitude it is because he has no way to measure pressure. If he has a way to figure pressure accuratly, he would want to use the pressure option instead of the altitude. Like was stated in earlier posts if the pressure is known then you would enter 0' in the altitude field on those programs that allow you to use both.

This program enters 0' in the altitude field for you if you opt to use pressure. That way you cant accidentaly enter both.

I know several RSI users that dont measure pressure. When they use the program they enter altitude and RSI defaults pressure to the standard at that altitude. Thats all mine does. If in the RSI program, and altitude and pressure are both entered, and the pressure at your location is 24.95 at an altitude of 5000' and 59 deg. you get slightly inaccurate but close numbers. If 0' is entered and the correct pressure is entered, then it will give you correct ballistics. My program automaticly issues 0' when pressure is used.

It is of course MOST accurate to use the pressure, but if the pressure is not known, altitude can be used and will be VERY close most of the time. The BARNES X program doesnt even give you the option to use pressure. Only altitude.

In short, if you know the pressure is 27.54 at you location it does not matter what the altitude is. Its raw pressure. You simply enter enter into the pressure field 27.54 at that point altitude is defaulted to 0' so as to not create inaccuracies in the ballistics and it will generate to you ballistics the same way as RSI, JBM, ect...

Please keep them coming!! I am getting alot out of this.