Check my understanding - Kestrel and Pressure

[ndking1126]

I'm pretty sure I want to buy the base model Kestrel to input my data into Strelok and 4DOF.

My very basic question is.. I would put "Atmospheric Pressure" from the kestrel app directly into the ballistic calculators ("Pressure, inHG" for Strelok and "Pressure" in 4DOF), and this information would be called station pressure?

I've downloaded the app, but since I don't have a kestrel device yet, I can't really get in to see what each piece of information is called. "Atmospheric Pressure" was what I saw on a youtube video, and it may not be the same verbage on the most current version of the app.

 

[ShtrRdy]

I am reluctant to reply because this topic is easy to get confused.

Basically there is the actual air pressure. I like go call it 'Absolute Pressure'. 'Atmospheric Pressure' may be the same thing.

The other term for pressure is Baromatic Pressure. This is the air pressure which is referenced to sea level.

I don't know if the air pressure terms used in the Strelok and 4DOF are specifically one or the other.

 

[ndking1126]

I am reluctant to reply because this topic is easy to get confused.

Thanks for chiming in. I think your statement pretty much sums up much of the shooting communities views on it. Lots of people will talk about it at a high level, but I'm struggling to find someone show specifics, unless you want to buy the top of the line Kestrel. Problem is that product does everything for you, so there's no transfering numbers to your ballistic app calculator.

Either way, my actual dope is coming in about 2-2.5 MOA less than what multiple calculators told me it should be at both 600 and 1000. If it was 2 MOA off at 600, I would have expected it to be 3, maybe for 4 MOA off at 1000. I was also expecting the numbers to be much closer.

I am confident with all the other inputs, so I figured it was about time to dive in and learn pressure. That might be what helps true up the programs output. I'm thinking range time with product in hand will be how I'll figure it all out. Thanks.

 

[ShtrRdy]

What altitude are you at? We might be able to figure out what pressure reading you're getting is.

 

[ndking1126]

1,020. I've been using 28.86, 28.88, (something right there, I can't remember the exact number) given the rule of thumb to subtract 1 inHg per 1,000 ft asl. What I don't understand is what temperature does 29.98 asl work? Seems to me, if that standardized measurement is at 60* as an example, I would have to increase the baseline pressure as it gets hotter. I'm typically shooting at 80, maybe 85* during the summer cause I shoot in the morning.

 

[dfanonymous]

I'll try to simplify it.

Since we are being simplistic, think more of the lines that you need the literal air density the will determine the drag on the bullet. That's what we care about.

Calculators and pressure:

What we want is absolute/station pressure, or another name is uncorrected pressure. A part that messes people up is when they look it up on their phone, we want it at our location not where it's being measured where ever they get that information from. Not where the weather man is, not in the town over, not at the local airport, not from the app, etc. Where we are at that moment.
atmospheric pressure just means barometric pressure, which is corrected pressure when talking about a kestrel depending on the settings. If the reference altitude is 0, you will have station pressure. That's all you need to know. 4 dof when the altitude is locked at 0 wants station pressure.

What you DON'T want to use is "corrected pressure." This is what the weatherman uses and the bullet doesn't see this. This is just something meteorologist like to use to compare things at different altitudes. Google it if you're interested in the details but this seems to have the most confusion on people.

 

[ndking1126]

If the reference altitude is 0, you will have station pressure. That's all you need to know. 4 dof when the altitude is locked at 0 wants station pressure.

This is really what I'm hoping to discuss. The rest I think I'm following. Hornady says to input "uncorrected atmospheric pressure" and my understanding is that's the output from the kestrel. Are you saying I'd have to put the elevation to 0 even if I'm higher?

 

[dfanonymous]

This is really what I'm hoping to discuss. The rest I think I'm following. Hornady says to input "uncorrected atmospheric pressure" and my understanding is that's the output from the kestrel. Are you saying I'd have to put the elevation to 0 even if I'm higher?

In the case of the kestrel on the bottom of the pressure screen you'll see a "ref altitude." That should be from the factory set to zero. You CAN mess with it to get corrected pressure, but since I and likely you are not a meteorologist or a weatherman, leave it or set it to zero as your reference altitude and the number the kestrel will give you is the station pressure, so in short yes.

This is the number you want.


The elevation on the kestrel is not going to be exact, it's not a gps. It uses pressure to estimate what the elevation is, again the bullet only cares what that air density is. It doesn't care if you're at 10000 ft. Just air density. The only time you need to change your reference altitude is if the calculator you are using require barometric pressure (corrected) only vs station pressure. Which is not the case with the calculators you've mentioned.

 

[Bob2650]

Most kestrels can provide you with both station pressure and the weather man version barometric pressure "corrected" for sea level.
What is actually needed by the ballistic calculator is "altitude density" That is how thick the air really is at your location. The higher end kestrel meters can show you that number too. The D3 Drop is the cheapest I can think of but it does not do wind.
Altitude density combines humidity, temperature and actual air pressure.
Ballistic calculators ask for the altitude so it can calculate the real air pressure from a weather man barometer number. Most kestrels and many other weather meters have a pressure sensor and does not need to know the altitude to provide data for the phone app to calculate a trajectory.

When you use altitude density you should find that most app programs no longer requires you to enter air temperature, humidity or altitude. You will still need "powder temp" but that is another matter.

 

[Bob2650]

They got the unlikely term Altitude Density from the people who fly airplanes. Minimum take-off distance and maximum take-off weight calculations are always made before take-off.
The runway may be too short if the temperature or humidity goes up or if there is too much weight.

 

[entoptics]

I wouldn't discourage anyone from buying a kestrel, as they are a handy tool, but if you have a modern smart phone they usually have a built in pressure sensor. There are plenty of free apps that will give you the station/barometric pressure.

I suggest experimenting with this and a simple thermometer before committing to the more expensive Kestrels with barometers. I use 4DOF and/or Ballistic, a less expensive Kestrel (3500?) with only humidity/temp/wind, and a free barometer app, and have had great results.

4DOF won't, but some ballistic calculator apps can even automatically pull the pressure from your phone's barometer.

 

[Bob2650]

I am using a kestrel 5400 heat stress with blue tooth and vane mount. They can be found new very cheap at times on ebay. (just checked, none for cheap at this time)
I play around with several different cell phone apps and at the moment Applied Ballistics is my favorite and that is one of many that communicates with the weather meter to automatically populate and update the environmental fields.
The heat stress meter is ugly with that stupid looking black ball on top but the heat stress function works very well and has come in handy several times.

I have no doubt that for the professional who needs to get that shot off as fast as possible the 5700 elite could make a big difference for the sucess of the mission. I am just a retired guy who is a NRA High Master and I like to plink at absurd distances, I am not in much of a hurry.

 

[ShtrRdy]

The kestrel can provide you with both station pressure and the weather man version barometric pressure "corrected" for sea level.
What is actually needed by the ballistic calculator is "altitude density" That is how thick the air really is at your location. The kestrel can show you that number too.
Altitude density combines humidity, temperature and actual air pressure.
Ballistic calculators ask for the altitude so it can calculate the real air pressure from a weather man barometer number. The kestrel and many other weather meters have a pressure sensor and does not need to know the altitude to calculate a trajectory.

When you use altitude density you should find that the program no longer requires you to enter air temperature, humidity or altitude. You will still need "powder temp" but that is another matter.

Not all models of the Kestrel provide Density Altitude.

 

[Bob2650]

You are correct. I should have been more specific.
Edits made, hope it makes sense now and is factually correct.