Calibrating weather meters for Density Altitude?

[BravoSix]

I searched this site as well as google for a while and I'm getting more and more confused. I bought a Brunton Pro ADC to use in conjunction with an FDAC to calulate firing solutions in the field. Among other things, the Pro ADC gives the user a Density Altitude reading without having to do any math. The Pro ADC has instructions for calibration of temperature, altitude, and barometric pressure. Can anyone tell me which of these measurements (if any) are required to be calibrated to get an accurate Density Altitude reading?

I'm pretty sure that temp will need to be calibrated. The other factors I'm not sure of.

Also, can anyone tell me how often the unit needs to be calibrated?

 

[Trigger Monkey]

Re: Calibrating weather meters for Density Altitude?

I have the ADC Pro and it will do density altitude for you but at the moment I can't remember how to set it. I also use the FDAC and honestly I barely look at my ADC Pro anymore, I just go by the manual chart on the back and it's been working pretty good so far.

I just messed with my ADC Pro a little more and if you go to the Altimeter/Altitude screen you scroll through the options by hitting the "Set" key. You'll see Ski Run, Min/Max Altitude, and then you'll come to Relative Air Density and the lower left corner of the screen will be the Density Altitude. I also found the instructions online that should help you out if all else fails.

http://www.retrevo.com/support/Brunton-ADC-Pro-manual/id/21329bh445/t/2/

 

[BravoSix]

Re: Calibrating weather meters for Density Altitude?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Trigger Monkey</div><div class="ubbcode-body">I have the ADC Pro and it will do density altitude for you but at the moment I can't remember how to set it. I also use the FDAC and honestly I barely look at my ADC Pro anymore, I just go by the manual chart on the back and it's been working pretty good so far.

I just messed with my ADC Pro a little more and if you go to the Altimeter/Altitude screen you scroll through the options by hitting the "Set" key. You'll see Ski Run, Min/Max Altitude, and then you'll come to Relative Air Density and the lower left corner of the screen will be the Density Altitude. I also found the instructions online that should help you out if all else fails.

http://www.retrevo.com/support/Brunton-ADC-Pro-manual/id/21329bh445/t/2/ </div></div>

Thanks, but do you know how accurate those numbers are without calibrating the unit for other factors. For instance, the altitude reading is always changing, even if just sitting on my coffee table. I found out that you need to calibrate the altitude before it will give accurate readings. I'm trying to find out what (if anything) needs to be calibrated to make sure I'm getting accurate DA readings.

However, maybe I'm focusing on minor variations of a number that doesn't really matter and I should just use the manual chart.

 

[mouse07410]

Re: Calibrating weather meters for Density Altitude?

I believe that on "decent" meters (like Kestrel) you don't need to calibrate anything - they're already fully & properly calibrated by the factory.

 

[Trigger Monkey]

Re: Calibrating weather meters for Density Altitude?

Without calibrating the unit to where you are the numbers are more than likely going to be wrong. When I got mine I adjusted the pressure and altitude to reflect where I was and it seemed to work pretty well. However, like I said since I got the FDAC I haven't really used the ADC Pro because the chart on the FDAC has been pretty accurate. I was shooting down in NC last fall and my buddy had his Kestrel out getting readings. Before I asked him what the DA was I calculated it to be about 500' and using his Kestrel he had 800'. This winter I was shooting up in WV and the temp was about 17 degrees and the altitude was a little less than 1000'. Using the FDAC I came up with about a -2000' DA and used that data on the FDAC with great effect to 730 yards. When I got home I checked the conditions on WeatherUnderground and plugged it into an online DA calculator and to my surprise the calculated DA was -2077'. The FDAC is plenty accurate so I would have a little faith in the system.

 

[mouse07410]

Re: Calibrating weather meters for Density Altitude?

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Without calibrating the unit to where you are the numbers are more than likely going to be wrong.</div></div>
Not quite correct, I think. This has to do with what barometric pressure one uses - Station Pressure (that requires no calibration) or Sea Level (corrected) Pressure. See here for a succinct explanation.

Ultimately what you need is Density Altitude (DA). Some ballistic calculators allow to enter it directly, good ones would also take Pressure Altitude and convert it to DA given the ambient temperature and RH. Elevation is unnecessary. So calibrating Kestrel for altitude is also unnecessary, unless you want to take the altitude for your logbook record from Kestrel altimeter.

If you choose to use SLP pressure (like what's reported by the weather stations) then you'd need to also know your actual elevation in order to determine PA...

 

[ToddM]

Re: Calibrating weather meters for Density Altitude?

Pressure would be the big issue in calibration, and depending on how the unit works altitude as well.

For example GPS units that calculate altitude by the pressure need to be calibrated at least once daily to be even remotely accurate. The same is typically true of pressure sensors. Whenever we use either in the field we calibrate every morning to known altitude/pressure measurements. Even then it's not uncommon for them to be off a couple hundred feet at the end of a day. With some units you can set the altitude and it will calibrate the pressure from the entered altitude, again of course depending on actual conditions there can be a lot of error there.

Ideally you'd want to calibrate all 3 for the most accurate readings, the problem always is that if you had calibration sources that were more accurate at hand during measurement you'd just use those

The reason the altitude reading is always changing is because it's based on the barometric pressure sensor. This is a problem because you can easily see major pressure changes due to weather fronts etc. which then "fool" the unit into thinking you are at a higher/lower altitude than you really are.

 

[sandwarrior]

Re: Calibrating weather meters for Density Altitude?

Bravosix,

Have you tried adjusting the altitude via the pressure before you start?

I have the older version of the Pro-ADC called the Sherpa. That's what I do.

FWIW, in aviation what we do is go by either the known field elevation and calculate (by charts) for density altitude. Or, we take the altimeter and set it at 29.92. Then you know the field density altitude....which is almost never the same as field elevation.

I'm thinking that on your Brunton, what it shows you is corrected pressure based off standard sea level standard pressure.

The whole corrected/uncorrected pressure thing is kind of confusing. Basically it boils down to an altitude that you are at vs. standard altitude.

 

[BravoSix]

Re: Calibrating weather meters for Density Altitude?

Thanks for the help. I was still a little confused, but I came up with a simple test. I left my Brunton outside for 30 minutes and used the weather data from www.wunderground.com to calculate the DA with this online calculator: http://wahiduddin.net/calc/calc_da.htm.

I compared the results to my Brunton's DA reading and they were almost exactly the same. I will do this same experiment a few more times at later dates to verify accuracy, but it appears that the Brunton does not require routine calibration to calculate DA.

ETA- The manual chart on the FDAC was pretty close, too.

 

[ggmanning]

Re: Calibrating weather meters for Density Altitude?

I set my Kestrel on station pressure and then use it for DA. Mine takes a few minutes to settle....it will vary up to 500' or so before it settles. This I can't explain because when looking at humidity, temp and pressure they are pretty stable. When it settles to +/- 50-100' i reference my DA shooting data and it is right on.

 

[mouse07410]

Re: Calibrating weather meters for Density Altitude?

ggmanning, your experience is what I would normally expect from Kestrel or another well made comparable device. We're on the same page.

 

[RobG]

Re: Calibrating weather meters for Density Altitude?

I have the ADC Pro also. I left mine outside for an hour then entered all the numbers into the above referenced calculator. There was a DA difference of around 400 between what the ADC read and the calculator came up with. Whats it all mean? Hell I don't know. Between the confusing pdf from Brunton and the cocktails, I can't figure anything out

 

[BravoSix]

Re: Calibrating weather meters for Density Altitude?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: RobG</div><div class="ubbcode-body">I have the ADC Pro also. I left mine outside for an hour then entered all the numbers into the above referenced calculator. There was a DA difference of around 400 between what the ADC read and the calculator came up with. Whats it all mean? Hell I don't know. Between the confusing pdf from Brunton and the cocktails, I can't figure anything out

</div></div>

I've checked mine several times over the last month and the ADC Pro has never been more than 100 feet off. Are you sure that you entered the correct altitude to begin with? (I used a topo map to get my altitude w/in +/- 5 feet.)

 

[Fieldman_.308]

Re: Calibrating weather meters for Density Altitude?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ggmanning</div><div class="ubbcode-body">I set my Kestrel on station pressure and then use it for DA. Mine takes a few minutes to settle....it will vary up to 500' or so before it settles. This I can't explain because when looking at humidity, temp and pressure they are pretty stable. When it settles to +/- 50-100' i reference my DA shooting data and it is right on. </div></div>

I'm going to try this..

 

[ctressler]

Re: Calibrating weather meters for Density Altitude?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ggmanning</div><div class="ubbcode-body">I set my Kestrel on station pressure and then use it for DA. Mine takes a few minutes to settle....it will vary up to 500' or so before it settles. This I can't explain because when looking at humidity, temp and pressure they are pretty stable. When it settles to +/- 50-100' i reference my DA shooting data and it is right on. </div></div>

I'm a little late with this but it pains me to see this happen. You don't have to set the Kestrel station pressure or set the reference altitude to zero to get the correct density altitude. This is the great thing about the kestrel, you don't have to know anything about anything to get great results, the Kestrel does it all you just have to turn it on and let it settle. I got this next part from the Kestrel website.

The density altitude measurement does not rely on the reference values in the altitude or barometric pressure screens. The density altitude is a function of temperature, relative humidity and pressure. Only the altitude and barometric pressure measurements rely on a reference.

Density altitude takes station pressure into account instead of barometric pressure, there's no need to set a reference for an accurate density altitude reading. (Station pressure is the actual pressure reading, and barometric pressure is the pressure reading adjusted to your geographic altitude.

 

[mouse07410]

Re: Calibrating weather meters for Density Altitude?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ctressler</div><div class="ubbcode-body">I'm a little late with this but it pains me to see this happen. You don't have to set the Kestrel station pressure or set the reference altitude to zero to get the correct density altitude. This is the great thing about the kestrel, you don't have to know anything about anything to get great results, the Kestrel does it all you just have to turn it on and let it settle. I got this next part from the Kestrel website.</div></div>
+1, absolutely!