Range Report Help with manually calculating DA

[Jo.]

Due to a failure to operate properly in cold conditions I'm returning my Kestrel 4500 to NK to be serviced or replaced.

In the mean time I would like to be able to calculate actual DA when I'm out shooting. My Suunto Core watch supplies me with absolute pressure and I have an old school thermometer for temperature.

I have successfully used this online calculator to calculate DA, but I would like to know how to calculate this manually as there are times when cellphone reception are absent leaving internet inaccessible.

I have found what I believe are good resources at this site but it's way over my head so I'm hoping that someone less mathematically challenged can help me create a simple formula to calculate density altitude using:

- Absolute/Station Pressure (hPa)
- Temperature (deg. Celsius)
- Relative humidity (or just use 50% as standard)

Output should be Density Altitude in meters and accuracy to the nearest 100m is more than adequate.

Thanks
Jo

 

[Bearwalk]

Re: Help with manually calculating DA

Jo,

Just to warn you, calculating DA is no simple task. It's also beyond one post, so I am going to refer you to the below links which should get you going. The NOAA paper is probably better.

As you get in to it, you'll see that you can make your life a bit easier if you are willing to live with assumptions along the way. Good luck.

http://wahiduddin.net/calc/density_altitude.htm

and

http://www.wrh.noaa.gov/slc/projects/wxcalc/formulas/densityAltitude.pdf

 

[Killer Spade 13]

Re: Help with manually calculating DA

These are great resources, but how does one go about getting the station barometric pressure?

It's not as if you're at the airport getting takeoff clearance and calling the tower for weather . . .

 

[ECH]

Re: Help with manually calculating DA

Here is what I use to get Station Pressure.

Brunton ADC Ridge

 

[sniperaviator]

Re: Help with manually calculating DA

You need a DA paper chart or one of those Aviation circular slide rules, or an Aviation electronic calculator. No one manually uses equations to get DA, there really isn't an equation that I know of, just tables of values.

You can do it pretty accurately with just temperature and Altitude. Barometric pressure is secondary.

Google "Density Altitude Chart"

 

[Lowlight]

Re: Help with manually calculating DA

Do you have this chart ?

Or at least one like it ?

Pretty much the simplest way to do it manually ? They have several versions and David Tubb has them more suited for the shooter ?

 

[Jo.]

Re: Help with manually calculating DA

Thank you guys for your replies and thanks Lowlight for the DA chart.

I've been looking for a metric chart and this one is a good step on the way as it has a celsius reference.

I have one of the earlier FDAC with the DA Estimation Table on the back, but it's a pain to convert the temperature.

These charts seldom covers really dense air as they are mainly made for aviation purposes. Here in scandinavia do we at winter time get quite dense, cold air.
I shoot in
-2000m (-6400ft) DA (-28°C /-18.4°F and 1035mb/30.57 in Hg) this winter and I've shoot in colder conditions and most likely will do so again.

I would love to have an all metric chart that covered those conditions but this will do fine until then

Thanks!
Jo

 

[Bearwalk]

Re: Help with manually calculating DA

What's a matter, you guys don't carry a spreadsheet in the field?? LOL

 

[DCR]

Re: Help with manually calculating DA

The calculation to convert feet to meters is way easier than calculating DA. I'd say use what you can that keeps the calculations to the minimum. Or isn't there a DA slide rule like the Mildot Master? Can't remember the name at the moment.

 

[Jo.]

Re: Help with manually calculating DA

Thanks everyone for your input.

I really wanted to get my head around this so I used a little time to find a way and have now made a chart in Excel using this formula from Wikipedia.

<span style="font-style: italic">where</span>
DA = density altitude in feet
P = atmospheric (static) pressure
PSL = standard sea level atmospheric pressure (1013.25 hPa ISA or 29.92126 US))
T = true (static) air temperature in kelvins (K) [add 273.15 to the Celsius (°C)] figure
TSL = ISA standard sea level air temperature in kelvins (K) (288.15 K)
b = 0.234969

<div class="ubbcode-block"><div class="ubbcode-header">Code:</div><div class="ubbcode-body ubbcode-pre" ><pre> =145442,156*(1- (B$1/1013,25/(($A2+273,15)/288,15))^0,234969)*0,3048</pre></div></div>

I used it as is just added a conversion from C to K in the formula and it now outputs DA in meters.
- Row One equals station pressure (P) in mb (hPa)
- Column A equals temperature (T) in Celsius.

It assumes dry air so Humidity is lost on this but I think it's is accurate enough for the distances I shoot and will do fine until my Kestrel returns.

I have only done a quick check with the Shelquist converter using 0% Relative Humidity and it seems to get the same results, but If anyone find an error please let me know.

This is how my chart looks after cleaned it up in Adobe Illustrator.