Why DA isnt Better than Pressure, temp and humidity in BC's

[_Windrider_]

Ive said a million times on here that using the components that make up DA will give you far better results than strictly using DA. That doesn't mean that DA doesn't have its place, i like it on hasty field cards, but i always have a temp associated with those cards for this exact reason. Just food for thought. This is from Applied Ballistics weekly newsletter.

Density Altitude (DA) combines: temperature, pressure, and humidity into a single variable. DA is the altitude corresponding to your current air density in the standard atmospheric model. For example, if you're at 1000 feet altitude but the air is hotter than standard for that altitude, your DA will be greater than 1000 feet. Since DA is accounting for temperature, pressure and humidity, it's actually representing air density rather than air pressure. If you use DA instead of temperature, pressure and humidity, the solver won't be able to account for the effect of temperature on the speed of sound (unless you input temperature and DA). For this reason, using DA for ballistic solutions can have some potential error. The graphic below shows a scenario where the DA is the same between two dramatically different altitudes, due to temperature and humidity. Notice that the solutions needed for a 1,000 yard shot are actually different for the same bullet at the same muzzle velocity. This is why Density Altitude is good tool for short- and mid-range solutions, but using the individual inputs for actual pressure, temperature and humidity is better for precise long range solutions.

​

Extremes in altitude, temperature and humidity can require different solutions even with the same Density Altitude value, as shown in the illustration above.

 

[LastShot300]

I'd say that if you run any Point-Mass based software the above is true, but it's not a general truth.

A couple of things are in order to understand why the above is a stretch.

a) Point Mass depends on the Mach number which in turn depends on Temp
b) If you resort to a simplified equation to calculate DA then you will see a difference
c) However whether the complete, full equation is used (Humidity comes into play too) then you will not see any difference and you should not by definition.

There is a very simple and straightforward way to prove this for good. Calculate air density (the only value that real matters to a bullet) using a simplified DA equation vs a fully fledged one, then compare to a calculation based on actual Temp, Pressure and Humidity.

Refer to the full ICAO document on how to do this. (which was written by real physicists not marketing guys)

All in all, such kind of statements/examples are indeed marketing based, not science based. No Point Mass-based solver maker will tell you the real truth behind their shortcomings.

 

[ma smith]

So is DA like some half-assed shortcut that isn't really a shortcut or?
is there some reason to use it?

 

[LastShot300]

So is DA like some half-assed shortcut that isn't really a shortcut or?
is there some reason to use it?

Makes sense only as a practical handy way, because you enter just one value, meaning less chance of input errors, other than that it all comes to a personal preference.

 

[_Windrider_]

I'd say that if you run any Point-Mass based software the above is true, but it's not a general truth.

A couple of things are in order to understand why the above is a stretch.

a) Point Mass depends on the Mach number which in turn depends on Temp
b) If you resort to a simplified equation to calculate DA then you will see a difference
c) However whether the complete, full equation is used (Humidity comes into play too) then you will not see any difference and you should not by definition.

There is a very simple and straightforward to prove this for good. Calculate air density (the only factor that real matters to a bullet) using a simplified DA equation vs a fully fledged one, then compare to a calculation based on actual Temp, Pressure and Humidity.

Refer to the full ICAO document on how to this. (which was written by real physicists not marketing guys)

All in all, such kind of statements/examples are indeed marketing based, not science based. No Point Mass-based solver maker will tell you the real truth behind their shortcomings.

Humidity error is minuscule. Between 0-100% it will change DA by max of 300 feet. You could set it to 50% and never change it again and there would almost be no measurable difference.

I know all about ICAO. Even in my job, we don’t use just use DA. Temp and pressure matter more.

 

[LastShot300]

Humidity error is minuscule. Between 0-100% it will change DA by max of 300 feet. You could set it to 50% and never change it again and there would almost be no measurable difference.

I know all about ICAO. Even in my job, we don’t use just use DA. Temp and pressure matter more.

Minuscule or not, it should be factored in for a complete DA solution. Most specially at ELR. Anyway, for medium range work it doesn't matter like you said.

 

[rmfield]

I like using DA as it really simplifies the process, but the temp issue is real at longer ranges. The problem with the ICAO standard atmosphere is that the temperature numbers tend to be way too low to match up with actual site temps. However, if you know what temps to expect at your site for any given DA, then you can use your own adjusted "standard atmosphere" numbers to get the temperature numbers very close to the location/actual elevation that you are shooting at. This makes your "adjusted DA" shooting solutions effective to much longer ranges. For example, these are the numbers I use for the Whittington Center (where I shoot at least 10 times/year) which is at 6630' AMSL:

To do this, you need a simple DA calculator like this one: wahiduddin.net/calc/calc_da_rh.htm
and the knowledge of what temps typically exist at your site at various DA increments. Its a trial and error process. Basically you set altitude and humidity to zero, temp to the typical temp at any given "DA", and then vary the pressure number (Altimeter Setting) until the DA lands on the DA you want.

BTW, I agree that humidity usually doesn't make much difference except at very high and very low temps. For example on a 95 degree day with humidity near 100% just after a rain shower. If you play around with the DA calculator, you can see this.

 

[spamassassin]

With a little thought and effort it's possible to select air temperature bands that double as rough and ready elevation bands. Referencing the attached image, as we move left one column in the drops we get the same basic effect as reducing the elevation by ~1000ft and if we move right by 1 column then it's the same as moving up ~1000ft. When I say "the same" this is not to be taken as being a precise to X number of decimals. It's an approximation true but it's an approximation which is precise enough while still being easy to use under time pressure. So far this temperature banding set has allowed me to use a single DOPE card while changing elevation from sea level to 6000ft elevation. This system is, of course, not infinitely extensible but it's functional for a surprisingly wide range of temp/elevation/pressure conditions. If it's raining then and the DOPE is for fair skies just drift 1 column further right.

An example usage: All of the data is indexed on 1000' elevation and I climb up to 4000ft and find myself in a place that's rocking a 60F temperature and I've got a target at 900yrds. Find the dope for 900yrds @60F (7.9mils), move right 3 columns and use that number (7.5 mils). Any fool can estimate with reasonable accuracy what the air temperature is and any fool can know to within ~1000ft what their elevation is by knowing where they are going ahead of time and looking at the range of elevations available in the area so it's possible, even with mathematics skills limited to what a 7yo child with a learning disability can do, cover a wildly variable set of conditions and locations with only a single dope card. DA is handy as heck, when I'm contemplating landing in the very near future. Otherwise, I don't find it to be so.

 

[MadDuner]

I just use the data as is. Knowing the supposed DA isn’t part of the corrections.

 

[_Windrider_]

With a little thought and effort it's possible to select air temperature bands that double as rough and ready elevation bands. Referencing the attached image, as we move left one column in the drops we get the same basic effect as reducing the elevation by ~1000ft and if we move right by 1 column then it's the same as moving up ~1000ft. When I say "the same" this is not to be taken as being a precise to X number of decimals. It's an approximation true but it's an approximation which is precise enough while still being easy to use under time pressure. So far this temperature banding set has allowed me to use a single DOPE card while changing elevation from sea level to 6000ft elevation. This system is, of course, not infinitely extensible but it's functional for a surprisingly wide range of temp/elevation/pressure conditions. If it's raining then and the DOPE is for fair skies just drift 1 column further right.

An example usage: All of the data is indexed on 1000' elevation and I climb up to 4000ft and find myself in a place that's rocking a 60F temperature and I've got a target at 900yrds. Find the dope for 900yrds @60F (7.9mils), move right 3 columns and use that number (7.5 mils). Any fool can estimate with reasonable accuracy what the air temperature is and any fool can know to within ~1000ft what their elevation is by knowing where they are going ahead of time and looking at the range of elevations available in the area so it's possible, even with mathematics skills limited to what a 7yo child with a learning disability can do, cover a wildly variable set of conditions and locations with only a single dope card. DA is handy as heck, when I'm contemplating landing in the very near future. Otherwise, I don't find it to be so.

For my hasty field cards I use a variation of this. Basically I get the pressure from my kestrel. I have cards for every .50” Hg from 0-100 degrees. I just find the pressure chart closest to actual pressure and then find the temp column. Gives me the DA and my drops. Humidity is assumed to be 50% always. This method adjusts my MV for the temp differences at the given DA.

the tables were inspired by your OG spreadsheets.

 

[spamassassin]

While I personally think of .5inHg might be a little more granular than necessary, nobody ever died from having too good of information. Also...

 

[_Windrider_]

While I personally think of .5inHg might be a little more granular than necessary, nobody ever died from having too good of information. Also...

View attachment 8120780

Yes you could easily do it every 1” but I have macros and v lookups and pivot tables in excel that auto creates the tables from a raw data import so I can literally do 20 spreadsheets for a gun from 31.92-20.92 in about 20 minutes.

then as I actually shoot in those conditions i can easily go back and manually enter true dope and refine the tables.

 

[spamassassin]

Yes you could easily do it every 1” but I have macros and v lookups and pivot tables in excel that auto creates the tables from a raw data import so I can literally do 20 spreadsheets for a gun from 31.92-20.92 in about 20 minutes.

then as I actually shoot in those conditions i can easily go back and manually enter true dope and refine the tables.

I've never thought I might fall in love with anyone but the woman I married but gotdamned if you're not making me rethink that

pivot tables, vlookups, macros... nerdgasm

 

[_Windrider_]

I've never thought I might fall in love with anyone but the woman I married but gotdamned if you're not making me rethink that

pivot tables, vlookups, macros... nerdgasm
View attachment 8120789

The hard part is getting the MV data across a broad range of temps.

 

[spamassassin]

The hard part is getting the MV data across a broad range of temps.

What I've found as the years go by and I have more historical data to analyze is that it's really not necessarily helpful to test at even 20deg intervals. It can lead to confused results if the number of rounds fired in each band aren't sufficient to expose outliers living in the 2nd standard deviation, much less the 3rd standard deviation. Lately I prefer to put a set of at least 10 and preferably 20 rounds up to ~120F, 80F, 60F, 30F, 0F. Control of ammo temperatures is simplified through the use of redneck engineered thermostatically controlled hot boxes for the hot side. I prefer ambient for the cold side but that's not always practical and it is sometimes easier to put a thermometer with remote display into an ice chest. Then just sit there and try to figure out how far the lid has to stay open to maintain X temperature and test as normal. This all specifically eschews measuring ammo temp directly. Measure the temp of the storage container instead. I also now draw lines between the min/max/avg sets which provides error bars that are much more telling than simple averages. Comparing the relationships of successive sets of error bars tells you where you're getting jumpiness (large shifts of velocity from small temp changes) or rattiness (inconsistency to the point of nonsensicality).

 

[_Windrider_]

What I've found as the years go by and I have more historical data to analyze is that it's really not necessarily helpful to test at even 20deg intervals. It can lead to confused results if the number of rounds fired in each band aren't sufficient to expose outliers living in the 2nd standard deviation, much less the 3rd standard deviation. Lately I prefer to put a set of at least 10 and preferably 20 rounds up to ~120F, 80F, 60F, 30F, 0F. Control of ammo temperatures is simplified through the use of redneck engineered thermostatically controlled hot boxes for the hot side. I prefer ambient for the cold side but that's not always practical and it is sometimes easier to put a thermometer with remote display into an ice chest. Then just sit there and try to figure out how far the lid has to stay open to maintain X temperature and test as normal. This all specifically eschews measuring ammo temp directly. Measure the temp of the storage container instead. I also now draw lines between the min/max/avg sets which provides error bars that are much more telling than simple averages. Comparing the relationships of successive sets of error bars tells you where you're getting jumpiness (large shifts of velocity from small temp changes) or rattiness (inconsistency to the point of nonsensicality).

Ammo is pretty stable these days across a broad range. I do my best to always have a the LabRadar running when I’m at the range. Since I live in Florida now, I just don’t get the cold extremes anymore so I may resort to doing what you do and creating that extreme…..even if it’s 95 out