Hi Frank great job on the podcast, I’ve seen some discussion regarding using Da vs station pressure and humidity for inputs with ballistic calculators. It looks like a bunch of he said she said. Do you have a preference on which input is more accurate? Or is there no real difference since both are actual measurements of your actual surroundings?
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Da vs station pressure
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I will talk about this,
I use both, station pressure, temp, humidity when i can, and DA in more field like situations
I record everything, so I have both sets of inputs available
I use both, station pressure, temp, humidity when i can, and DA in more field like situations
I record everything, so I have both sets of inputs available
Frank, when you cover da can you touch on how to ensure your software and non Bluetooth windmeter are using the same inputs. E.g. ab or 4dof and a kestrel 3500
I’ll give my 2 cents. I think it will match what frank says.
When you you use SP, Temp and RH you are building a more specific firing solution. When just using a DA you are Just lowering the accuracy. For example any given DA can be comprised of multiple combinations of SP, temp and RH. Each of those elements effect the bullet in their own way. So a generic DA of just 6000 could have a different solution than say a DA that is 6000 but you input 24.87, 52 degrees and 38% RH (no clue if that is actually 6000 DA) just illustrating an example.
Either will get you on target but the one is just a hair more precise and much more necessary for the ELR distances.
You're correct in the sense that DA does not provide the same accuracy as each individual value... though I think that opens a can of worms/confusion that deserves some clarification.
Density altitude(DA) actually does not even consider relative humidity(RH). It is simply a formula containing pressure and temperature (see blow). That pressure can be entered in several forms, including station pressure(SP) in the example being discussed. I know it's coming so I'm going to get out ahead of it... some DA calculators/solvers include a spot for dew point or humidity and adjust the answer accordingly... they are WRONG! I'll explain why later in a moment. First let's take a moment to look at the real DA formula:
DA= pressure [120 * (current air temp - standard air temp)]
Now, where relative humidity comes into play is air density. This is where most DA solvers, including most apps, reside. They actually calculate air density and then translate that density solution into an altitude number. This is a product of most DA solvers being designed with aviation in mind. Almost the entire aviation industry uses the same vernacular where density altitude and air density are used interchangeably. Doesn't make it correct, but it works for the industry as true air density is all they care about.
So, it really comes down to knowing your equipment and how it works. Is your ballistic program looking for true DA or is it looking for you to enter air density? Once you understand your software the accuracy provided should be the same as it doesn't matter whether you combine each component into a single number or the solver does it behind the scenes. The outcome should remain the same.
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That’s the aviation formula that is taught. its shortahnd and is close enough for aviation. A true DA formula has a RH component. It is very very small which is why most shooters when using a ballistic calc set it to 50% and leave it. But, to be accurate, you cannot get a true DA without including RH.
Even in talking with others much smarter than myself it seems pretty commonly accepted that DA does not include RH, but I'm not opposed to being proved wrong and learning something... can you provide a scholarly source outside of the aviation field for this DA definition and calculation with RH?
Ill start off simple with the great and powerful wiki:
Both an increase in temperature, decrease in atmospheric pressure, and, to a much lesser degree, increase in humidity will cause an increase in density altitude. In hot and humid conditions, the density altitude at a particular location may be significantly higher than the true altitude.
And then i can prove it with my ballistic calculator (Here is AB Analytics):
59F
28.92
0% RH
DA = 1159
59F
28.92
50% RH
DA = 1271
Try it on any DA Calculator. Here is a link for one:
https://wahiduddin.net/calc/calc_hp_abs.htm
The change is minute. in reality it can be ignored, but to say RH isn't part of the equation is 100% wrong. By setting RH to 50 at all times in your BC you cut the error in half at its most extreme. say RH was zero and you have it set at 50, thats 50% of the error if you had it set to 100%. I encourage you to try it.
I know that aviation formula well. There are also 2 or 3 other quick ways to guesstimate DA. in fact there is another thread here where myself and a few other aviators hash it out. Even ICAO standard atmosphere has a RH component....29.92, 59F and 0% RH. there is a reason for that, that equals 0 feet DA (actually 2 feet but for all intensive purposes its 0). if you increase just the RH to say 75%, your DA is now 165. So it absolutely is part of the official mathematical equation. This page has a long scientific explanation...
https://wahiduddin.net/calc/density_altitude.htm
Again, absolutely nothing wrong with the formula you shared. For what we do its more than accurate enough. Just know, RH matters.
Thank you for providing 3 refrences... though all are avaition based.
Our disagree appears to be focused on the semantic of is there a difference between density altitude and air density... and we seem to agree there is a possibility for DA to be calculated differently i.e. short hand vs true calculation(in your words). Sound about right?
Earlier you stated DA was less accurate and yet you argue that it contains all of the necessary environmental componets. Why do you consider it less accurate?
Hum, sorry but with today’s tools, this entire arguments is in the weeds.
No disrespect, but we all tend too make to much of this part unless your running at ELR distances
No disrespect, but we all tend too make to much of this part unless your running at ELR distances
Because you can derive a DA with multiple different combos of the factors. Each affects it differently. A high temp low pressure da may have a different solution than a higher pressure lower temp da even though they both might be the exact same da. But as was just pointed out, this is an exercise in knowledge. Unless we are going super long distance it’s kind of just not that bag a deal.
In aviation, RH is only important when it comes to reciprocating engines. A jet engine doesn’t give a rats ass about humidity but it will wreck havoc on a C172. Even my old NATOPS ignores it as well as all my college texts but it is a factor.
FWIW, I have tested this DA vs Temp, Press, RH many times, and provided your software does it right, the difference is simply non existent and accurate to the last digit. Some software makers reccomend going with the "usual ones" vs DA because they need Temp to account for Mach, but all software were not made the same. I also presume some software are using the short hand version to come up with DA.
That is the important take away from our "in the weeds" discussion. Understanding how your software uses the components you input to calculate the solution as they don't all do it the same way.
For instance, it's important to change the temperature and relative humidity separately, and ensure that the program is changing the output appropriately. Keeping in mind that higher temperature should result in a lower required elevation setting for a given shot, and that a higher relative humidity should also result in a lower required elevation setting.
Also experiment with getting your program to perform the conversion from barometric pressure to station pressure. If, for example, you put in a condition of barometric pressure of 29.85 at an altitude of 6000 feet, and compare that with the results of a calculation made using a station pressure of 23.85 at 6000 feet, you should see approximately the same results from the elevation calculation. If not, something isn't right.
Then change the station pressure figure to, say, 5 inches of mercury less than what you usually have. When the program performs the calculation, you should note a marked decrease in the calculated elevation at distance.
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