I used several atmospheric density calculators on the net - they give different answers! It seemed that, in 2011, it is not clear how to compute the density of air! I know now also why people mention pressure, temperature and humidity and not density - it is not trivial to compute. I researched more and found an article called

"Revised formula for the density of moist air (CIPM-2007), Metrologia 45 (2008) 149–155"

I have a prototype implementation of it. Soon I will release a BfX function that calculates the air density and expand BfX_c to include humidity as an option. (humidity effects, however are very very small).

(CIPM = International Committee for Weights and Measures)

The calculations confirm the earlier calculations menitionned in this thread for the density for which Ingalls tables are valid, as well as its relative density to icao.

For BfX it was a day with improvements.

Now the calculation of drag based Pejsa's drag coeficients is also done at the standard ICAO pressure. I have used the opportunity - now that I know how Pejsa's drag function is based on Ingalls Mayevski's parametrizations of deacceleration - to introduce the Ingalss Mayevski drag function to BfX. Of course the associated drag is also calculated at ICAO conditions.

I have reproduced Ingalls table and will put the spreadsheet online soon. By doing so I found a bug in BfX_Xv (it didn't want to parse "fps") and repaired it. I encountered also a wrong conversion constant from inchHg to Pascals. Furthermore I introduced the PSI (pounds per square inch) to BfX.

These modifications had impact on the GettingStarted spreadsheet and hence I had to modify it a bit.

You can download the updates. Soon I will extend my site with a discussion on atmospheric conditions and drag calculations

Delved deep in the issue, Pejsa was not clear about it. However on page 76 in his book he stated that he adjusted his A function to equal the Mayevski a function at 2600fps.

this means that he uses the Ingalls conditions P=30inchHg, T=60F, humidity=67%.

The (ICAO air density)/(Ingals air density)=1.00371. Although the effects will be minor to the point of being neglible, I will adapt Pejsa's drag calculations in BfX to ICAO. Furthermore, I will add Mayevski Ingalls (under ICAO conditions) drag functions.

Lapua has radar measurements for many of its bullets. From this their specific drag functions has been calculated. I downloaded the free Lapua QuickTarget ballistic calculator from www.lapua.com and made a quick comparision of its output for the Lapua GB478 Scenar 6,8 gram (105 gr) 6mm NormaBR bullet. Later, after I have sorted out things with Pejsa's calculations (did he use US Army Standard Metro or ICAO conditions plus 78% humidity?) I will do a more elaborate calculation. I think the results are pretty good. The question is how well the QuickTarget calculations match the exact trajectory? Ideally one wants to compare calculations with the actual path properties too!

it works with my excel2007 on a 32 bit machine:

can you show me your =bfx_help(1)?

yes, bfx_u allows that the user can specify his/her own units for in and output. This is not only handy for the metric/imperial units, but also if MoA or a distance is desired.

Nice!

have look at =bfx_u(output_unit, value, input_unit)

... delving deeper in the book of Pejsa, he has used Ingalls, Us Standard Metro and ICAO data / tables. Which one he used to compute the (de)acceleration has to be reverse engineered...

BfX_C works as it should!

Bug in BfX_C is fixed

Dear all,
Lately I was confused which atmospheric conditions I used in BfX - it was too long ago to remember and it seemed I also forgot the physics behind the drag functions.

Okay. Drag functions are independend of atmospheric conditions.


In the calculation of drag you have to use a density of air. In BfX the ICAO conditions are used (15 degrees C and 101324,6 Pascal 0% humidity), which are common for present day ballistic programs. Just checked the code - I am sure again - it holds for any BfX drag function

There is also the US Army standard meteo conditions (15 degrees C and 99991,6 Pascal), frequently used in the past. BFX_C which is used to convert ballistic coefficients, contains a bug thatI will soon fix. It incorrectly converts the ballistic coeficient as it assumes that BfX uses the US Army standard meteo conditions. if you enter
=BfX_C() and you  see the output BfX_C(T=15 [Celcius]; P=99999.76 [Pascal] []) then this is erronuous.

The new version will show BfX_C(T=15 [Celcius]; P=101324,6 [Pascal])

Expect the correction soon.

I have added, inspired by a posting of jrmy_1, the iphy (inch per hundred yards) and cphm (cm per hundred meter) units of angle  iphy=tan(angle)*100*0,9144/2,54) and cphm=10000*tan(angle)

e.g. =bfx_u("iphy";1;"moa")

to install the update you can use the setup or simply replace the .xll file

by the way, one of the most commonly ignored issue to achieve a one shot one kill setup is the calibration of the sights. Is the scope according to the specs?

I have a scope - 230 euros - that has a reasonable optical quality, and has an excelent mechanical quality. I see the black dot in the sight and that is good enough for me. Anyhow, the optical quality was good enough to become 1st in the Dutch 2008 300-500-600 yards (yes yards in the Netherlands too!) F-Class championship and third in 2009. In 2010 the beautiful military shooting range was (probably temporary) closed for us  .  Mechanically the scope is very constant in its moa/click and has no hysterises I know of. But instead of being 1/4 moa per click it is 0,21 or so. And that is an issue if you have to depend on calculations (the rifle was zeroed at 100m and I shot a lot at 500m).

I have a workbook illustrating a scope calibration.

regarding humidity, I did not capture its effects in BfX_C.