Shaking the Technology Monkey: The Field Density Altitude Compensator

To say I’m not enamored with high tech gizmos would be an understatement (kindly overlook the fact that I write a blog).  They seem to cause people’s brains to shut down and to love tyranny.  The fact that I’ve been getting my ballistic data from an unconnected smart phone has bothered me a bit, although it seems to work alright.

When I saw a product that could allow me to access trajectory data while taking account for density altitude, without batteries, I became intrigued.  That product is the Field Density Altitude Compensator (FDAC).

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As you can see the FDAC is similar in construction to the Mil-Dot Master, which happens to be another cool and useful piece of gear.  It would have been nice if they had made the FDAC more strikingly different than the Mil-Dot Master in appearance.  It can be confusing to pull out gear that looks just about the same and choose the correct one every time.

Density altitude simplifies several different atmospheric factors that affect how easily the bullet travels through the air and it brings it all down to one number, which I believe represents the theoretical altitude at which the air density at the observed location would be considered a “standard atmosphere”.  What’s nice about density altitude is that you don’t have to keep track of all the atmospheric factors separately and try to somehow “average” them into some kind of voodoo elevation setting.  You have one number to mess with.  I’m not a science guy so I’m glad that paragraph is over.

You might wonder how you figure out the density altitude at any given location at any given time.  One way I have used is a Kestrel meter.  I love my Kestrel.  It’s a portable weather station.  It does about 50 things in addition to calculating density altitude and wind speed.  It looks really cool because it is OD green and has a red backlight and hangs on a lanyard loop.  I’ve had my Kestrel 4500 NV for 4 or 5 years now, but I think it cost me about $300.  It takes 2 AAA batteries and the battery life seems pretty good.  It takes a few minutes for the unit to get a steady reading.

The folks who came up with the FDAC, Adaptive Consulting, came up with a shortcut to calculate density altitude.  It probably isn’t as precise as the Kestrel, but they say that the battery life should be better.  To use it you have to have an idea of your location’s actual altitude and the temperature.  That shouldn’t be too tough.  I like having to pay attention to my environment.  Here’s how the FDAC inventors came up with a way to come up with density altitude:

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Find the number at the bottom that is closest you’re the temperature in your location, which in this example is 15°.  Follow the vertical line up until you reach the diagonal line that has the number that is closest to your actual elevation, which in this example is 5500.  Follow that line horizontally to the left and it will indicate your approximate density altitude, which is 4000 in this example.

Here are the rest of the instructions from the FDAC itself:

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Following our example, here’s how you’d read the dope for your conditions:

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A closer look:

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Here is what the entire card looks like when removed from the FDAC body:

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You have a couple questions that I am able to predict by the use of a complex artificial intelligence algorithm.

What if my load is not the same as whatever load they came up with?

That’s a good question, since all the slides are made for the 175 grain .308 Sierra Matchking.  What they did was figured out what other combinations fit the dope for various muzzle velocities of the 175 grain Matchking.  They put out a compatibility matrix found here.  They list 16 other .308 bullets other than the 175 Matchking at a range of velocities.  They advertise that this data should get you within 0.1 mils (about a 3rd of a minute of angle) of your bullet’s actual trajectory.  They offer a different version called the Milspec XR FDAC for more efficient long range cartridges.

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Your second question was “What if I don’t use milliradians?”

Oh yeah, I forget about you people sometimes.  If you flip the card insert over, it reads in MOA:

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One nice thing about having all the dope data on a card is that you can compare the data for different density altitudes and get an idea of how much the dope changes.  As you start to remember your dope, your ability to “fudge” the standard dope and apply it to the elevation setting for your current altitude and temperature will improve.

My first impressions of the FDAC were that it makes what can seem overly complicated about as simple as it should be.  It’s just reading dope off a chart in a familiar way, rather than doing a scientific experiment every time a shot is fired.  I was impressed at the ease of use.  As I said before, I wish it looked more different than the Mil-Dot Master.  I have a lead fishing weight hanging off my Mil-Dot Master and I still get them confused.  I wish they had used a 10 mph full value wind rather than a 5 mph, because I think it makes the math just a little easier.  I wonder how well it works to use a generic load and a generic scope height to come up with the number needed to get a hit.

I like the concept of the FDAC and will be sharing my experiences with it in the field later in the month.

9 thoughts on “Shaking the Technology Monkey: The Field Density Altitude Compensator

  1. I’ve never seen that, thanks! I have just started using the Whiz Wheel, from Accuracy 1st, and am pretty impressed with it.

    • The comparisons I have read between the FDAC and the Whiz Wheel is that the FDAC is a lot easier to use and the Whiz Wheel has more features. That would make sense that I’m using the FDAC and you’re using the Whiz Wheel, based on what we have established on our respective intelligence levels (me failing the quis and all).

  2. RS,

    Haha. I’ve never heard that comparison, so I think you just made that up. I have to say, the wheel is very easy to use, once you are shown how and try it once or twice. Watching the tutorial video didn’t really help me at all, and I thought it was much more difficult than it was. The FDAC looks complicated to me, based on the pictures. You’ll have to play with it in May.

    • They have different inserts. If memory serves they only offer a few “cartridge” choices, but there will be several different cards for different muzzle velocities. They have a cross reference chart for other cartridges that basically match up. I just can’t find it at the moment 🙁

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