Acoustic chronograph

[Roe]

I'm testing an acoustic chronograph with my Hornet in this film. According to the commercial very accurate, for certain compact and not dependent on light. Last two very good for me as I live above the Arctic circle and carry everything on my back (need the space for warm clothes in the dark season).

HUGE sensor area, very easy to use but also noted it took readings from my 338 (V0 was way off) when I was shooting well off to the side of it. This might be problematic on a range with many shooters I would think.

It was a really interesting product to test and if longevity and accuracy is there it will be great for me. I have yet to shoot "known loads" over it.

FILM: Acoustic chronograph

 

[wwbrown]

Last Spring I worked up a mathematical model for an acoustic chronograph the model included the angle the sound propagates from the projectile when it moves super sonically, the path of the bullet in 3 dimensions and the location of the microphones. I built the model to see how accurate everything had to be to achieve a given error metric. The biggest issue was that the path of the bullet across the acoustic chrony had to be very close to parallel to a line connecting the two microphones. The error in muzzle velocity climbed very rapidly when the angle between the bullet's path and the microphones deviated from 0.

Before I did the analysis I thought an acoustic chronograph would work very well as the sensing method would be more consistent than one that relies on solar illumination. and it is if you align the gun and microphones very closely. Given the need for closely aligning the bullets path and sensors I chose not to develop an acoustic chronograph for field use.

 

[Roe]

Guess I know what the spirit level is for now. Thanks for an extremely interesting reply Wade.

I actually read the user manual now. It claior at a 99% or better accuracy and "...to get the most precise reading, you have to shoot parallel with or at a very shallow angle to the sensors. If not, the bullet will travel on a path longer than that between the two sensors and the reading will not be correct. This applies to any two-sensor setup, whether it is acoustical, optical, electromagnetic or radar."

It also includes a table (printed on the back of the chronograph) showing optimal height for the bullet to travel over the chrono.

How does that fit with your calculations? Can I trust the brochure?

My initial reaction is "1% is a lot". I see that I will have to shoot the same bullet over both the acoustic and optical chronograph next time. I will also tip the chrono at an angle to see what it does. I have the basic Chrony model and it has worked well for me (except in the dark season) because I shoot a large number of shots and get my average over time/ from maybe 50-100 shots.

Donovan -
LINK to Norwegian shop Vertebrae dot no These guys ship to USA. Cost is NOK2495 which is USD 440. Deduct 25% from that (national sales tax not applicable with international sale) and add $10-20 for shipping.

 

[wwbrown]

Actually, the importance of paralleel paths does not apply to optical or radar methods of measuring the velocity. The difference between sound and the other methods is that sound propagataes away from the bullet in a sonic cone. If inrtherted further in the sonic cone go to http://en.wikipedia.org/wiki/Sonic_boom and look at the picture of the plane flying on the right hand agea of the page and read the accompanying text.

I am not as enamored with chronographs as I used to be, I tend to focus far more on how they perform at the target at the range I intend to use the load for.

Roe, I would be interested in seeing what your results are for varying angles, theap os slways a possibility (sometidso merge and sometimes small) that I am wrong.

 

[mikecr]

No matter the form of propagation, relative sensor distance would always be a factor affecting measure.
So yes, sensor angle w/resp to bullet path matters, and this can be seen with an Oehler as well.
Path is simple enough near the muzzle, but difficult downrange.

Thanks for bringing this up

 

[wwbrown]

You are correct that the relative sensor distance does make a difference regardless of the sensing method. The big difference between acoustic and light based methods is the of the speed of sound is on the same relative size as the muzzle velocity whereas the speed the light is much greater than the MZ, so the path differences has a much greater impact in the acoustic case.

 

[Roe]

The chrono changes to fps with a button touch if that's what you're asking.
I'll definitively try tilting the chrono at different angles to see what it does, and hopefulle do simultaneous measurements with the Chrono but there's less than 3 hours light now and not full at that.

 

[Roe]

Found this

http://www.ebay.com/itm/SuperChrono-Acoustic-shooting-chronograph-/271087938582?_trksid=p5197.m1992&_trkparms=aid%3D111000%26algo%3DREC.CURRENT%26ao%3D1%26asc%3D14%26meid%3D3667313144234816544%26pid%3D100015%26prg%3D1006%26rk%3D2%26sd%3D110979628334%26

Looks like it is the same shop I got mine from, $368 + $18 shipping.

 

[Roe]

I've tried tilting it and wow, my Hornet suddenly does 5500 fps! ;D

I'm not quite familiar with it yet, but it certainly is more user-friendly in a dark environment like mine. Really like the quick setup and pack size, but I have not yet determined accuracy. A 50 shot average put my Hornet loads to 747 msec (I think it was?) which was the same as the Shooting Chrony said.

Another mistake I do in this film is placing the acoustic chronograph behind the Chrony - this means sound shadow can affect the accuracy. I'll switch them around next attempt.

FILM ACOUSTIC CHRONOGRAPH PART 2