Two-Box Chrono

[adamjmac]

It's not for everyone. Every chrono is unique and has its pros and cons. I built this chrono for one reason - because it's produces good data.

If you are trying to reduce your SD through testing then technical performance is critical and convenience features are nice to haves. There's a lot of shooters out there, but a smaller subset who will do anything to turn 3/4 moa into 1/2 moa, and this chrono is for them. It's for the shooters that lose a point to elevation and know that it means homework. It's for shooters who plan their week around range trips where every test session is a step towards a better score next weekend.

It needs to come with instructions - how to improve your SD. And why SD is important. How many shots to fire and how to analyze the results. That's what my blog is for. This chrono is critical piece of the puzzle and other chronos just don't cut it. So if you are on the fence or don't really see why you should use this chrono, follow my blog and we will get there.

It just takes so.... much time to write those blog articles

 

[jlow]

Agreed! The very fact that this 2-box chrono allows a 15 foot spacing sets it apart from most chrono so well worth looking at even if it is not for every possible situation where a chrono is used.

 

[Hopkins]

It may be enlightening to show the increase in accuracy of ES of theoretically identical velocity shots as the distance between boxes is increased from 8" to 15'. Based on previous discussion I believe the system will give .5 fps +/- accuracy with the trigger boxes 15' apart. What would the accuracy be with the same boxes set up 2' apart. I would guess every 2 sensor system increases accuracy capability with an increased separation so long as the triggers both pick up the shot.
I think this system is a serious improvement over the rest of the available systems for getting a true picture of SD's of any supersonic load I'm working up.

 

[adamjmac]

It may be enlightening to show the increase in accuracy of ES of theoretically identical velocity shots as the distance between boxes is increased from 8" to 15'. Based on previous discussion I believe the system will give .5 fps +/- accuracy with the trigger boxes 15' apart. What would the accuracy be with the same boxes set up 2' apart. I would guess every 2 sensor system increases accuracy capability with an increased separation so long as the triggers both pick up the shot.
I think this system is a serious improvement over the rest of the available systems for getting a true picture of SD's of any supersonic load I'm working up.

The sensor and hardware timer itself has a useful resolution of about 1/4 microsecond, and the travel time at 2 feet is about 700 microseconds. So that's a resolution of 1 fps. The errors are not all due to hardware resolution, and in practice I would expect the SD of 0.5 fps would increase to about 1.0. As far as measuring your ammo properly, this error would cause a true SD of 6.0 to be observed as 6.1, which requires thousands of shots to be statistically significant.

What's more relevant than raw precision is that it won't just throw one shot off by 10 fps. With a small sample size, that can throw off your ability to draw conclusions a lot more. The fact that it's so simple and detects the bullet reliably is the real value to your testing.

A good reason to stick with 15 feet is that with a 15 foot sight radius it's much easier to align the pitch angle. You need 7.5 times the precision in the height adjustment of each box. Just lay them on the ground at 15 feet - fire and forget.

 

[Hopkins]

Could you please explain the need for 7.5 times the precision in height adjustment for each box? I am guessing it has to do with establishing the plane the sensors are in so it is parallel with the bullet path and the bullet path is roughly 2' above that plane while separated by 15'

 

[adamjmac]

If the boxes are 15 feet apart, then to adjust the alignment by 5 minutes of angle, you need to raise one box by 1/4 inch.

If the boxes are only 2 feet apart, then that same angular adjustment would be to raise the box by only 0.035 inches (just less than 1 mm).

It's harder to get the alignment right with a shorter spacing. It's reasonable to try to get the box within 1/4" vertically at 15 foot spacing and the average velocity will be within 10 fps.

 

[Hrsmn51]

Will the chromo record shots fire suppressed? Also will it record rounds fired over 3999 fps?
H

 

[adamjmac]

Suppressors: yes. It measures the bullet shockwave, regardless of the muzzle. The limit is set to 5000 fps.

 

[kountryboy2]

How will it work,if my bench is 4 ft above the two-box system?

 

[normmatzen]

Kountryboy

If your two box system is set at 15' spacing, the bullet at ~2800 fps takes about 5 thousandths of a second to traverse that distance. The box detects the passage by the sound shock wave impinging on the microphone on the boxes. Sound waves take 2.66 thousandths of a second to travel 3 ft to the boxes.
If one box is 6" higher than the other, the error in time of your reading is .444 thousandths of a second which is 8 per cent. That would make your reading off and indicating an error of +/- 224 fps!

On the other hand, with one box 1" higher than the other, the error is about 38 fps and with 1/4"
set-up accuracy, the accuracy of reading is +/- o.oo6%! Bear in mind, this is a consistent error and will not mess up ES calculations too much. At least no where as much as absolute Mv accuracy.

I leave math proof of my calculations to any one who wants to make me look foolish!

 

[adamjmac]

How will it work,if my bench is 4 ft above the two-box system?

4 feet above is fine.

@Norm, It doesn't matter how far above you are, the angular pitch sensitivity is the same. Your math is correct but the geometry is not as simple as you describe. The sound wave travels in a diagonal direction forward and down from the bullet, based on the angle of the shock cone. The angular sensitivity is in the right ballpark though, 10 fps for 1/4 inch.

It is easy to be within 1/4" and therefore 10 fps. Setting it up right is just a matter of taking 30 seconds before you shoot to do it.

 

[MJT]

4 feet above is fine.

@Norm, It doesn't matter how far above you are, the angular pitch sensitivity is the same. Your math is correct but the geometry is not as simple as you describe. The sound wave travels in a diagonal direction forward and down from the bullet, based on the angle of the shock cone. The angular sensitivity is in the right ballpark though, 10 fps for 1/4 inch.

It is easy to be within 1/4" and therefore 10 fps. Setting it up right is just a matter of taking 30 seconds before you shoot to do it.

Adam - does it calculate stats (like SD) or do I do that? Also, have you tried putting 2 or 3 of your systems side by side to see what the results are? they should be almost the same I would think?

 

[Barlow]

Will you take my 2 box Tepeco chronograph from the 70's in trade? Barlow

 

[adamjmac]

Adam - does it calculate stats (like SD) or do I do that? Also, have you tried putting 2 or 3 of your systems side by side to see what the results are? they should be almost the same I would think?

It doesn't calculate for you. You can only clear, save and load the shot memory later. I decided not to include calculations because it would make the remote control more complicated and I'd also have to add the ability to remove individual readings from the data set. Fire a group, write the numbers down, either as you fire, or later by scrolling through the list.

Yes I put 10 in a row and that is how I verified the precision of the sensors. Incredibly precise as long as the trajectory of the bullet is consistent.

I regularly shoot over two just because I have them lying around. Although the two side by side never disagree by more than 1 or 2 fps.

 

[MJT]

It doesn't calculate for you. You can only clear, save and load the shot memory later. I decided not to include calculations because it would make the remote control more complicated and I'd also have to add the ability to remove individual readings from the data set. Fire a group, write the numbers down, either as you fire, or later by scrolling through the list.

Yes I put 10 in a row and that is how I verified the precision of the sensors. Incredibly precise as long as the trajectory of the bullet is consistent.

I regularly shoot over two just because I have them lying around. Although the two side by side never disagree by more than 1 or 2 fps.

that's really good to know. thanks!