Importance of Aligning a Chronograph

[dg]

imagine a set of rugby league goalposts, in which we have 2 vertical and parallel uprights and a horizontal cross bar which is 90 degrees or normal to the two uprights.

regardless of the spacing of the uprights, the length of the cross bar represents the shortest distance between the two uprights.

now draw or imagine any other line (call it &), drawn or positioned so it cuts both uprights at any angle other than 90 degrees.

the length of this angled line (&) regardless of where it is positioned or cuts both uprights will be longer than the crossbar.

the mathematical length of this angled line (&) at any angle theta, is = length of crossbar /(divided by) the angle theta

so, on the assumption that the uprights represent the parallel line of sensitivity or projectiles pick up zone for the chrony sensors.

and the crossbar represents the sensor spacing of 12 inches for a chrony.

the angled line (&) represent the trajectory for any projectile as it passes over the chrony.

so for a line of trajectory (&) that is angled at 10 degrees to the chrony, it will have a length between the sensor parallel lines of = 12/cos10 degrees = 12.185119 inches.

as 12.18511 > than 12.0 ,then I would assume that the projectiles would take longer to travel the extra distance, which would result in the electronic calculator indicating a slower or reduced average velocity.

But as has been mentioned previously, this is all theoretical and supposition.

The inherent and unknown errors in the sensors spacing, electronic timer, sensor pick up zones and angle and the fact that the chrony axis is not perfectly parallel to the line of flight of the projectiles, probably support the view that any indicated velocities are approximate at best.

cheers
dave g

[stinkitup]

Ummmm Trev has this one. Rotate the chrony say 45 deg and now 12inches is 6 inches.

The sensor points up not stays perpindicular to the angle say of the face of the chrony. There fore said velocity will always be high if you angle the chrony vertically. Also be same if you twist it as well but can still get a reading.

When horizontal chrony when fully horizontal is a rectangle with the trajectory. Angle the chrony and the 12 inch horizontal is now the hypotenuse of the a triangle formed with the trajectory. Thy hypotenuse is the longest side of the triangle but the trajectory is now measured over less than the hypotenuse so is now shorter than 12 inches.

Should just draw a pic but rabbitsz already did.

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[Seddo]

I was expecting a pic of a chrono with a hole in it.....

[GJS]

I enjoy a challenge and a puzzle. I'm not convinced. Can someone put a picture up that shows a path shorter than the line parallel to the crony?

[Seddo]

I enjoy a challenge and a puzzle. I'm not convinced. Can someone put a picture up that shows a path shorter than the line parallel to the crony?

It's not possible, all other angles are longer.

[justjeff]

Rabbitz drawing has it perfectly. You guys are assuming the sensors only sense in a purely perpendicular line from the axis of the chrony. This isn't so, they have a 'window' of a number of degrees, so it is perfectly possible to shorten the distance between the two sensors, unless you go and input a truly ridiculous angle.

Jeff

[stinkitup]

So did this in cad easy to copy and tilt/rotate.

Blue chrony is level/horizontal with red lines the sensor vertical. Green is the same chrony but tilted superimposed over the horizontal you can see the red sensor lines are inside the blue ones therefore shorter. White is the trajectory.

Hope it helps.

chrony tilted.JPG (16.94 KiB) Viewed 1196 times

[trevort]

When it's tilted the chrony becomes the hypotenuse and the bullet flight path the shorter side of the triangle


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[GJS]

Jeff's theory that the sensor beam has a wide angle rather than that of a laser has merit but believing the beam stays perpendicular to the bullet path and not the Crony housing is well lets just say a little unlikely

[dg]

today at around lunchtime with the sun pretty close to directly overhead (not sure if this is a relevant factor or not, but in the previous test the sun was behind and very low, much later in the afternoon).

as best I could, I aligned the chrony straight to the target and inclined to the horizontal at an angle of approx. 20-30 degrees, with the front of the chrony being the lowest point.

a greater angle makes the chance of shooting the overhead plastic screens more likely and also the fuzzy image of the screens enters the scope picture.

the readings were 1st shot clean barrel 2717 (always slow) 2749 2747

the chrony was then adjusted to be as near as possible parallel to the line of flight, next 5 readings were 2863 2861 2869 2853 2861.

just an opinion, but I could not imagine anyone attempting to use any chronograph, when positioned, so obviously out of alignment

cheers
dave g

[Camel]

Better take the spirit level with me next time I borrow one. From the evidence presented it seems o make a fair difference.

[trevort]

Give me 300 for it Humpy so I can buy a magnetospeed


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[Camel]

Give me 300 for it Humpy so I can buy a magnetospeed


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Well bugger me that job of yours advising people how to invest money must have addled your brain, or was it the Guiness, which ever, last year it was only 200


P.s., don't forget to put my press in the ute this weekend. Or is the freight on it going to go up by 33 percent as well.

[stinkitup]

I think the biggest thing to realise is a chrony is a batch of numbers eg setup shoot 20 loads....next day do same thing in same spot same loads will probably result in a slightly different velocity. Possibly same stats eg es sd

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[justjeff]

Yes guys, if you miss align the chrony that much, you will get an increase in the length, but if it's just a few degrees, the few degrees window in the sensors will overcome it, giving in effect the drawing Rabbitz did. Considered in the other plane, looking down from above, you could make the distance either shorter or longer, depending on the alignment of the chrony centre points compared to the bullet path.

I don't think after 10 years of range officer duties, I have ever seen someone have a 20 deg, let alone a ten degree off centre chrony, which is probably surprising considering some of the things you see at the range. Stink had the right idea, it's irrelevant. Never had a pig look back and say it wasn't going to drop because that bullet was too fast or slow.

Jeff