How To Double Your FPE

[rsterne]

After somebody expounding on another Forum about how the slots in the top of the sensor housings on a Chrony prevent light entering on an angle, so can't cause a velocity error, I ran a quicky experiment to prove this diagram has merit....



If you use a single indoor light source above the Chrony, instead of two, you shorten the pellet path, and the velocity reading will be artificially high.... How much difference can that make?

I set up my well proven, undamaged Chrony and checked the velocity of my Grouse Gun at 1500 psi with 18.1 gr. pellets.... 720 fps, as expected.... I then set up a single light source, centered (by eyeball) between the diffusers, and 27" above the sensors.... I shot about 9" above the sensors and got (drum roll, please)….

1180 fps....

Pretty much proves that using a single light source is the cheapest way to double the FPE of your gun (or more)…. 

Bob

[Hobbyman2007]

Hahahahaha, in that case my 392 pumper makes almost 120 FPE on ten pumps . Thanks for the tip Bob . And to think all this time I wasted , changing springs , hammer weights , port sizes and buying bullet molds .

[wimpanzee]

I always use a fluorescent double tube light longwise over the chrony.

I've always gotten consistent results, agree with outdoor lighting and other chronograph reports (PA 10 for 10, JSAR testing on dreamline, Motorhead testing on my marauder) even though some people say not to use those types of lights because of the refresh rate or something.

[JungleShooter]

Nice, Bob, that was enlightening with that single-source light over the chrono.  Clearly, an engineer's mind is thinking here, my own mind wouldn't have come up with this. 

And your clickbait worked like clockwork, I saw the subject (How to double you FPE) and my mouse felt magically drawn to it. Click.   

Matthias

[wimpanzee]

Seems you could increase it even further by putting the pellet path closer to the light source too! Even more power!

[rsterne]

Correct.... The higher the pellet path, relative to the light source, the faster the velocity the Chrony will indicate.... A couple of years ago there was a thread on another Forum about somebody with an MRod that was obviously shooting way too fast.... The video showed a single light source about 3-4 feet above the Chrony, and the pellet path was closer to the lights than the sensors.... 

Bob

[mr007s]

I find this hard to understand.
The pellet passes over front sensor and it reads object, then passes over rear sensor and calculates distance and time.
How does the light placement have any effect? The sensors do the calculating.
 I dont see how the placement of light source matters, if anything the error message would show.
Case in point: In the illustration looks to me like if the front sensor did read the object, it would be read as it passed the rear sensor, not when it broke the plane of light indicated on the drawing.
Just my thoughts on this.

[rsterne]

Light travels as a straight line.... If you only have a single light source, the path of the light is shown by the red lines.... If you have two light sources (or the sun, which is 93 million miles away), the light entering the sensors is travelling parallel and vertically, along the black lines.... The pellet path (blue) is above the sensors, and it is obvious that the path between the red lines is shorter, and the higher the pellet path above the sensors, the shorter it will be, relative to the path between the vertical, black lines....

Since the chronograph calculates velocity by measuring the time difference between the shadow of the pellet passing over the sensors, and assumes that the distance travelled is a constant, if the path length is shorter than it expects, and therefore the time is shorter also, then it calculates too high a velocity.... It is the interruption of the light source that is detected by the sensor....

Bob

[RichH]

Light travels as a straight line.... If you only have a single light source, the path of the light is shown by the red lines.... If you have two light sources (or the sun, which is 93 million miles away), the light entering the sensors is travelling parallel and vertically, along the black lines.... The pellet path (blue) is above the sensors, and it is obvious that the path between the red lines is shorter, and the higher the pellet path above the sensors, the shorter it will be, relative to the path between the vertical, black lines....

Since the chronograph calculates velocity by measuring the time difference between the shadow of the pellet passing over the sensors, and assumes that the distance travelled is a constant, if the path length is shorter than it expects, and therefore the time is shorter also, then it calculates too high a velocity.... It is the interruption of the light source that is detected by the sensor....

Bob

If light travels in a "straight line" how can I move all around an empty room, with a single overhead light, and see it at all times from anywhere in the room? Is the "straight" line of light moving?



BTW-I agree with the analysis, but not because of the "straight line" of light.
It's not the "light" traveling in a single line it is the sensor's view of said light source, and the position of the projectile when it intersects that path.

[wimpanzee]

If light travels in a "straight line" how can I move all around an empty room, with a single overhead light, and see it at all times from anywhere in the room? Is the "straight" line of light moving?

This nifty little thing called reflection, which allows us to review our thoughts and statements, as well as making light visibly bounce off of everything except Vanta Black.

Do you think the Earth is flat, too?

[rsterne]

If light travels in a "straight line" how can I move all around an empty room, with a single overhead light, and see it at all times from anywhere in the room? Is the "straight" line of light moving?

Gee, I thought you would have cast a shadow that moves with you, from a single light source?.... Unless you are a ghost, of course.... 

Reflections can also cause Chrony errors, they warn you about that in the instructions.... I was trying to simplify the concept so that everyone can understand it.... I can see that some would rather argue the concept, and the experimental proof of it, rather than accept that a single light source can cause high Chrony readings.... I guess I was wrong to bring it up.... 

Bob

[Buldawg76]

Does having the sunscreens in place have any effect on fps reading if single light source is used. If the single light source is high enough above the sunscreens so they intercept the straight line of light does it still make a difference.

I do all my chrony work outside on sunny days when the sun is pretty much directly overhead ( 11am to 2pm approx. ) or on sunny overcast days to help minimize any errors from indirect lighting. Also shoot of porch railing with chrono on tripod to minimize height variations of pellets flight path with chrono 1ft away from end of muzzle.

Mike

[RichH]

If light travels in a "straight line" how can I move all around an empty room, with a single overhead light, and see it at all times from anywhere in the room? Is the "straight" line of light moving?

Gee, I thought you would have cast a shadow that moves with you, from a single light source?.... Unless you are a ghost, of course.... 

Reflections can also cause Chrony errors, they warn you about that in the instructions.... I was trying to simplify the concept so that everyone can understand it.... I can see that some would rather argue the concept, and the experimental proof of it, rather than accept that a single light source can cause high Chrony readings.... I guess I was wrong to bring it up.... 

Bob

If it's just a "straight line" as you say then woldn't I completely block it? I mean you are making it sounds like a 1/4" "straight line" beam of light comes out of the bulb.......which ISN't true......is it?

[RichH]

If light travels in a "straight line" how can I move all around an empty room, with a single overhead light, and see it at all times from anywhere in the room? Is the "straight" line of light moving?

This nifty little thing called reflection, which allows us to review our thoughts and statements, as well as making light visibly bounce off of everything except Vanta Black.

Do you think the Earth is flat, too?

Nope Earth isn't flat......but what's that got to do with light sources? If "reflection" comes into play how on earth does a chrony even work? Wouldn't the chrony then be getting constant light?

[wimpanzee]

If light travels in a "straight line" how can I move all around an empty room, with a single overhead light, and see it at all times from anywhere in the room? Is the "straight" line of light moving?

This nifty little thing called reflection, which allows us to review our thoughts and statements, as well as making light visibly bounce off of everything except Vanta Black.

Do you think the Earth is flat, too?

Nope Earth isn't flat......but what's that got to do with light sources? If "reflection" comes into play how on earth does a chrony even work? Wouldn't the chrony then be getting constant light?

Top quality trolling. I almost believed it.

[HunterWhite]

Bob is right.

I have built light sensor circuits that were probably very similar to the circuit in a crony.
The projectile doesn't completely block the light, but it does dim it a bit.

Imagine that you are on a golf course,  and the wind is directly from the tee to the cup.
A small cloud passes over head. It doesn't go completely dark, but there is a little less light while the cloud is overhead.
If another golfer is standing by the cup.
If you were talking on a cell phone and used a stopwatch to determine how long it took from when the shadow first appeared over the tee until the shadow first appeared over the cup.
You now have time over distance, which can be converted to any unit of measure you like.

In this case you are the first sensor, and the golfer standing by the cup is the second sensor. The stop watch is counting hundredths of a second.

The sensors in a crony are not 500 yards apart,  most are 1 foot. The stopwatch in a crony counts millions of counts per second.

When the shadow is first detected at the first sensor it starts counting. The count is stopped when the shadow is first detected at the second sensor. The higher the count, the slower the projectile. 

It's not sensing a blackout, just a fluctuation in the intensity of the light.

Bob mentioned that the Sun is about 93 million miles from earth, this means that the triangle still exist, but it is very small. The Ray's are almost parallel. And the Earths atmosphere is a really good diffuser.

The description that Bob provided is accurate.

BTW, I worked in electronic instruments and measurements fo over 30 years.

[rsterne]

The diffusers provide an overhead light source on a sunny day, when the sun is not directly overhead.... They would also provide overhead light sources indoors from a single light IF the area between them is blocked, so that is the strongest light source instead of the straight lines between the single lamp and the sensors.... You don't need the diffusers on a cloudy day, because the light is coming from "all" directions.... The Chrony uses the brightest source (generally overhead, but in all cases parallel), and detects the dimming of that source as the pellet travels over the sensor.... If it can't do that, you usually get an ERR message....

It is a tribute to current Chrony designs that they work as consistently as they do in varying light sources.... Still, I have found that I get the best results outdoors by using the diffusers with quite bright lights mounted above them.... just like I do indoors.... I have the sides of my Chrony, between the diffuser support rods, covered with plastic sheets, painted flat black on the inside, to eliminate side illumination, which I found increased the number or ERR messages.... As long as my shots are more or less centered above the sensors, the velocities I measure are consistent, and therefore I take that to mean accurate.... I have two Chrony's which measure within 3 fps of each other at 900 fps....

Light does travel in straight lines, other than a couple of special effects, such as diffusion, diffraction, or bending by intense gravity.... We don't see the latter here on earth (unless observing a distant star, where the light is passing a black hole)…. We do see the former, because of dust and water vapour in the air, which causes the light to "diffuse" or spread out, but even that is mostly due to it reflecting off the dust or water particles (or the plastic of your diffuser).... In space, in a vacuum, your shadow would cast a sharp line, bright around it, completely black inside it.... The reason that is not the case in a room with a single light source is that the light is reflecting off the walls, other objects, even dust particles, and "filling in" your shadow to some degree.... Diffraction is a different effect, that happens when light passes the edge of a solid object, or through a pinhole.... Within a few wavelengths of light, it interferes with itself, and creates bright and dim areas on a microscopic scale.... but this is not something you can see without magnification, with a few exceptions, such as the "moire" patterns on a window screen.... Again, this does not concern us with the operation of a Chrony…. where light essentially travels in straight lines from the source to the sensor, interrupted by the passage of the pellet....

If the brightest light source indoors is a single light source, relative low down and between the sensors.... you can get the effect I drew in the diagram in the OP.... If you don't believe it, test it for yourself.... I did, and I was able to "fool" the Chrony into getting a velocity of 1180 fps instead of 720 fps.... This is the reason for some of the ridiculous velocity claims you sometimes see.... THAT is the reason for this thread, to educate you to that possibility....

Bo

[HunterWhite]

Okay,  maybe the Earth IS flat,  but that would cast a straight line shadow. Since the shadow on the moon only seems to have a straight line every 14 days then I theorize that the Earth is flat only every fortnight.

🤔

[HunterWhite]

Okay, just kidding.

Nice graphic and explanation Bob.

Now I'm going to lower my crony.

[Thane]

I am far more interested in hearing Rich explain how a chrono works...