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PostPosted: Wed Aug 29, 2018 9:44 pm 
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For those who think a single bulb light source is OK for a Chrony, please view this YouTube video. Please pay close attention and view the entire video before jumping to any preconceived conclusions. :wink:

https://www.youtube.com/watch?v=lXtdVOlLHZo

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PostPosted: Wed Aug 29, 2018 9:56 pm 
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Excellent demonstration of a well known phenomenon.... If he had been firing higher above the sensors, the drop in velocity with a single light source would be even more dramatic....

Image

Note the shorter pellet path with a single light source.... which produces a higher velocity reading....

Bob

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PostPosted: Wed Aug 29, 2018 10:02 pm 
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That explains my problem =)

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PostPosted: Wed Aug 29, 2018 10:06 pm 
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A couple of takeaways from that video worth noting. One is that you might be able to get away with a single light source if you make a cover to prevent the single light from shining directly on the sensors, and two I like the idea of narrowing the effective width of the diffusers with metallic tape.

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PostPosted: Wed Aug 29, 2018 10:29 pm 
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Look at the rear sensor of this unit and it is glaringly apparent that the plastic which facilitates the the slot loading is gone missing and broken away . The opening to the rear sensor is 4 or 5 times wider than the front sensor .For whatever reason the rear housing is broken and missing the parts of the plastic housing that give the rear sensor the slot loading it was engineered to have (probably shot and hit by to many low shots , look at the dent in the rearmost part of the housing ) .It also is apparent that shiny silver tape has been used to cover this damage ! How can you possibly blame the errant readings on the light source when the chronograph is obviously been compromised like this ? This example is not a valid test with an obvious broken chronograph .While a single light source may not be optimum an undamaged instrument will not give the errors given in this video . The diagram makes no account for slot loading and I do not know of any light sources that would project two beams as depicted .

"Just because you saw it on the internet does not make it true " Abraham Lincoln .


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PostPosted: Wed Aug 29, 2018 10:55 pm 
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Quote:
slot loading

Please explain what you mean by that?....

I can assure you that the phenomenon demonstrated in this video is real, and not caused by a damaged Chrony…. This is well known and documented, and has been for many years.... There is one video on the Internet where there is a single bulb about 3 feet above the Chrony, and the pellet path is about a foot below the light.... some 2 feet above the Chrony, way above where the diffusers would be (none were fitted)…. The guy was claiming supersonic performance with lead pellets in a Marauder.... Guess why?.... :roll:

Yep, don't believe every thing you see on the Internet.... However, in this case, the concept of errors from a single light source are correct.... So are errors from using a fluorescent light source.... or LEDs running on poorly filtered, pulsing AC....

Bob

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Airsonal;
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Proud Member of the 2000+fps Club!


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PostPosted: Wed Aug 29, 2018 11:01 pm 
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Well, for whatever reason mine shoots faster with a single light indoors. I'll try the cardboard and see what happens. My light was only a foot or two over the sensors so this would exacerbate the problem as well.

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PostPosted: Wed Aug 29, 2018 11:04 pm 
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Thanks for the post Bob, further confirmation of the problem. (looks like I was posting at the same time)

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PostPosted: Wed Aug 29, 2018 11:08 pm 
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The light path is straightened by placing the sensor below a slot in the housing . This prevents any light from entering from an angle as depicted in the drawing and only allows light from directly overhead to enter . This discrimination is determined by the size of the slot and the distance from the slot to the sensor The further from the slot and the narrower the slot the more discriminate the path will be .

Here are the pics of the chronograph used to demonstrate his theory . I am sure anyone can see it is very damaged and the slot loading of the rear sensor has been completely compromised .


Attachments:
rear.JPG
rear.JPG [ 13.11 KiB | Viewed 325 times ]
front.JPG
front.JPG [ 11.94 KiB | Viewed 325 times ]
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PostPosted: Wed Aug 29, 2018 11:19 pm 
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Yes, you can use a single light source, PROVIDING you shade the space between the diffusers.... Taping them up to narrow the range of angles where the pellet can be detected will improve the accuracy even further.... What you are doing is illuminating the diffusers from above, and being diffusers, they "bend" the light, some of which then goes straight down to the sensors.... The combination of the taping and shading the area between effectively changes the single light source into two....

I still have no idea what you mean by the term "slot loading".... :roll: …. Yes, the slot somewhat restricts the angle from which light can enter the sensors, but it certainly doesn't restrict it to "straight above"....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Wed Aug 29, 2018 11:31 pm 
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Bob I am confident someone a intelligent as you can measure the slots and the distance below the slots and come up with a theoretical distance as to when the sensor can not see the light source as it will be to close to directly illuminate the sensors as the slot loading will prevent a direct path to them . It is after all just addition and basic trigonometry . But back to my point the Chronograph is obviously broken so the experiment and conclusion are invalid .



Slot loaded wave guides are like venetian blinds Pointed up you can see the sky . When parallel to the horizon you see straight out . When pointed to the ground you can not see the sun directly . This is the purpose of the slot above the sensor . It is to mitigate the effects of the height of the light . When it has been broken away , like raising the blind it's effect is gone .


Last edited by pelsby on Wed Aug 29, 2018 11:50 pm, edited 1 time in total.

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PostPosted: Wed Aug 29, 2018 11:45 pm 
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Even better, I just tried a single light source over my well proven (and completely undamaged) Chrony…. With the light 27" above the top of the sensors, and not spending more than a couple of minutes playing with it until I got velocity readings.... I got 1180 fps using 18.1 gr. JSBs from my Grouse Gun at 1500 psi, shooting about 9" above the sensors.... roughly a third of the way up between the sensors and the single light source....

Wanna know what it REALLY shoots?....

A nice smooth bell curve from 700-720-700 fps over 16 shots from 1900 psi down to 1200.... :roll:

Yeah, those slots in the top of the sensors really do a lot of good making sure that only light coming from straight above can enter them.... NOT !!!

Would you accept this impromptu experiment as valid, or will you dismiss this as well?....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Wed Aug 29, 2018 11:53 pm 
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Bob I am not questioning your set up without pics . I am pointing out the deficiencies in the posted example It is a broken Chronograph .

Btw are your results not an inversion of the expected outcome ?


Last edited by pelsby on Wed Aug 29, 2018 11:59 pm, edited 1 time in total.

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PostPosted: Wed Aug 29, 2018 11:58 pm 
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No, you were saying that the slots in the sensor housing prevent stray or angled light from entering and being recorded by the sensor.... and that his damaged Chrony allowed that and hence was the source of the error in velocity....

Quote:
While a single light source may not be optimum an undamaged instrument will not give the errors given in this video .

In fact, had he not been shooting so low over the sensors, his recorded velocity from the single light source would have been much higher.... and further, any change in the height of the pellet path will alter the velocity recorded from a single light source....

If you think I am going to assemble the single light setup again just so that I can take a photo to convince you, you are sadly mistaken.... I have no reason to lie about the results.... :roll:

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Thu Aug 30, 2018 12:12 am 
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I will surmise that if one wishes to use this gear when obviously broken condition and well outside of the manufacturers recommendations while denying the laws of physics then you will get corresponding results .


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