CED M2 Project

[AlanF]

A few months ago, I decided to make one last effort to get my CED M2 chrono working reliably - it has often let me down with unexplained results. I want to get an accurate assessment of velocity spreads - not so interested in absolute velocity. When I first bought it about 3 years ago, it just had sky screens i.e. no infra-red. I already had a F1 Chrony and would set them up in series as a check against weird readings. Some days they would agree well, not so much in absolute velocity, but the difference would stay constant to within a few fps. Other times the difference varied by 20 fps or so. Based on what the figures were, I suspected the CED, but had no definite way of proving it. I heard that the IR screens would make the CED more reliable because it made them independent of sunlight, so I bought a set. Unfortunately I was still getting weird readings quite often. Rod Davies mentioned that he'd had similar problems with his, so I borrowed it and tested them in series. Again I couldn't get consistent agreement and seemed to be getting more signal strength (E0 and E1) errors than with the old sky screens. So Rod's went back. Then I heard someone say you can put them in an enclosed box to keep out interference from the sun. This sounded promising so I asked Rod if he'd sell me his - he said I could have it! So I made an elaborate box with IR screens at the bottom and sensors at the top. Both the sensors and IR screens could be adjusted longitudinally to enable velocity calibration and to position the screens for maximum signal strength. It was a real disappointment. I just could not get both CEDs working simultaneously - there was always one of them with either an E0 or E1 error . My conclusion is that the IR screens do not emit an even glow over the screen surface, in fact I suspect some of the tiny IR emitters weren't working, but I can't tell without an IR viewer. Anyway, I then decided to abandon both the IR and sky screens and put the sensors in an enclosed box with white interior and bright light bulbs. First signs were very good - no E0 or E1 errors. But then I looked at the figures and the velocity differences were varying dramatically, in the order of 50 fps! Apparently the white background wasn't bright enough . I could have tried even brighter bulbs and a brighter white surface, but decided instead to go right back to square one and use both CEDs with sky screens in bright overhead sunshine. I set them up in series with the old F1 as shown in the picture below. After a bit of spacing calibration of the CEDs, here are the readings for some different 7mm loads :

57.0gn CED1 2844fps CED2 2845fps F1 2846fps
57.5gn CED1 2860fps CED2 2860fps F1 2859fps
58.0gn CED1 2902fps CED2 2898fps F1 2900fps
57.2gn CED1 2857fps CED2 2859fps F1 2860fps

As you can see, all 3 chronos are giving the same readings to within a few fps! And there was not a single E0/E1 error ! So what have I learnt? I will now do all velocity spread testing on clear days when the sun is high with just the sky screens. If they're always run in series then it'll be obvious from the results when the light is too low.

<img src="http://www.ozfclass.com/forums/pic/chronoqueue.jpg">

[Brad Y]

Alan, taking an assumption you got from your testing there. You must have a darn good F1 or maybe they are better than I thought. Might have to use mine a bit more!

[BATattack]

thats a pritty serious piece of engeneering alan!! what do the boys think when u wheel that sucker onto the range??

considering the very small aiming angle changes we need when doing long range testing i was thinking of setting up a magnetospeed chronograph directly in front of the barrel OR setting up an attachment to the front of the forend OR mounting a bracket off the joystick top so it moves with the gun.

seems that the magneto speed chronograph gives much more reliable readings the only downside is they change of bullet impact and change barrel harmonics. . . . . some say they dont change harmonics but they do.

ive done ladder testing several times at 5-700yds using a chronograph and a person down the buts plotting my shots. genrally we havnt seen any correlation between a low velocity on the chronograph and a low shot on the target which leads me to believe ther is more error in the chronograph than there is in my load practices. at the end of the day i shoot the ammo that gave me the smallest group at long range.

it would be interesting to test your set up with a reasonable load and see if your slower/faster shots are landing in their appropreate positions on the target?

[Barry Davies]

It does not necessarily follow that a slower shot will strike low and a faster shot will strike high --if that's what you mean by appropriate.
Barry

[Bindi2]

I have a F1 Chronny. One of the early ones . I bought it new 30 odd years ago.
Two things i have learnt
error messages are either flat batteries or the sensors need blowing out with a compressor. Dust is a curse.
The Chronny should be in light shade never in direct sunlight.This is a long story that did have serious problems till the testers were finally proved wrong.

[Chopper]

Just a thought , what happens on a constant overcast day with testing? Chop.

[IanP]

Interesting project Alan!

IR leds in your screen each have a molded lens and give a designated angle of dispersion maybe anywhere from 15 to 30 degrees. I built my IR screens so I know the angle of dispersion and designed the spacing to suit the height of the screen and coverage needed for the detector at the bottom. I'm sure CED have designed to illuminate as required for the sensor and screen dimensions.

The sensor is a photo diode or photo transistor which toggles an op amp and provides a start or stop signal to the micro controller to start or stop the timing. Its a very low tech setup and should be reliable and each detector could be easily tested for operation. Photo detectors are sensitive to infra red light within a certain wavelength so IR leds are a perfect match, much more so than "white lights". A single led not illuminating I dont think is at all possible. The led array in the screen will be wired in series to ensure the current thru the individual leds is the same and the illumination is as close to identical for each individual led.

You can easily check your led array by turning it on in a dark room and viewing it with a digital camera or video camera. I checked mine with an old video camera set in night mode and was amazed at how much light the array gave out.

The CED uses an angled lens to gather the IR light from whatever source and focus it onto the detector and I wonder if this has become dirty or misaligned slightly on your chronograph. My advice would be to have a look at the IR array with a digital camera in a dark room to see if it is working. Check the lenses on the detectors for dirt or misalignment. Then check the output of each detector to see if its working. This requires a digital oscilloscope but is an easy test to perform by anyone with a knowledge of electronics. I doubt the microprocessor will have a problem with its timers for calculating and displaying the output in fps tend to think its probably more a detector issue but without having it to test I dont know for sure.

If in doubt buy the new MagnetoSpeed V2 with an adjustable height bayonet for use on all barrel diameters. The pulse detection principle and sensors that dont rely on light are the way to go. Real muzzle velocity and easy as pie to set up with repeatable results.

Ian

[AlanF]

Just a thought , what happens on a constant overcast day with testing? Chop.

No testing .

[AlanF]

Ian,

Thanks for the info. I will try the video camera - I have 4 IR screens so can compare the ones that give errors with those that don't.

It seems I can get by without the IR. I'll do velocity spread testing at home into a bank with both CEDs. It only takes a few shots to eliminate a bad powder/primer/seating depth combination, more to confirm a promising load. I'll probably just use the F1 at the range where accuracy is not as critical. Can I borrow your Magnetospeed in Raton?

Alan