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
![Brick wall ](*,)](./images/smilies/eusa_wall.gif)
. 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
![Brick wall ](*,)](./images/smilies/eusa_wall.gif)
. 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
![Cool 8)](./images/smilies/icon_cool.gif)
! 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">