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Registered Member #544
Joined: Wed Feb 21 2007, 11:20PM
Location:
Posts: 5
hi everyone... i recently completed my 2kJ coilgun and in order to measure the velocity of my projectile i purchased a rifle chronograph. rated for speeds of 30 - 7000fps. my velocity measurements thus far have been ridiculously (and quite obviously) high. i tested it with a .22 rifle and made sure it was calibrated correctly. the apparatus is all setup correctly and is being used correctly so thats no problem. the only thing i can think of is the difference in the length of the projectile compared to the length of a given rifle bullet. my projectile is 5cm long. at higher speeds and in that short distance, the small difference in lengths would significantly alter the velocity readings depending on how the optical sensors operate. my question is has anybody else had this problem? any suggestions or ideas would be very helpful and much appreciated thanks in advance! -ap
Registered Member #156
Joined: Sun Feb 12 2006, 07:04PM
Location:
Posts: 23
Maybe the electronics of chronograph is disturbed by the em field of the coilgun. I had this problems with coil- and railguns when the distance between gun and sensor was to small.
Registered Member #167
Joined: Mon Feb 13 2006, 06:41PM
Location: Waterloo, WI
Posts: 54
I know the problem! your experiment is slightly flawed. The surronding lighting is critical. I am assuming when you tested the 22 you were outside? and when you tested the coilgun you were in the basement or something.
Registered Member #544
Joined: Wed Feb 21 2007, 11:20PM
Location:
Posts: 5
nope. both shots tested inside under the required shooting lights. no fluorescent lights were on and all windows were covered. i took all of the lighting precautions to prevent any problems of that sort.
Registered Member #544
Joined: Wed Feb 21 2007, 11:20PM
Location:
Posts: 5
contacted the company about it and they notified me that their chronograph did not work properly in the presence of a magnetic field. is this a common problem? i see a lot of people using chronographs for their velocity measurements so there surely must be a way around this. any ideas for shielding the box? an enclosure with a path to ground possibly?
Registered Member #544
Joined: Wed Feb 21 2007, 11:20PM
Location:
Posts: 5
Yes the case is grounded. I built a steel enclosure for the coil over the weekend and my results have been mixed so far. I agree with the idea of extending the distance between the chronograph and the coil, but my lab space is fairly limited right now for a variety of reasons. When space frees up I have also decided to get out the air track and get a velocity measurement using conservation of momentum, mainly to see if my photogates are giving me an accurate reading.
Registered Member #158
Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282
My Pal Digital Chrono wouldnt work within a few feet of mine either. Would spit out numbers over a thousand fps. I forget the exact lengths, but keeping it maybe 5 feet away or so and it worked fine. The problem I had was accuracy, and ended up destroying the sun visor things and also breaking the faceplate of the unit, since a little over 5 feet away the projectile was hitting the unit occasionally.
The length of the projectile shouldnt matter, most all commercial chronos should be capable of measuring a .22 or a 30" long arrow, mine measures shotgun shot accuratly (within a couple feet per second of the number stamped on the box of shells) as well as arrows from my bow just fine.
Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
I had never problems using the bjorn's pic 16f84a speedtrap. It works reliably, just set some hysteresis on the detectors (used a comparator with positive feedback and adjustable threshold)
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Chronograph problems in magnetic fields
