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Yes, it's been some time since my last installment of The Chronoman Report and from my observations, a few of you have suffered from my absence. So, let's go over some of the basics of chronography.
The chronograph, no matter which brand you use, senses the shadow of the bullet as it traverses the start and stop "gate" of the unit which, through electronic wizardry, gives you a display reading in feet per second (meters per second for our metric friends).
Now being a highly sophisticated piece of electronic gadgetry, it works best when its environment is as consistent as possible. Things such as temperature, humidity, light intensity and angle, and planetary alignment (for gravitational and coriolis effects) can adversely affect it's readings. Pistol bullet flight times aren’t long enough for the rotation of the earth to become an issue (coriolus effect).
Now out on the range in the heat, cold, wind, rain, dust, and earthquakes there isn't all that much we can control other than light angle and intensity. These two items have the greatest affect on recorded velocities. In my own independent testing, I have had repeatable differences of over 100 fps using the same equipment on the same day just by switching in and out of a lightproof box. We were using six different guns and all of the test gun owners commented that the out-of-the-box velocities were way too high and power factor calculations confirmed this. It was a very bright and sunny Sacramento day.
The biggest complaint we receive at the National's Chronograph Station (besides from my bad jokes) is that our velocity readings are significantly different from those recorded at home. Aside from the fact that "home" may be 6000 feet higher, 80 degrees cooler, and 300% less humid, after questioning the competitor a bit we find that s/he has been using direct sunlight, usually right after work in the afternoon. The light intensity and angle over the screens being the culprit for inaccurate yet consistent readings.
We can correct this by situating the chronograph(s) in a light-proof box and use either home fabricated incandescent lights mounted over the sky screens or factory made lights that replace the sky screens. Both Oehler and CED make excellent light packages for their equipment. Care needs to be taken to insure that the whole set up will operate on your range of choice with batteries in the event that commercial power is not available.
Depending on your own personal chronograph, its manufacturer may recommend a specific wattage for indoor (in box) use. This can range from 40w on up to 100. When I use this type of set up, I'll place the lights directly over the sky screen, being careful to avoid direct contact as the plastic sky screens will melt or burn. (Burning down the chronograph station is usually frowned upon by the upper match staff and is covered in the rule book as "Range Equipment Failure".
The best incandescent bulbs I have used are display case bulbs. These are usually in the 40-60 watt range and are elongated. Aquarium bulbs may be used also but are usually rather pricey due to the special "fish light" that they emit. RV lights may be used when commercial power is not available as they operate on 12VDC.
Another set up precaution that will help with consistency of readings is to be sure to erect a blast shield for the bullet to pass through before it passes the first Sky Screen. This is especially helpful when shooting sub-sonic rounds as the muzzle blast from the gun can beat the bullet to the Sky Screen and give false readings. The blast shield also protects your down-range equipment from blast damage.
On the chronograph box (The Coffin) in particular, make it big enough to accommodate more than one model of chronograph. There are differences in height and width. You might consider making it big enough for two units to operate in series in order to cross-check your equipment with another unit like we do at Nationals.
As far as construction materials are concerned, anything from marine plywood to foam-board can be used. Keep in mind that the lighter in weight the box is, the more portable it is. Adjustable legs are handy for alignment with the shooting position as well. The access door to the chrono coffin should open from the side. It's extremely difficult to place the equipment in and remove it from the top of a chrono coffin without falling into the box and there by being declared a part of the range equipment and having to stay there the entire match.
For bullet entrance and exit holes in the end panels (at Nationals we refer to these as the bullet g'zinta and g'zouta holes), a 1-lb coffee
can makes an excellent template. Locate the holes so they’re centered in the triangular path above the sensors and below/within the sky screens. This will usually keep the bullet's flight path 2-4" above the sensors and below the light emitters, avoiding expensive replacement parts and the ire of the equipment sponsor.
If you have the urge to paint the chrono coffin, by all means do. The exterior can be any color your heart desires but UNDER NO CIRCUMSTANCES PAINT THE INTERIOR WHITE! A white interior, while pretty, will bounce light all over the place and give no end of false and error filled readings. In fact, the chronograph may refuse to work at all inside a white-painted box. A flat-black painted interior works quite well.
On a final note, if you're in an area which is "chrono-box deficient", a place on the planet from which absolutely no boxes can be found or manufactured, one of the older chronograph manufactures use to recommend to take your chronograph readings throughout a given day and then average your readings -this being their cure for varying light conditions. It seems a rather time and ammunition intensive fix but perhaps they came from one of the aforementioned box deficient areas of the world. If you have any chronograph related questions or comments please feel free to contact me at: Mygunwerks@aol.com
Chronoman
June 28, 2008
by: Greg Lent
TY14427"
This post has been edited by geezer-lock: 19 October 2009 - 12:39 PM
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