Chronograph tips

by B.B. Pelletier

I’ve probably said these things before, but they’re fundamental and bear repeating. With more of you starting to use chronographs, you need to be aware of some of the basic operational tips for the equipment.

Before we go any farther, know that I’m limiting my remarks to the Shooting Chrony-type of chronograph that uses an incandescent or natural light (sunlight) source. There are various chronographs that use infrared light sources and these notes do not necessarily apply.

Lighting outdoors
Outdoors you want an even source of light, with an overcast sky being the best. The worst is direct sunlight falling on the skyscreens. That condition is what the diffusers are for, so use them. A day where clouds are being blown all around is a tough day to chronograph outdoors.

Lighting indoors
Fluorescent lighting does not work with a chronograph. A fluorescent light flickers at speeds imperceptible to the human eye (most of the time), but the sensitive skyscreen will be set off. When that happens, you’ll get spurious readings, errors and half-readings. That can be a big problem, now that many households are converting from incandescent lighting to fluorescent. You may have to kill all the lights in the room to get the chronograph to work.

Mercury vapor lights found in warehouses and workshops can also be problematic. Whenever your chronograph starts firing on its own, you probably have a lighting problem.

Direct light sources are another way around indoor lighting problems. There are commercial skyscreen lights you can buy or you can make a light bar of your own with parts bought at a hardware store. A much simpler way, if you shoot in a room that has a ceiling painted a solid light color, is to reflect a bright light off the ceiling and let the skyscreens look at that light. This is how I do it in my office, which has a 10-foot ceiling. I use a photo light, but you can use a 500-watt halogen work light shined upward.

Chronograph tips
Here’s the big one. Keep the muzzle of your gun at least a foot back from the starting skyscreen. This is especially true when testing the powerful super magnums, such as the new Benjamin Trail NP XL1100 I tested for you on Monday. In fact, it was while testing that rifle that the idea for this report was born. I held the muzzle of the gun too close to the start screen a couple times and got several shots that measured 300 f.p.s. slower than they should have. That’s not the gun acting up. That’s the chronograph operator’s fault.

If we had a super-fast video camera filming the muzzle of a spring gun, you would be able to see a ball of pressurized air that comes out of the gun ahead of the pellet. If the pellet travels at 900 f.p.s., this ball of air goes about 1100 f.p.s. for a couple inches. If the muzzle is held too close, the skyscreen senses the ball of compressed air and starts the clock. Once the clock has been started the pellet has no effect on it anymore. Of course, the pellet passing over the stop screen stops the clock and now you have a longer interval on the clock than the pellet really should have registered. More time equals a slower pellet transit time, hence the readings are slower than they should have been. Just by backing the muzzle up 12 inches from the start screen, you take care of 100 percent of this problem with all spring guns. Maybe with some powerful pneumatics like the Condor you should back up 18 inches. And certainly with a big bore I would back up 4-5 feet. The pressurized air will still be seen by the skyscreen, but by backing up you allow the pellet/bullet to trip the sensor first.

Tip #2–stay level
Both skyscreens look in the same direction. If the chronograph is flat on a table, both skyscreens should look directly upward so they are set up to calculate the passage of a pellet that flies perpendicular to their line of sight.

If you angle a shot through the line of the skyscreens, the time it takes to trip them will be longer that it would have been if they went through at a perfect perpendicular angle. That’s because an angled line through two planes is always longer than one that passes through perpendicular.


The slanted line through the top chronograph is a longer path, resulting is lower indicated velocities. The bottom chronograph shows how the rifle should be fired.

Maybe the explanation is difficult to follow, but look at the drawings. I can slow down any gun by slanting the line of the pellet through the skyscreens. Try it yourself and you’ll see what I mean.

Tip #3–clean those skyscreens
If your chrono has any age to it, your skyscreen shields are dirty. Those are the clear plastic “lenses” that cover and protect the real sensors. Use a Q-tip to clean them, and your numbers will be easier to obtain.

Tip #4–watch your angle
As well as watching the up/down angle through the screens, you also need to be careful of the sideways angle. You will get an “Error 2″ message when you miss screen two, which is the most common error you’ll see, because the start screen (screen 1) is closer to the gun and harder to miss.

Well, those are some things to think about when you use your chronograph next time. They’re wonderful instruments that respond best if a little care is used during their operation.

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