by Tom Gaylord
Writing as B.B. Pelletier

This report covers:

  • How it works
  • The evolution of the personal chronograph
  • The advent of the skyscreen
  • Advantages of skyscreens
  • Fly in the ointment
  • Screen placement problem one
  • Screen placement problem two
  • Shooting perpendicular to the skyscreens
  • Lighting
  • Odd chrony info
  • Summary

Reader Buldawg asked for this report, but several other readers chimed in and seconded his request. I have written about this before, but we have so many new readers that it is probably time to do it again. There are several definitions for the word chronograph, including a very accurate wristwatch, but the chronograph I am talking about today is one that measures the velocity of a projectile. We are interested in pellets, mostly, but the chronograph was created to measure the velocity of bullets.

How it works

A chronograph works by counting time between two events. In today’s discussion those events are the passing of a projectile through two electronic screens. The first one starts the clock and the second one stops it. So we call these screens the start and stop screens. The counter parses time into very small intervals. In the case of my Oehler 35P chronograph the counter divides each second into 4 million units. So time between the start and stop screen is measured in 4-millionths of a second.

There is no difference between this and a pneumatic hose laid across a road to count traffic, or across a gas station to ring a bell when someone drives in. When Bill Gates invented that device (for the roads, not the gas stations), he told buyers it could count traffic and it could also measure the speed of vehicles, if the distance between the first and second hoses was precisely known and remained constant. Radar guns were more accurate, though, and didn’t have the problem of the hoses moving, so Gate’s invention was relegated to counting axles.

The evolution of the personal chronograph

Before the 1960s, ballistic chronographs were laboratory instruments manned by a staff of trained personnel. Chronographs became affordable to private individuals in the early 1960s. Back then the technology was crystal clocks that pulsed at speeds in the 10-100khHz range. Their results were displayed on nixie tubes that had to be interpreted through tables. Direct velocities were not given.

The screens used at this time were paper with wires running through them. The wires were attached to connectors on both side of the screens. When a bullet passed through the screen it broke several of the wires, changing the conductivity through the screen. This was sensed by the circuitry in the chronograph and served to turn the “clock” either on or off. There was a start screen that turned the clock on and a stop screen that turned it off. It was a brute-force approach to the problem, though, and suffered from imprecise results. If you went through a hole left by a previous bullet, there was no reaction. And after awhile, the screens were shot up and had to be replaced.

The advent of the skyscreen

It was Oehler who first gave us the “skyscreen” — a screen that used a photo optical sensor to detect the passage of the bullet/pellet by sensing the reduction in light (the shadow) as the bullet passed over the narrow aperture of the screen. I am going to call this aperture the optical curtain. You are already used to these sensors being used to automatically open doors whenever someone approaches, so this is just a different application of the same technology.

Advantages of skyscreens

The skyscreen changed everything for shooters. Not only did it eliminate the need to buy replacement screens, it also did not get shot up in normal use. A chronograph with skyscreens can function for hundreds of shots without intervention. Suddenly the $300 that was paid for a chronograph (1970s money, so $1,000 today) didn’t seem like such a burden!

skyscreens
The skyscreens have to be exactly spaced for the chronograph to work correctly.

Fly in the ointment

Ah, but even skyscreens aren’t perfect — and therein lies the gist of this report. It is possible to get spurious readings when using skyscreens, and the problem is — most people will never know it. Today’s chronograph shows a velocity in both feet per second and meters per second on a large digital screen. The problem with that is whenever most people see a number on an instrument display — they believe it. And most of the time that number is wrong — just as the speedometer in your car displays the wrong speed most of the time. To get the right speed (in either the car or on the chrono) everything has to be rigidly controlled.

Screen placement problem one

Screen placement is of paramount importance. Think of the skyscreen aperture as a thin optical curtain that extends straight up from the screen. When the pellet pass over (through) this curtain, it lowers the light hitting the photo receptor in the screen. That triggers the clock to either start or stop.

skyscreen tilt
If one screen is not parallel to the other, the readings will be incorrect.

If one of the skyscreens is not parallel to the other screen, the space between the thin curtains is different than the computer in the device expects it to be. A difference of one-eighth-inch (3.175mm) spacing will give a result that is a serious percentage different than the correct number, depending on the clock speed.

Let me make it simple You will see the number 850.5 f.p.s. on the display, when the true velocity is 829.7 f.p.s. You will dutifully record the number you see and you will be a little more than 3 percent off the actual velocity — all because one of your skyscreens isn’t pointing where it should. Incidentally, in this case, the amount you are off will vary, depending on how high or low the muzzle is when you shoot through the screens. because the screens aren’t parallel, the separation varies with the distance above the sensors.

Screen placement problem two

The screens are supposed to be a specific distance apart. Let’s say that distance is 12 inches. But what if one of them is 1/8-inch closer than it should be to the other screen? Again, you will get the wrong number. This time the screens are parallel, but they are spaced incorrectly. This time the amount of error won’t change as the pellet rises or falls against the sensors — just as long as the pellet’s flight through the screens is perpendicular to the sensors! And there lies the next, and by far the most common, chronograph problem!

Shooting perpendicular to the skyscreens

When you shoot you have to hold the barrel of the airgun perpendicular to the optical curtains extending up from the skyscreens. If you don’t, the pellet will take longer to trip the stop screen, which will lower the recorded velocity. This is the most common fault shooters make when using chronographs. They get careless because the chronograph seems so forgiving, but what they are really doing is helping it to lie to them more convincingly. The displayed number looks so official!

bore off
If the bore is not perpendicular to both skyscreens, the reading with be incorrect.

Lighting

Everyone soon discovers that the lighting has to be constant. If it pulses like fluorescents do, the optical screens will trip and the chronograph display will go haywire. But what people don’t appreciate is that the skyscreens also need even constant light to function correctly. A cloudy day is perfect, as long as the clouds aren’t moving around. If they are the chronograph will go crazy.

Direct sunlight will kill the skyscreens. It’s like looking directly at the sun with your own eyes. That is why the translucent shades are provided with the instrument. Most of the time they aren’t needed, but when they are you’d better have them!

In my office I use a white ceiling and I bounce a 500-watt photo light off it. It works perfectly. Are my numbers correct? Who knows? The best I can tell is they are consistent — reading to reading — because I always set up in the same way. Those numbers look impressive when you read them and I never get called on them, but I would not bet a lot of money that they are 100 percent correct.

Odd chrony info

The faster the clock speed the more precise the number you get. Screen spacing is still important, but if it is exact, the numbers are pretty close to reality.

Screen spacing is important. My Oehler separates the start/stop screens by 24 inches, which is give more precision than my Shooting Chrony Alpha Master that separates the screen by 12 inches. If the clock speed were the same, the larger screen separation would triumph. I used to own a British chrono that attached to the muzzle of the gun. It had about 2 inches separation between the screens. It still gave me a number, but how precise could it be?

There are chronographs that use infrared light and there are chronographs that use induction (the passage of a mass that produces an electromotive force close to a sensor). I can’t comment on the induction chronos because I have never tested one. I’m sure they give you numbers, but how accurate they are is for someone else to say. The IR chronos work fine, but watch out for the sun if you use them! The sun through a skylight can cause a problem.

Summary

These are the basics of using a chronograph. The most important thing to remember is that all chronographs will give you a number. The question is — do you trust it?