by Tom Gaylord
Writing as B.B. Pelletier
Shooting Chrony Alpha Master chronograph has a control box with display that separates from the skyscreens by 18 feet. These chronographs are shown with their light diffusers attached, but I don’t use them. Read why in the report.
This report covers:
- What about the false triggering of the skyscreen?
- Start screen — stop screen
- Atomic blast wave
- The solution to false triggering
- How I use the new chronograph
- Battery operated
- Light diffusers
As you may be aware, I shot my Shooting Chrony Alpha chronograph with the Benjamin Bulldog big bore air rifle. It wasn’t the rifle’s fault — it was mine. There were other shooters on the range, so I had to place the chronograph about 10 feet in front of my position to get the skyscreens out from under the metal roof that hangs over the firing line. For range safety rules, I could not get closer to the chronograph. So, I was pointing the airgun to shoot over the skyscreens.
Unfortunately, the Bulldog is a bullpup design that has a heavily shrouded barrel. It doesn’t point very well. On one shot, I pointed too low, and a .357 bullet hit the plastic skyscreen enclosure and the metal case.
To its credit, the chronograph continued to function. Even with the shielding blown away, the skyscreen sensor wasn’t damaged and still sensed the passage of the bullet, but I was concerned how long it would last. I asked Pyramyd AIR for a replacement chronograph, and this time I requested the Alpha Master, which has a control unit that separates from the skyscreens by 18 feet. That would allow me to shoot at the range in safety. In fact, I’ve been to the range 2 more times with the new chronograph, and it works exactly as planned.
What about the false triggering of the skyscreen?
I’ve touched on this topic several times over the past few weeks. My new chronograph has had some false starts. The first mention was when I tested the Diana 45 after the tune. I got some velocities that were hundreds of f.p.s. too slow in the middle of strings that otherwise seemed normal. If I’d never seen this sort of behavior before, I would have easily been tricked into thinking the gun was at fault. But back in 1994, when I wrote the R1 book, I had a similar experience that was due to the early triggering of the first skyscreen.
Start screen — stop screen
The skyscreens are photoelectric sensors that detect small changes in light. When a pellet passes over a screen, it casts a slight shadow and the sensor detects a slight drop in the light it’s receiving. That’s why lighting is so important to chronograph operation.
The first skyscreen tells the onboard computer to start counting the time, and screen 2 tells it to stop. Since the distance between the 2 screens remains fixed, it’s possible to count the time it takes the pellet to pass from the center of screen one to the center of screen two — and use that to calculate how fast the pellet’s moving. If the first screen — also called the start screen — is triggered early, the computer counts a longer time before the pellet crosses the stop screen, and the pellet’s velocity is calculated to be slower.
But how can an optical sensor “see” anything before a pellet passes over to cast its shadow on the screen? The answer is distorted air waves that get past the pellet as it leaves the muzzle. You’ve all probably seen something like this, but you didn’t know what you were seeing.
Atomic blast wave
When an atomic bomb detonates, a white ring of energy moves out from the blast at supersonic speed. It’s visible in the air and on the ground for several miles. This white energy ring is compressing the air it passes through at supersonic speed. The air the energy wave passes through condenses when it’s compressed, and that’s the white color you see.
When a pellet gun fires, the compressed air escapes around the pellet at the muzzle. While it’s similar to the energy wave from the atomic blast, it isn’t the same. In this case, it’s just the air that was compressed in the barrel behind the pellet. For a short distance, this compressed air appears as a visible distortion in the air. The first skyscreen can sense this distortion for a few inches from the muzzle, and that’s what sometimes triggers it to start the counter. Since the distorted air moves faster than the pellet, the start screen starts early.
I’ve tested my new Alpha Master for this phenomenon with other pellet guns, and I’ve found the skyscreens on the new instrument to be very sensitive — much more sensitive than the screens on my old Alpha chronograph. Sensitivity probably varies from sensor to sensor and is not specific to any certain model of chronograph. I’ve also seen the same phenomenon with my Oehler 35P chronograph, as well as with a very old Shooting Chrony I used to have.
The solution to false triggering
The solution is to keep the muzzle of the gun back from the start screen far enough to allow the distorted air to dissipate. Shooting Chrony recommends 3 feet in their manual, but I’ve found that 2 feet is far enough for my new chronograph. Firearms shooters rarely see this phenomenon because they usually shoot from at least 10 feet back. At that distance, even the much more pronounced shock wave of expanding gasses from the burning gunpowder dissipates. But I shoot in my office, where I used to hold the muzzle 12 inches from the start screen. I have to be farther back with my new chronograph.
How I use the new chronograph
I normally don’t need to separate the skyscreens from the control box when testing in my office. I have a table next to my desk with the chronograph permanently set up because I usually chronograph 100 to 200 shots each week. So, that 18-foot cable remains coiled up on my desk. All I do is position a 500-watt halogen light to shine on the white ceiling above the skyscreens, and everything works perfectly.
When I do go to the range, it’s a simple thing to uncoil the cable and set up the chronograph to work remotely. I can then set out the skyscreens on the range and operate the chronograph — including turning it on and off — from the safety of my shooting bench.
All of the mathematical functions of the Alpha model are present in the Alpha Master. I can record up to 32 shots in a string; delete any numbers that I don’t want in the string; and calculate the average, fastest shot, slowest shot and the standard deviation of the string. The power switch is on the remote unit, so I’m in complete control of the chronograph, even when it’s downrange, where I can’t go.
The Shooting Chrony registers velocities under 1,000 f.p.s. as decimal fractions, so I get numbers like 732.4 f.p.s. I don’t report these decimals in my blog articles. I round them up or down in the conventional way (0.5 to 0.9 is rounded up to the next higher whole number and 0.1 to 0.4 is rounded down to the lower whole number).
All Shooting Chronys run on one 9-volt battery, and they last a reasonably long time. You’ll get a low-battery indication before the instrument dies. So, pay attention to the display, and you’ll never be without a chronograph.
Every Shooting Chrony comes with 2 white plastic light diffusers to let the skyscreens “look” at even lighting. You see them on their posts in the first picture. I never use them. They’re for those days when there are fast-moving clouds in a bright blue sky or for when you’re in direct overhead sunlight or under overhead cover that casts uneven shadows. Indoors, I point my skyscreens at a white ceiling, and outdoors I use either the bright blue sky or an overcast day. Overcast days are ideal for skyscreens.
When it comes to chronographs, shooters never had it as good as we do today! When I was young and personal chronographs were new, they didn’t have skyscreens. They had paper screens with wires embedded in them. The instrument registered the breaking of the wires as small changes in conductivity, so you only got a few shots at a screen before it had to be replaced. Buying replacement chronograph screens was a considerable investment for a shooter.
The chronographs of that time didn’t read out in numbers, either. They had lights arranged in columns — sort of like the computers from a 1950s Buck Roger’s movie. When you fired, some of the lights illuminated, and you had to look up what they meant in a table. You didn’t get an exact velocity. You got a range of velocities, like 2,140-2,148 f.p.s.
You paid $300 and more in 1960 dollars for these instruments and counted yourself fortunate to be able to own one. Just a decade earlier a chronograph cost about $100,000 and took a trained person to operate! See Smith’s Standard Encyclopedia of Gas, Air and Spring Guns of the World for that one!
So, shelling out $100 and change in 2015 is peanuts for what you get. I was amazed to discover the difference between the Alpha model I had and the new Alpha Master model I got is just $7.04. I would pay a lot more to get the ability to control the skyscreens remotely.