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Calibration of a chronograph is interesting....

At one time, NISt had a container of speed standards, but when they opened it, and the standards sped off into the bush, and have not been seen since. That's a joke, of course. But it is true that there is no such thing as an NIST speed standard.

Speed is a derived quantity, derived from distance and time. We do have NIST traceable standards for those quantities.

Distance between the photosensors is fixed.

Crystal controlled oscillators are used to measure time. Even a cheap crystal is very stable. The crystal will age a bit when first put into service, but will not change enough to matter. If you want to check the oscillator frequency, you can make a little pickup loop, place the loop near the circuitry, and feed the signal to a frequency counter. But that won't be very revealing, since the crystal is very time stable, and you'll always get an answer close enough to nominal that the difference won't matter.

Once the chronograph is calibrated at the factory, it pretty well stays calibrated.

Now the LabRadar works very differently. The outgoing signal is mixed with the return signal to create a "beat frequency", and it is trivial to measure that to four or five significant digits. Ken Oehler has said that getting the effective spacing between photosensors to better than 1/8" was difficult. The LabRadar overcomes that problem.

That is interesting. What, in your opinion, would be an indicator that a chronograph is damaged or otherwise out of spec? Could a component other than the photosensor or oscillator be damaged and cause the unit to display an erroneous reading?

For the most part, the circuitry either works or it doesn't. If it's not working, you don't get a reading.

For conventional chronographs, there is one condition that will definitely cause an error, and that is lighting. If you are using your diffuser screens and a cloud comes by, your readings will shift. IIRC, it's about 25 FPS with a rifle cartridge.

To do good chronographing, you have to control the lighting, and you have to stick a thermocouple to the barrel just forward of the receiver, and control barrel temperature. If you let the ammunition sit in the chamber for about a minute, the ammunition will effectively come to barrel temperature, so you've controlled both ammunition and chamber.

Those are conditions external to the chronograph.

For the morbidly curious:

The passage of the bullet makes a little dip in the light that the photocell receives. This signal then passes to a Schmidtt trigger, that provides a clean starting edge for the counter. When the counter gets that clean starting edge, it starts counting cycles of the crystal oscillator. When the second sensor gets its edge, the counting stops. The Shooting Chrony oscillator runs at 12 MHz, so if the counter gets 4,000 counts, 1/3 millisecond has elapsed from start to stop. The sensors are 1 foot apart, so the speed is 1 foot / .33333 milliseconds, or 3,000 feet per second, if I've done my math right.

The system is pretty bullet proof. Alas, the chronograph, not so much.

I've compared several Pro Chrono chronographs against other, far more expensive units. They've all been spot on. I think it's foolishness in the day and age of $99 chronographs not to use one. It's the biggest indicator of safe handloads when comparing against pressure tested data.

Thanks, Denton.

Brad, I agree for the most part.

Perhaps chronographs, regardless of manufacturer, should be used by more people to confirm safe pressures and not just velocities. It's probable that this group shoots more than the average Joe. I would not say that absolutely everyone has a need. If you need 10 cartridges to take to deer camp, following a manual should be fine. I know in the past there were a few times when inaccurate data made its way into print, but these days, it's unlikely.

denton,

I have owned two Shooting Chronys which declined in accuracy over time--and also have a local friend who had the same thing happen. They did not just quit working. Instead two started showing velocities that were obviously much higher than they should have, and changing batteries didn't help. The third started showing variation between sunny and cloudy readings of 100-150 fps. Sent that one back to get fixed, and it worked OK for a while, but I was starting to mistrust them.

Back then I was doing a LOT of shooting with a wide variety of cartridges, and eventually concluded the muzzle blast of some of the bigger rounds was shaking something loose. That's when I acquired my first ProChrono, and those problems ceased. (Purchased another PC maybe two years later, in large part to see if it worked as well, and also tested it against my Oehler 35P while shooting a wide variety of cartridges from .17 to .416 caliber. The second one worked just as well, despite me using the plain old overhead filters, instead of the furnace filters that Al Nyhus has used to good effect.

But the first also just kept trucking, so I gave the second one to a young friend with a growing family. This was several years ago, and he's still using it, with cartridges as large as the 33 Nosler--with the chronograph less than five feet from the muzzle.

John

Interesting and useful information.

Yes, muzzle blast can have an effect on readings. Besides the optical possibilities, disc ceramic capacitors (among other components) tend to be microphonic. You can strap one across the input of a high gain audio amplifier, tap on it, and hear the tapping at the output. So muzzle blast can possibly mimic/interfere with the effect of the passing bullet.

I used a Shooting Chrony for years, and did a formal analysis of the Probable Error. IIRC, the real precision (ability to get the same answer, given the same stimulus) of the system was between 1 and 2 FPS.

That said, in addition to the passing cloud issue, if you use only one section of rod to support the screen, you will get a different reading vs. if you use both sections of rod. So I had a selected set of conditions for chronographing, and carefully followed those. I got very consistent, credible results, but could only do measurements when conditions were just right. So now I have a LabRadar.

I'm a little nonplussed by your experience with declining accuracy. That should be un-possible. But anything that happens must be possible. There was a design change part way through the production run, with early units running a 4 MHz clock instead of 12 MHz. My unit was a later unit. Wonder what else they changed, and if that had any effect?

denton;
Good evening to you sir, I hope the day's been a good one for you and all who matter to you are well.

Two years ago now, the Shooting Chrony which I'd had for years began to show excessive readings at higher velocity. Strangely enough I was there with a buddy who was testing his which was a newer model of Chrony - a green one with the metal diffuser stands. He said he was getting odd readings from it on occasion as well, so we set the two up in tandem.

As a matter of practice and likely something that John has written recommending if I'm not wrong, I run a few shots with a .22 with the same box of ammo on all chrono days.

The crazy thing is that they stayed correct or at least within 20fps of where it should have been and both chronographs confirmed that.

When we chronographed a .17 Rimfire Mag my Chrony was a little bit too fast however. Then when I ran a few loads from a 6.5x55 my Chrony reading was 150fps too fast. Lastly I ran a few rounds through my .308 Norma which were loaded maybe 5 years back and in all subsequent chronographings had been consistent until that day when my Chrony readings were way too fast.

After a bit of correspondence with John and on his recommendation I picked up a Pro Chrono from a retailer here in BC who had one in stock.

All of the "known" speeds went back to "normal" again with the Pro Chrono.

Anyways that's my experience with a Chrony losing it's way, but again it was a very old one with the cardboard light diffuser holders - pre metal rods - and it'd served me for decades so I got good life out of it.

Hopefully that made sense and was useful to you or someone out there.

All the best.

Dwayne

Edit;
Both were Chrony's but different ages.

Well, anything that happens must be possible. And now we have another example of one of the older Shooting Chronys going bananas. Hmmmm......

The fundamental system is extremely basic: A start pulse, a stop pulse, a count of the clock pulses in between, and a distance divided by measured time to get speed. Such a system either works right, or not at all. The only guess I can come up with is loss of hermetic seal on the oscillator unit, or some kind of internal contamination that slowly diffuses. I've never actually seen that happen.

So I am perplexed.

I wonder if damage to a circuit board or other component would account for the declining accuracy? The reason I asked about damaged equipment earlier is because my experience has shown that unserviceabilities like cracks or impacts from rough handling do not always stop something from functioning, but will degrade performance. Muzzle blast, heat from the sun or improper handling may cause problems.

When I was in the military, I had to get regular physicals as part of my overseas duty. The medical people had a new fangled blood pressure gauge that automatically inflated and took readings. I was almost removed from duty because my pressure was recorded as being 200/95 on several tries. I asked if these BP units were properly calibrated. I was rudely told that they "self calibrate" and cannot give bad readings. Ah...

At my insistence, the doctor used an old fashioned BP gauge with a manual bulb inflator. Strangely, using the old school BP gauge, my reading was 116/74. As a technician, I know that calibration is a process of verifying and, if necessary, adjusting a piece of equipment to ensure it is operating within its design parameters. Sometimes, things can go wrong, but not disable equipment completely.

Steve, in the example of +/- .5% of 3,000 fps, the actual range of velocities displayed could be from 2985 to 3015.

Good shootin' -Al

Steve, good question. The crystal oscillator is only as good as the components that control it.



Capacitors and resistors can change values with age which will change the total output and accuracy of the circuit, and transistors due to the NPN or PNP junction also decline in response time with age. They just start getting erratic until finally giving up the ghost. So you can have all kinds of errors introduced due to voltage changes and/or bias changes to both the oscillator and the output circuitry. It’s common.


Oehler just happens to give you all of that.

Another variable that hasn’t been mentioned with the Shooting Chrony is the photodetector separation. Thus far, we’ve assumed that the separation is fixed, but inherent in the folding design is the hinges wearing over time. The sensor housings can shift slightly, as well. These factors can induce error in the readings. For a bullet travelling at a true speed of 3000 fps, and an assumed sensor separation of 1 ft, a 1/8” shift in the true sensor separation, whether due to wear and play in the hinges or to a shift of the sensor housings themselves, or both, is enough to cause an error of about 31 fps. Which amounts to about 1% of the measurement.

Ummmm.... I don't think so.

A crystal oscillators frequency is controlled by the physical dimensions of a little piece of quartz, that acts like a very high Q resonant circuit. With some effort, you can "pull" the frequency a bit, but that is on the order of 1 part per 1,000, not enough to matter in this case. If the bias isn't right, the oscillator stops. It doesn't shift frequency. Unless you are running the transistor very hot, or doing reverse breakdown, transistors go practically forever, and if they do fail or degrade, the oscillator stops. Solid state crystal oscillators either work or they don't. Most broadcast receivers use a synthesizer referenced to a crystal oscillater. You don't see those frequency drifting.

Just wild speculation: Maybe they cheaped out and used an LC oscillator rather than a crystal. The cheap way to do it is with a capacitor and a slug tuned inductor. With repeated vibration, the slug can come loose, and without the slug, the frequency of the oscillator goes up, and with it, the indicated bullet speed. If the slug works its way farther in, the opposite happens. That's a bona fide SWAG for you. If that happens to be the case, it's a poor design.

This sure turned into an interesting thread.
I'm sure learning alot about chronographs, and how they work.

Yes, it's a real issue. Ken Oehler has commented that it's hard to get the sensors aligned so that the accuracy of the effective spacing is better than 1/8". Still, a 1% error is not a show stopper, especially if it is consistent.

The LabRadar does not have that problem, and may be more precise and more accurate than older designs.

Capacitive loads affect frequency. Crystal oscillators must be matched to varying loads which account for the wide variety of crystal components available.

Maximum drive power must also be considered to prevent over-driving. If a quartz crystal is constantly exposed to overdriven conditions in excess of the anticipated maximum power, it can age quickly.

Even placement on the board can affect capacitive load, whether it's relocating the crystal itself or rerouting of other components. All of these are potential sources of mechanical resonance which have to be accounted for and tested in the oscillator circuit design process.

Alteration of the power supply load can cause a change in the effective resistance of the oscillator circuit leading to frequency drift. One solution is to use a regulated power supply, ensuring that the output voltage will always stay at the rated value of the power supply, regardless of the current that the device is consuming.

That would seem to say that any and all changes on the PC board due to degradation of other components, corrosion or heat can actually cause oscillator drift.

I always defer to the engineers for the explanation, but starting with a Shooting Chrony, and later, a Pact, a couple of Pro Chronos and a MS, all demonstrated their strengths and weaknesses.

Comparing the Pro Chrono and the MS to the Oehler showed me that these two were excellent, cheaper alternatives.

Bringing this back around to one of the original points of the thread, a Pro Chrono or MS should make most people happy.

I would like to thank men like Ken Oehler! His work was extremely important to the advancement of ballistics.

These less expensive chronographs were the result of the work he had a big hand in, and shooters have clearly benefited. You can get good results for less.

Those arguments sort of work until you start putting numbers on them. Then they don't.

A pal is working with the new AndiScan Micro doppler unit and is pretty impressed with it.

Good shootin' -Al