Resolution of chronographs?

B

Boyd Allen

Active member
Does anyone have any data as to the resolving power of ballistic chronographs? While I agree with the generalization that greater precision in weighing powder charges results in lower extreme spreads of velocity, some of the claims being made seem to ignore the differences in accuracy of the best scales and chronographs, sort of like mixing rulers and micrometers. The other factor that seems to be ignored is that the pressure charge weight relationship may not be linear. Data? Comments?
 
while ohler is a std of the industry, they have old slow electronics.
best when used on longer spreads of the screens.

no idea on the new labradar.
 
A good math story problem

So if you have two chronos, A and B and you have a specific load for a specific rifle that has a very low standard deviation. Let's say Chrono A gives readings within plus or minus 3 percent of actual velocity, but the average is the same every day. Chrono B gives readings that vary only 1 percent on a given day, but one day they might be three percent higher than average, one day three percent lower, or anywhere in between. Which would you rather have? Myself, I'd pick Chrono A. We are told frequently that low standard deviation doesn't necessarily equate to small groups. I gain more confidence from a consistent average.

This is all hypothetical and I'm no expert. I've only ever had one chronograph, a Shooting Chrony Beta Master. I had some frustration with non-readings in certain light conditions. When I got LED screens, that all went away and I've been very happy.

Those of us with one chrono aren't thinking consistently. Why shouldn't chronographs be like rifles, in other words why not have one of each popular model and maybe two or more of your favorites? That's the only way to get a real sense of them.
 
CMaier said:
while ohler is a std of the industry, they have old slow electronics.
best when used on longer spreads of the screens.

no idea on the new labradar.
Click to expand...

You make a good point. We all know how rapidly electronics change these days, getting better and faster and less expensive all the time.

I use a chronograph with guts designed rather recently. They brag about their 48mhz clock speed and quarter of a percent accuracy. They also claim that each unit is accurately calibrated at the factory. Who knows how true this is, since most of use only own one chronograph, but I suspect newer is better.

I see guys with antique chronographs which were very expensive when they purchased them. I find it hard to believe that the old unit with a clock speed about equal to my heart rate can be as accurate as even the cheapest modern chronograph. I'm reminded of my old Tandy model III computer with a huge 32K (yes that's "K") of memory for which I shelled out over $3000, enough to buy a nice car at the time. :cool:

Furthermore, I think the low ES and SD values some of these "experienced" shooters brag about are a result of the inability of their ancient chronographs to resolve small differences in muzzle velocity.

As for the charge weight vs pressure linearity the OP asked about, I have no way to measure pressure. But just for grins I graphed the MV vs Charge weight for a batch of 5 shot groups I tested the other day. You can see it's pretty linear. I'm guessing the deviation from linear is more due to the variations in my loading procedure and component selection rather than chronograph errors, but again I have no real proof.

MV vs Charge Weight Graph
 
At one point or another

At one point or another I have tested the calibration of most chronographs that came out. One thing became evident in that no one seemed to be able to tell me what they were calibrated to. I tested the calibration with test equipment that I knew the tolerances of. The most erratic were those that used sky screens. None of them allowed the user to test the calibration of their unit and most still don't. I don't know of any that are thermally shielded or voltage stabilized. Even battery operated units should be. So I wouldn't place too much emphasis on the data extracted from them. Just use them as another indicator among the various types the accuracy game uses.
 
THE OHLER 35 does use 2 sets of screens, compares data and allows the user to decide
when to be told there is a suspect reading.
i an looking at the labradar, anyone have info
on their reading quality?
 
CMaier said:
THE OHLER 35 does use 2 sets of screens, compares data and allows the user to decide
when to be told there is a suspect reading.
i an looking at the labradar, anyone have info
on their reading quality?
Click to expand...

Their claim is 0.1%, which, at 3000 FPS, should be within 3FPS. RG
 
(This post is related to short range Benchrest!)

I don't know ANYTHING about chronographs....don't know why I'm posting here! On the other hand, what could be better, or cheaper, than simply measuring the group you just shot? I'm really asking as I can't see how knowing what speed the bullet is traveling, how consistent the speed is, or whatever...has any bearing on what we're trying to do here. If you can find the better load more quickly and save some shots, a chronograph would pay for itself rather quickly...I just don't know what the deal is.

Wait...I do know something about chronographs! You can't leave one setup at the Super Shoot while you go load more bullets. Somebody will shoot it!!!
 
Hi Wilbur,
This has very little to do with short range benchrest. The whole expensive scale, weighed charges to a piece of powder thing belongs to the long range game. Heck, some of those guys have been known to weigh primers. Relating to that sport, I was simply curious as to how much sense it makes to discuss the effect of single grains of powder given the accuracy of the tools that we do the measuring with.
 
Would tell me

R.G. Robinett said:
Their claim is 0.1%, which, at 3000 FPS, should be within 3FPS. RG
Click to expand...

After testing one of these 3 screen instruments I was curious as to what standard they calibrated the chronograph to and what the tolerances of the gear they calibrated it on were. They didn't have the courtesy of returning my call or letter. Would have liked to have tested three or four of these units along side one another and compared the data. Chances are the manufacturer would not like it at all.
 
Andy Cross said:
After testing one of these 3 screen instruments I was curious as to what standard they calibrated the chronograph to and what the tolerances of the gear they calibrated it on were. They didn't have the courtesy of returning my call or letter. Would have liked to have tested three or four of these units along side one another and compared the data. Chances are the manufacturer would not like it at all.
Click to expand...

Andy, I was referring to the LABRADAR claim, which is 0.1%. According to the specifications supplied with them, my original Oehler, Mod. 12, @ 5 foot screen spacing is supposed to be (+/-) 1%; the newer Mod. 35 and 35P are supposed to (+/-) 0.5%. I cannot attest to calibration standards, or, tolerances of Oehler's test gear. At those precision levels, I have always considered chronograph DATA as relative, as opposed to absolute - especially with regards, to minimal DATA collection, such as one five-shot string. If truly within 0.1%, the LABRADAR is a different animal. RG
 
R.G. Robinett said:
Andy, I was referring to the LABRADAR claim, which is 0.1%. According to the specifications supplied with them, my original Oehler, Mod. 12, @ 5 foot screen spacing is supposed to be (+/-) 1%; the newer Mod. 35 and 35P are supposed to (+/-) 0.5%. I cannot attest to calibration standards, or, tolerances of Oehler's test gear. At those precision levels, I have always considered chronograph DATA as relative, as opposed to absolute - especially with regards, to minimal DATA collection, such as one five-shot string. If truly within 0.1%, the LABRADAR is a different animal. RG
Click to expand...

BOOM!

Lissen to this man! REALLY lissen......

"data, relative"

As one who can and does shoot over 4 systems at once I can state that truer words were never spoken.

I, for one can state that while one of my cheap chrono's routinely reads about 25fps "faster" than the other two it just ain't worth the hour it would take to "make it match the others".......

for me



al
 
Thanks Boyd!

I did see a couple of folks sharing one of those Labradar deals while I was at the Roanoke range. Looked like it was doing a pretty good job.
 
Oehler 35P

If I recall correctly, the 35P's timing is run via a 4.0MHZ crystal oscillator......for BOTH screen spacings.

I remember making my conduit 9 feet long........max length to fit my old '97 Grand Cherokee. :eek:

BTW......isn't the LabRadar's max speed limit 3900fps?

Kevin
 
Radar

When we start talking radar then you enter a whole new world of pain. Bottom line is control the reflection obtained from the target subject or you will end up with data that simply isn't relative. Not sure how you go about controlling the reflective signal from a bullet given the number of variables involved.
 
The Labradar worked pretty good all in all! I think it noted the speed of every shot fired but don't know anything beyond that...except the guys that were using the Labradar didn't win...
 
The crystal oscillator is the final accuracy component.
0.0001% is not hard to get.
With a temperature compensating unit you can hold 0.01% over a decent range.

The math is not all that complicated.

distance between sensors in feet.
frequency of the oscillator in cycles per second (Hertz).
velocity comes out in feet per second

velocity = (distance * frequency) / (counter value)

I used 10 MHz since that is common oscillator.
10.24 MHz become common later on. Would have made the math easier.

You need full integer/binary math to preserve accuracy.

16 bit counter (and hardware to read it and reset it).
Signal conditioning circuitry.
Optical sensors make a small signal.

Even a GHz processor is not going to count 10 MHz well enough.
Hardware counters chain (only takes 4 chips).

Optical screens have shot-to-shot variation.
Their 'sensitive zone' is NOT uniform across the area.
It effectively changes the distance slightly.

At a high enough bullet speed the math breaks down since 1 count = 1 foot/second.

Error analysis is not hard.

There is count uncertainty, distance uncertainty, frequency uncertainty.

I tested with a single long trace on 3 inch x 5 inch printed circuit boards.
Each good for a few shots by bridging the hole with fine wire.

No issue with muzzle blast using a .22 RF rifle a few inches from the muzzle to board.
Tested with blanks just to be sure.

After a few shot the board broke up and had to be replaced.

Once everything was working I switched to optical sensors.

An optical transistor and some simple plastic lenses.
Easy to polish and bend to shape.
 
Radar

The Legal system in this Country approves the use of Radar technology to enforce speed limits on streets and highways. Been that way for a very long time. Mandatory calibration,of detection equipment, is required to assure (accurate) speed measurement.

Is the same concept/tehnology used in the latest radar(Labradar) chronograph?

I'm asking in the event I get a speeding ticket. There might be something in these responses I can use in court for a defense.:)

What say you?



Glenn
 
Chism G said:
The Legal system in this Country approves the use of Radar technology to enforce speed limits on streets and highways. Been that way for a very long time. Mandatory calibration,of detection equipment, is required to assure (accurate) speed measurement.

Is the same concept/tehnology used in the latest radar(Labradar) chronograph?

I'm asking in the event I get a speeding ticket. There might be something in these responses I can use in court for a defense.:)

What say you?



Glenn
Click to expand...

The various diodes (Gunn, IMPATT, etc.)) used to generate the high frequency signal are notoriously noisy signal sources.

'Negative resistance' describes the circuit behavior that leads to microwave frequency oscillation.

One of the big jobs is selecting the desired frequency and setting up feedback at THAT frequency.

Then locking to a quartz oscillator at a much lower frequency to reduce the phase noise in the measurement.

Tuned cavities are often used at higher frequencies.

Making a capacitor that still behaves as a capacitor is a real chore at the highest frequencies.

Welcome to the land of tuned stubs.
 
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