Chronograph comparison

[taliv]

shot some rounds over two chronographs at the same time this week. The chronos were a "Chrony" and a "CED". They were positioned less than an inch apart and a little more than 10' from the muzzle.

The rifle was a 338LM and the breaks between the lines below indicate different loads as my friend with the 338lm is in the process of load development with several different bullet weights. So while the loads aren't all that great, they are informative about how the chronographs compare. I'll leave it to the reader to draw any possible conclusions.

The max difference across 15 rounds was 18 fps. That is 0.63% of the mean of the 15 rounds.
The mean difference of the 15 rounds was 5 fps. That is 0.18% of the mean of the 15 rounds.

chrony CED delta
2928 2926 2
2938 2929 9
2928 2928 0

2494 2494 0

3007 2989 18
3012 3000 12
3009 3002 7
3014 3009 5

2704 2696 8
2681 2674 7
2679 2678 1

3043 3039 4

2833 2817 16
2813 2804 9
2796 2790 6


I found this interesting because I did this test using a different shooting chrony in 2005. The only difference in tests was that they were spaced about 6" apart. However, I found in that test the CED recorded consistently higher velocities, with the summary as follows:

A sample 15-shot group has the following
Max difference 45 fps
min difference 35 fps
Mean difference 39.5 fps
Std. dev. 3.4 fps

Next week, a friend is supposed to receive an RCBS chrono and I plan to shoot over all 3. I'll report back if the results are interesting.

 

[MCMXI]

taliv, I'm surprised at how close the output of those two chronographs is based on my own experience. When I was working up a .308 Win match load a couple of years ago using a Shooting Chrony Beta Master, the typical avg MV was around 2,750 fps. When I upgraded to a CED M2, the avg MV of the same load was consistently around 2,650 fps. Based on theoretical and actual comeups at 200, 300, 400, 500, 600 and 800 yards, the CED M2 seems to be the more accurate chronograph. I put it down to the fact that the CED has a faster clock speed and the sensors are further apart.

I would be interesting to test 10 or more of each.

 

[taliv]

yeah, i hear you. i also guessed the CED would be more accurate due to the sensors being farther apart and potentially really far apart as you can buy the folding connector bar in different lengths. I didn't realize the CED has a faster clock speed though.

Just off the top of my head, those seem like two distinct types of errors.
1. perhaps if they both had the same clock speed, and the clocks were sync'd all shots would be the same distance. e.g. one is always 12 fps faster than the other
2. calibration of the individual units might move that deviation closer to zero.

 

[R.W.Dale]

Interesting test for sure but don't you think 10' is a little close for a cartridge with the blast of a 338lm. I wonder if chrony proximity to the muzzle might be tainting your results slightly.

Personally I strive for closer to 20' from the muzzle to the chrony


Tapatalk post via IPhone.

 

[ny32182]

I would hope that clock speed would not be a factor in whether the device is accurate or not. As a commercial product it should perform within the stated design parameters.

Complete layman opinion regarding the manufacture of chronos here, but I would think that consistent sensor spacing from one device to the next would be the most important factor in real world accuracy.

Also I would think the angle at which the bullet is going over the sensors would matter, and would be difficult to get exactly parallel (bullet path to sensors) all the time. Same with shooting over two chronos at once.

To me all it says is that for folks such as competition shooters who need to meet a specific velocity minimum, make sure your load is reading high enough on your chrono to account for ~50fps in possible difference when shooting over someone else's chrono. It is an interesting comparison.

 

[taliv]

RWDale, could be. monster muzzle brakes probably help quite a bit though.

I know muzzle blast has caused errors before where the chrony displayed ERR or just didn't pick up the shot. However, could muzzle blast cause it to give a distorted answer? would it be faster or slower?

ny, i'd hope. i don't know what the clock speeds are, but i'd think it's not hard to get enough samples to read at least a tenth FPS. i think the chronos do display tenths at low velocities (like under 100 fps might show 85.9 fps or something). maybe i'll do the math tomorrow if i get bored to see how many samples you'd need

 

[ny32182]

I wouldn't assume that the sample rate equals the clock speed. The clock is probably faster.

 

[MCMXI]

According to CED's website, the micro-controller runs at 48 MHz. My knowledge of the specifics is lacking, but if I have my math right, a bullet moving at 3,000 fps will pass through a 1/4" wide sensor "plane" in around 7 microseconds. So that would mean that a chronograph running at 48 MHz has 333 chances to detect the bullet to start the clock and the same to stop it. This is where I would think that clock speed could make a difference but maybe it doesn't. I imagine that the controller assumes a fixed distance between the sensors, and regardless of whether it starts the clock anywhere between cycle 1/333 and 333/333 and stops the clock anywhere between cycle 1/333 or 333/333, it calculates velocity by dividing the assumed known distance between the sensors by the time elapsed between the start and stop modes of the clock. In other words, the shortest time would be start at 333/333, end at 1/333 and the longest time would be start at 1/333, end at 333/333.

 

[jpwilly]

In other words, the shortest time would be start at 333/333, end at 1/333 and the longest time would be start at 1/333, end at 333/333.

So how much fps difference would there be between the two extreems?

 

[MCMXI]

So how much fps difference would there be between the two extreems?

I have to walk my dogs so I look forward to reading the answer when I get back. By the way, I don't know how wide the actual sensor "plane" is. Maybe it's a lot less than 1/4". The lenses look like they're about 1/4" or less wide. Maximum velocity for the CED is 7,000 fps so there may be a clue there (to actual clock speed).

Another thing to think about is that the bullet has length. So you can think of the sensor plane being very, very thin (maybe that's how it is) and the tip of the bullet cuts the plane and then passes through the plane. So the clock has to detect the bullet while some part of the bullet's length is passing through that very thin plane. You could calculate the time elapsed between the tip and the tail passing through the plane and the number of clock cycles during that time. Hopefully, someone who's very knowledgeable about chronographs and how they work will come along and explain it all. I find this sort of thing interesting, that's why I'm babbling on a bit.

 

[Jeff82]

tag

 

[taliv]

I wouldn't assume that the sample rate equals the clock speed. The clock is probably faster.

i'm not. samples usually take some number of clock cycles. best case would be 1, but it might take 3 clock cycles to take a sample. in the latter case, 1858's number would be adjusted to 111 chances

I find this sort of thing interesting, that's why I'm babbling on a bit.

same here.

 

[MCMXI]

same here.

So babbling is allowed?!

I've been thinking about this some more. The flattened cone shaped lenses above the sensors on the CED M2 are approximately 1/4" wide (front to back). A 178gr A-MAX is 1.322" long. The sensor doesn't care which part of the bullet it "sees" so the time for detection starts when the bullet tip enters the detection plane and ends when the tail leaves it. For the A-MAX, that distance is something like 0.25" + 1.322" = 1.572". The time for a bullet moving at 3,000 fps to move that distance is 43.7 microseconds. If the processor has a clock speed of 48 MHz, it is capable of taking a measurement 2096 times when the bullet is in the detection plane. How many consecutive measurements are required to start or stop the clock is anyone's guess. It could be 3, 10, 20, 50, 100 etc. An electrical engineer familiar with IC, op amps and circuits in general would probably know.

It's hard to find information detailing exactly what components are in chronographs but my best guess is that the two light sensors are photodiodes operating in reverse bias photoconductive mode. I was reading about photodiodes and found this information.

Photodiodes - photocondutive mode
In this mode the diode is often reverse biased, dramatically reducing the response time at the expense of increased noise. This increases the width of the depletion layer, which decreases the junction's capacitance resulting in faster response times. The reverse bias induces only a small amount of current (known as saturation or back current) along its direction while the photocurrent remains virtually the same. For a given spectral distribution, the photocurrent is linearly proportional to the illuminance (and to the irradiance).[2]

We've all seen those plastic lenses over the photodiodes (which are small) and read the manual with the chronograph that suggests shooting directly over the sensors if possible. The plastic lenses, shaped like a flattened cone, focus and magnify the illuminance onto the photodiodes which monitor a cone shaped plane that has some thickness (approx. 1/4" for the CED M2). The processor running at 48 MHz or whatever is constantly monitoring the current produced by the photodiodes (steady state). The steady state current is based on ambient lighting conditions i.e. if it's bright, the saturation or back current would be higher. So now the bullet passes over the sensor and the illuminance changes slightly which changes the current slightly and it'll take some number of counts before the processor starts or stops the clock. There would probably have to be an op amp in there somewhere to monitor the rate of change of the output current. A bullet will cause a small but rapid change in output current whereas a change in ambient light would cause a larger but slower change in output current.

No wonder we sometimes get ERR messages and it makes sense that the quality of the components, the lenses, the photodiodes, the processor etc will affect the accuracy of the system. I've read that "common" chronographs vary by as much as 8%. I'm not sure if that's 8% from the lowest to the highest but there are plenty of opportunities for error with these things.

 

[gamestalker]

I also read them at about 20' so I don't have to worry about muzzle bast distorting the number's.

 

[taliv]

So babbling is allowed?!

if it wasn't, my post count would be 1/10th

So today was not so good for the CED. I compared it to an RCBS. The CED was closer to the muzzle and about 2" away from the RCBS. The fedex guy dropped off the RCBS about an hour before the shots below were fired, so it is brand new.

So on the 7th shot, the CED failed to pick up the shot. On the 8th shot, it registered something pretty wild, and then it didn't register another shot for the remainder of the day. I don't know why this would happen, but it was fairly late in the day and it's possible the sun moved down far enough just at this time that it wouldn't work. I was not using the diffusers on either chrono and it was sunny today, though as I said, the sun was low in the sky.

CED RCBS delta
2952 2907 45
2944 2936 8
2930 2926 4

3029 3025 4
3065 3005 60
2994 2990 4
---- 3018
3148 3026 122

(I fired another 10 or so shots after this and the RCBS picked them up but the CED didn't so there's no point in listing them)

so what is interesting here obviously, though the sample size is small, is the 4-8fps delta on most of the shots, with 3 of them just wildly different.


btw, 1858, i cross-posted this on snipershide and there are some interesting posts there that sum up some of the clock speeds of various chronos. According to info posted there the clock speed on my older CED is only 4mhz but the new M2 doesn't support the 4 or 8 foot sensor-spacing.