Accuracy of Chrono Readings

[Bob3700]

All,

I have encountered an interesting issue that isn't discussed much.

The accuracy of your chronograph. Many threads state loading data and then provide velocity numbers based on their chronograph readings. I am here to tell you that at this point, I am suspect of many chronograph readings. Here is why.

During testing of my 6 mm BRX, I have had access to four chronographs,three Oehler 35ps and a Chrony.

Two of the Oehlers would repeat identical loads within 10 fps of each other. One Oehler regestered 90 fps slow, the Chrony registered 125 fps high, and the third Oehler reads 90 fps high. So which velocity do you believe, the two that match up, or the low or high reading units. All of the Oehler models have a proof channel.

Now don't misunderstand, this is not a bash against Oehler as I believe they make the best units out there, but the issue is calibration. Unless you electronic box has been verified against known standards, you can be generating data that isn't worth the paper it is printed on.

It would appear that unless you unit was recently calibrated and verified,with a factory load), all readings are suspect.

Any more when individuals post velocities for their cartridges and loads, I assign the level of truth that they are due.

What has been you experience in this area?

Bob

 

[memilanuk]

Bob,

A few years back I was testing a pair of CED Millenium chronos after getting tired of hearing the consistent nagging from the Oehler crowd,sorry). Put them back to back, swapped the skyscreens around, varied the spacing, etc. and the end result was that one unit,the CPU) read on average 24fps slower than the other. Given that offset, the two chronos would be a few,4-6fps) off each shot, sometimes one way, sometimes the other. Otherwise... very consistent.

Later, when I had reason to wonder if my chrono was going wonky on me, I tested both units back to back again. This time the 'offset' was about 35fps. I haven't tested them recently to see what the difference is now. Then there is always the question of 'which one, if any, is right'?

There are supposed to be ways to 'check' your chrono using a load known to generally be very consistent in most guns... like shooting some honest-to-goodness olympic match .22 LR ammo through your smallbore rifle of choice. The velocity of this ammo is *supposed* to be very consistent. I haven't personally ever got around to this so I can't say for certain.

For the CED chronos, the 'test results' they cite are not available,I asked) and indicated the readings get progressively more and more 'off' as the velocities increase. Whether that is typical of just the unit tested, the particular model, or consumer-grade chronographs in general, it's hard to say.

Well, time to step aside for the lemming rush of Oehler fan-boys.

 

[JERRYHM]

Bob,
I agree with most of what you said. I have shot or monitored many thousands of rounds fired across chronographs with palma rifles, about 100 different rifles. The rifles were from the US Palma team and the US Veterans team.
I found that the most important condition is the light. Light changes drives the chrono's crazy. The three Oehler chronographs I've used for testing were very close to each other in the MV's as long as we did not have light changes.
The Chronys I've compared were all over the place on the MV's.

The more I shoot over chronographs the less I trust the actual readings until the test is repeated several times over different days.

Jerry

 

[Mikem]

Bob, I certainly cant speak for anyone else, but when I post a bullets speed its the average bullet speed over at least five rounds, and some times ten.
And then there's the ES and SDs, both of witch are just a calculation of the last strings number of fired round, with there speed converted into standard deviation,SD) and extreme spread,ES).

Every thing involved, is a stored know constant, except the time it takes the bullet to pass between the two screens.
Its simply time converted into speed, with some pre programmed math calculation.

As far as calibration is concerned, you dont have to calibrate a calculator or a watch either, and there dead on. So I would imagine the speed of the bullet is compared to a clock circuit, that is quite fast and accurate. And Im sure the mathematics calculation are dead on.
If you suspect an error, it should show up in your recorded speed. ES or SD.

Now this is not to say that varying lighting condition may cause some inaccuracy,,or failure to record) but I would also think the shooter would pick up on that right off. But I personally haven't seen this happen.

You might test your theory by setting up two,or more) Crony's tight together and in line, so that both,or all) will read the same bullet, then compare the recorded speeds. Then reverse them for a comparison. Investigate it more in depth than your multiple crony testing. You might also get some good info on this from the manufacturers, they designed and built them, and can tell you what the error % might be.

I feel comfortable that my reading are quite accurate, there speed is what I would expect, and my ES and SDs are low.
I have a remote reading Crony with printer, and like and trust it. JMO
M.

 

[Bob3700]

Gents,

Thanks for your replys.

I believe that one reason for varying velocity readings with Oehlers is the screen spacing. The timing circuit is only as accurate as the distance it is measuring across. A 1/4 inch means a lot here.

The fold up in a box type chronos have this beat as the screens are a fixed distance and the operator cannot alter that.

The Oehlers screens are placed on a rail by the operator and there is where the error takes place. Too long a distance and your Vs are low, too short a distance and your ego is inflated.

Calibration can take place as you say, firing known factory ammo over the screens. Most in my neck of the woods will use Federal GM 308 as that is the constant many will use. RF match could be used the same way.

I am going to check some screen spacing and see if I can determine where the issue resides.

milanuk, I know what you are saying concerning the Oehler crowd as I am one of them. That said, I have not heard anything bad against a CED. My only issue, when the 35P was discontinued, was the price of the extra infrared screens that most said were required to have absolute confidence in the CED readings. My opinion, the CED then got a little pricey with the 'Extra' screens.

I do think the feature of the more modern chronos, connecting to your computer, is something that I would appreciate. The Oehler is simply a speed trap and doesn't offer these upgraded features. It is older, but very reliable and accurate, technology.

Bob

 

[alf]

I've been using an Oehler 33 since they first came out. In the directions, it was stated that the further apart the screens, the more accurate the readings. So I bought a 10' piece of conduit and have used that every since for my spacing. Inside the unit is a switch to set to the spacing you use. I seem to have repeatable numbers from day to day, and year to year.

Al

 

[Mikem]

Bob, I noticed you use Federal GMM in 308 as a standard, but didn't mention what that MV was.

Back on 10/28/04 I conducted some MV tests on this commercial ammunition contained, 168g SMK bullets, 44.0g of Varget and Fed GM-210 primers, fired from my H-S Precision HTR, 24' cut rifling barrel.
With my Crony, the avg MV was 2637 fps ES of 66.63 and an SD of 20.04 form 10 rounds of Factory Federal GMM.

After dissecting the above factory ammo, I then reproduced it, using a 168g Sierra MK, 44.0g of Varget, and using Fed. GM-210 primers with the following results.
On 12/10/04 an Avg MV of 2644 fps, ES 18.49 SD of 13.45.

With a MV variation between the Factory load and my hand load of only 7 fps, I would say my load did duplicate the Federal load, but were more consistent, indicated by lower ES and SDs, and I also believe my Crony preformed well.

Is this close to what you were using as your standard MV for checking various Crony's MVs?
Mike.

 

[Ackman]

My chrono is a Pact Pro. Some readings have got me to wondering. But plenty of other results were the same as similar loads through similar guns read by an Oehler 35. A friend compared that Pact Pro with his Oehler 35 and both chronos gave essentially the same velocities.....they were within about 15fps of each other.

 

[RDavies]

As JerryHM says, I have found light does wierd things. If I use it on a bright shiny day, the readings are all 150 fps high. I remember a few times thinking my 284 was shooting 2950fps and my 22BR shooting 3100 fps,heavy bullets) and thinking wooppeee big velocities, I must be doing something realy good here. Then when I tried it later with cload cover, the velocities are 150 fps lower. I have tried this a few times since with the same results. As for accuracy of each shot velocity, I give up as my most accurate loads often did not have the realy low ES and SD like everyone else sems to post. Then this was with a bargain basement Shooting crony.

 

[Mikem]

Low,single digit) ES and SDs are vary difficult to get consistently, there are too many variables, other than your reloads, but they must be of top quality, in every respect. I have tested some of mine that were in single digits, but not every round from every batch.
Look at the ES and SDs of the Federal GMM 308 I tested and posted above, you would be hard pressed to buy better commercial ammo, and its ES was 88.?, hardly low.
If you can keep your ES below 20.0 and your SD below 10.0 your way ahead of the game. That's what you should strive for, but you wont always get there.
M.

 

[reubenski]

I have noticed this as well with two CED M2s and an ProChono. I finally got tired of messing with them and just started shooting the load that fell within my acceptable accuracy that should have the highest velocity based on charge at 800m. I used my true fired data to reverse engineer or manipulate the MV in my ballistic software. I would change the MV until the software reads my fired elevation data. I call this my 'effective MV'. The only catch is how accurate your BC is. I try to get them from Bryan Litz. The only thing I use chronos for now is ES and SDs.

I was taught this by a competition shooter/ training instructor.

 

[LongRanger]

Reubinskey....

You got it right.

If you are a long range shooter....over 600y....the only thing with respect to velocity that matters is ES. Mathmatically, you can't use SD to calculate anything you need to do to get better accuracy. SD is nothing more than a 'confidence factor' that MUST take ES into account.

20 fps ES variance fastest shot to slowest shot still equates to about 5' vertical at 1000y, using anybody's chrono.....that's what you really want to reduce in a 20 shot string, right? Vary your powder/powder charge/seating depth/barrel harmonics until you get ES under 10.

Regardless of the external conditions of light, angle of sun, etc, what you get as a readout from any chrono of a single shot string is factual for that particular load and the conditions when it was shot. Spend a few hours with yours at your local range and learn what it is telling you.

Those who want to argue the merits of the absolute factual accuracy of their chronos should compare any one or all of them under the same standard set of 'in house',enclosed building) standard conditions of temp, light, etc. Such a comparison may reveal differences from one brand to another 'assuming' the loads are absolutely the same. But then, again, what and how was the 'standard' against which they were compared?

Use your chrono not to prove a point about how 'good' or 'accurate' it is versus another machine, but to give you factual, at the moment, differences in your loads. Sure, out on the shooting range, the 'same' loads might chrono 80 fps faster this afternoon than they did this morning. If you are trying to ascertain the speed of your loads, such as when making a change from one lot of powder to another, or finding out how much faster 1 more grain of powder will launch you bullet, try to make all tests under the same environmental conditions....very, very difficult. But the read-outs of your shot strings within any of your morning or afternoon or evening tests showing a 20 fps difference reading from fastest to slowest shot in any particular string is STILL going to show up as 5' of vertical at 1000y....under the same conditions.

Frank

 

[FTRshooter]

A lot of the variances encountered with chronos have to do with the setup. It is vitally important the bullet path be at the same distance from the sensors from one day to the next. Also, the chrono must be absolutely level and the bullet path must be level also, parallel to the sensors. Beyond that the screens must be perfectly aligned with the muzzle of the rifle and the target. What we are trying to achieve is the exact repeatability of each trajectory, EVERY TIME. If the screens are crooked or the path is not the absolute shortest distance between the screens, the results will be off.

Also, the Chrony has the nasty habit of not unfolding completely flat if you don't use some 'persuation.' It is critical that it be unfolded perfectly every time, lest you get a higher velocity reading.

Lastly, it is also important to measure the velocity with the first screen at the exact same distance from the muzzle, every time. I use tripods with levels for attitude and tape measure for distance to muzzle. I try to aim for a path about 4 inches above the sensors. EVERY TIME.