Something happened to me I can't explain. first off, at my home range, I have to put my F1 Chrony about 17ft away from the muzzle to gather some light as its a covered range. I never have issues unless its too dark in there.

So I take my chrony to a friends range, and we do some shooting with the chrony about 6ft-8ft from muzzle. My 7mmSTW with 74gr of IMR7828 and 160gr accubond read I was shooting 2820 to 2900fps. I expected maybe 3100fps. I shot 78gr of H1000 last year thru the same chrony at 3050fps.

Ok, most of my guns showed wild velocity performance. My 45-70 was pretty well right on maybe 50fps slower than usual-. My 308 was about 70fps slower than normal. My buddies 7mmremmag was about 100fps slower than it should be. And my STW was 150fps maybe 200fps slower than I would expect.

Called Shooting Chrony and they said the reason was it was too close. And the more muzzle blast slower it may seem.

Anybody do any tests that showed this?

Chrony is rightm no test required. Chronographs don't actually measure speed, as such, they measure time and translate that to speed.

At a short distance, your muzzle blast is triggering the first screen well ahead of the bullet's arrival. So, when the bullet goes over the second screen and stops the timer the time will be much longer than actual and that will give you a slower reading. A greater muzzle blast will start the count sooner than a lesser muzzle blast, simply a fact of life.

Go back to your original screen distance and everything will return to normal.

Yes, chronos actually measure time. Unfortunately their accuracy can vary over time and temperature due to the cheap R-C circuit usually used to create the clock reference signal counted to yield time by the chrono. The cheaper the chrono the more likely these variations are.

If vendors used a crystal clock in their units, we'd see very much smaller variation over time and little with temperature ... but the equipment would be more costly.

(And yes ... I have worked as a circuit designer.)

Thanks guys, I am an electrical engineer myself. I have designed quite a few timer circuts. And know that crystals are dog cheap. Why don't they use em? I have no idea. I guess in the corp world, pennies add up.

Perhaps you Electrical Engineers could explain why changing the distance to the Chrony would would affect the resistor capacitor time constant. This is a chance to learn somthing.
If you think muzzle blast is causing an error move back another five feet and try again.
Good luck!

Not an EE so I probably shoud shut up but will the muzzle blast get in front of the projectile at 7 feet?
Don't know the answer but it does not seem logical to me.
Not, let's hear from somewone who knows.

Man, I really should PROOF the reply BEFORE I hit the send button-look like an idiot.

Not an EE so I probably should shut up but will the muzzle blast get in front of the projectile at 7 feet?
Don't know the answer but it does not seem logical to me.
Now, let's hear from somewone who really knows.

I can't speak for Shooting Chrony, but must inject a couple of comments.

1. I haven't seen a chronograph made in the last 40 years using RC oscillator instead of a crystal.

2. The problems with accuracy are not caused by the oscillator or time counting circuits, but by the screens not telling the counter exactly when to start and stop counting.

3. The muzzle blast wave is traveling near the speed of sound and leaves the muzzle at the same time as does the bullet. This means that it typically causes problems only with bullets near or below the speed of sound (approximately 1200 fps). With subsonic bullets, the blast can affect the skyscreens far beyond seven feet from the muzzle.

4. Changing light conditions can do nasty and unpredictable things to skysceen performance.

KenO

quote:

Originally posted by KenOehler:
I can't speak for Shooting Chrony, but must inject a couple of comments.

1. I haven't seen a chronograph made in the last 40 years using RC oscillator instead of a crystal.

2. The problems with accuracy are not caused by the oscillator or time counting circuits, but by the screens not telling the counter exactly when to start and stop counting.

3. The muzzle blast wave is traveling near the speed of sound and leaves the muzzle at the same time as does the bullet. This means that it typically causes problems only with bullets near or below the speed of sound (approximately 1200 fps). With subsonic bullets, the blast can affect the skyscreens far beyond seven feet from the muzzle.

4. Changing light conditions can do nasty and unpredictable things to skysceen performance.

KenO

Buy an Oehler

quote:

Originally posted by gab:
Not an EE so I probably shoud shut up but will the muzzle blast get in front of the projectile at 7 feet?
Don't know the answer but it does not seem logical to me.
Not, let's hear from somewone who knows.

If you look at the formula for computing recoil, the velocity assumed for the gas ejecta is around 4000 FPS. I think it very capable of passing the bullet, at least for a short distance before it deceleraters too much.

I place my Chrony so the front screen slot is 10' in front of the front edge of the bench. It seems to be working properly so placed, even for large-caliber BP rounds.....

Welcome aboard Ken.

muck

KenO,

It is an honor having the godfather of modern chronographs post on this thread. Before I go any further let me say that 2 of your products have made my handloading hobby so much richer; the models 35 and 43PBL.

Getting back to the original question. I thought the skyscreen sensor had to "see" a bullet to trigger the detector. Isn't that why I often need to blacken my shiny Nosler bullets? Then how can a skycreen trigger with just the muzzle blast? What exactly does it see?

I haven't tore into a chronograph. But it doens't make sense to use a RC circuit for timing since quartz came on the seen like decades ago. Radio shack carrys em for dollars.

I imagine the problem is manufactures try to get real good dynamic range over light/temp conditions and you get slight delays in the response. Thats the only reason I can think of why you get varying velocity readings over light conditions. I suppose its a TTL and you have to make a decision to the Eprom or flip flops or whatever on did it trip or not?

I have a feeling that with my STW, its not the blast in front of the bullet but the blast laggin the bullet.

If that dad gone oehler wouldn't have cut us off, we might not have this issue.

Dave -- Shock waves cause false triggers at skyscreens because shock wave is like a spherical lens moving thru the air at the speed of sound. Light is diffracted by the "lens" and causes a change in light received at skyscreen. Dumb skyscreen can't tell lens from bullet.

eldeguello -- It's true that some gas might leave muzzle at 4000 fps or faster, but ballistic coefficient of this gas is horrible and most velocity is lost within inches of muzzle. An old rule of thumb is that the bullet is ahead of muzzle gas after traveling 25 bore diameters from muzzle. It's a rough rule and I won't attempt to defend it.

KenO

Mr. Oehler, perhaps we are differing somewhat on sematics. Your definition of "muzzle blast" seems to be restricted to the blast of powder gases alone. And perhaps that's true in a pure sense. But my working definition of muzzle blast includes all gases emitted from the muzzle at a shot. That would include the air in the bore ahead of the bullet.

Bore gas (air) is real. It will be ejected down and out of the bore as a very high pressure, high velocity blast. That focused air blast and its density/wave change would seem to have the potential to trigger an optical screen even if the bullet could be stopped at the muzzle without exiting. ??

If so, and if the first screen is sufficently close to the muzzle, the "start" trigger would occur long ahead of the bullets passage. Thus, the higher velocity cartridges would be more likely to generate a false start trigger than a low velocity/pressure cartridge. Or have I missed something?

All the high-speed photos I've seen showing a bullet leaving the muzzle includes density shadows of blast well in front of the bullet. I have taken this to mean that, once the bullet has exited, the light weight gases which are still under pressure will immediately exceed the speed of any bullet for a short distance as it disperses. If so, would not the higher pressure and larger volume gas blasts (7 STW for instance) would produce this effect more than often than a smaller, such as a 308. ??

I do know that I can safely place my screens closer to my bench when the wind is in my face than I can with it's at my back. With a hot 7mm Rem and wind at my back I must put my screens at 15 ft, when the wind is in my face I can get good readings at a 10 ft. distance. Inside that 10 ft. distance I can't get consistant readings at any time.

And I agree that proper light is critical at all times. My unit needs a small taped-on sun shield to shade the top slots in my screens or stange things happen to the numbers.

I'm not an EE, just an old, tired space electronics instrument tech. who never had the privilige of seeing a schematic of a modern chronograph but did work on lots of electronic counters in my time at the Cape. It do seem unlikely that a modern chrono maker would not be using very accurate crystal controlled clocks.

For the non-electonic types out there, our basic chrono accuracy depends on the accuracy of the clock and its counter gates, plus or minus one count, AND the proper focus and spacing of sensitive optical screens. Even the inexpensive crystal clocks and counters are pretty good today so excellant screens are very important.

OK, so we were typing at the same time! I see what you said and accept it. For now! ;-)

I don't pretend to understand all the nasty things that can go on near the muzzle. I just try to protect the skyscreens from all the interfering crap. In general, the shock wave moves far and in a predictable fashion. All the moving gas is hard to analyze, but doesn't usually get to the first skyscreen if you are spaced the suggested 8 - 10 feet from muzzle.

KenO

Ken,

Glad to see you on the forum. Lots of folks aren't too pleased to see some of their old Oehler chrono's become unsupportable. I suspect the sad fact is that availability of parts is the issue.

Certainly if the original poster saw a significant change in response over a short period of time, the clock design is not the culprit ... and Ken is very likely to be correct and blast, shockwave, or compression wave is to blame. Is always better to get a 10 or 15 foot velocity that can be trusted than a 6 foot velocity that varies wildly.

As to the clocks ... you'd be amazed at the simplifications that are sometimes done to reduce parts costin cheap units. If a Chrono shows an increasing variance from other trusted units over a longer period of time (like years) is really may be the clock. You might think of it as drift from a standard.

I have actually seen this happen when comparing my chrono results on IPSC ammo at home versus chrono results against the new Oehlers in use at the Nationals. After checking the main clock with a scope and triming it ... the results came back together.

I haven't seen the schematic of the Chrony, but if there is a modern unit around that MIGHT have cheap out parts, its low cost would make me suspicious.

quote:

Lots of folks aren't too pleased to see some of their old Oehler chrono's become unsupportable.

Support for all Oehler chronographs continues. We still have the required parts along with our proven desire and ability to support our customers. Check at oehler-research.com or call 800.531.5125 for support.

Thanks for the kind comments.

KenO

Ken,

Welcome aboard. Beware of the political forum!!

And thank you sharing your experience with us.

Mike

Ken,

Thank you for correcting the misperception about support. The comments of concern that have been made to me were based on the presumption that the discontinued product would not be as well supported anymore.

My apologies for inappropriately stirring a still pot! My goof.

mstarling; How can you make such positive statements on subjects you are ignorant on ?.
Interesting!

Take Care!

Question on velocity readings. I have an Oehler 35p. While checking my loads in the morning, (temp about 40)my loads seem to average about 50 fps faster then the same load shot in the afternoon, on the same day (temp about 45-50)so what data would be the most accurate? Note: all loads were shot 12 ft from muzzle.

quote:

Originally posted by bigcountry:
Something happened to me I can't explain. first off, at my home range, I have to put my F1 Chrony about 17ft away from the muzzle to gather some light as its a covered range. I never have issues unless its too dark in there.

So I take my chrony to a friends range, and we do some shooting with the chrony about 6ft-8ft from muzzle. My 7mmSTW with 74gr of IMR7828 and 160gr accubond read I was shooting 2820 to 2900fps. I expected maybe 3100fps. I shot 78gr of H1000 last year thru the same chrony at 3050fps.

Ok, most of my guns showed wild velocity performance. My 45-70 was pretty well right on maybe 50fps slower than usual-. My 308 was about 70fps slower than normal. My buddies 7mmremmag was about 100fps slower than it should be. And my STW was 150fps maybe 200fps slower than I would expect.

Called Shooting Chrony and they said the reason was it was too close. And the more muzzle blast slower it may seem.

Anybody do any tests that showed this?

I have always set up mine so the start screen is 10' from the muzzle, and have never had your problem, either with modern rifles or ML up to .73 cal. with 150 grains of FFg.......

Watch a slow motion video of a bullet exiting the muzzle and you can see the "shock wave" move ahead for a short, sometimes very short, distance. When I got into chronos I had the same problem. Noticed the chrono was moving after each shot and velocity was all over. Figured the blast couldn't be good for it and gave it another 10 feet of breathing room, no problems.

[QUOTE]Originally posted by KenOehler:
Support for all Oehler chronographs continues. We still have the required parts along with our proven desire and ability to support our customers.
KenO

Of all the toys that I have purchased over the course of my reloading adventures (45+yrs)only one has been serviced without question and without charge. If everybody supported their products like the man from Luckenbach, what a wonderful world this would be. Jim

quote:

eldeguello -- It's true that some gas might leave muzzle at 4000 fps or faster, but ballistic coefficient of this gas is horrible and most velocity is lost within inches of muzzle. An old rule of thumb is that the bullet is ahead of muzzle gas after traveling 25 bore diameters from muzzle. It's a rough rule and I won't attempt to defend it.

KenO

I see. I didn't know how far the gases pursuing the bullet actually went after exiting the muzzle. But I can certainly understand that their ballistic coefficient is around "0". Thanks!

I haven't conducted tests on having the unit 6-8 feet from the muzzle, but shooting a 7mmSTW it amazes me that the muzzle blast didn't blow the chrony over. I get my best readings from 10-12 feet from the muzzle. I am also curious about your readings that were taken 17 feet from the muzzle. How did those turn out?

Mr. Oehler, welcome aboard. You'll find this forum is filled with some pretty decent folks, opinionated as hell, but decent. There is an absolute wealth of information to be learned from this forum. Tom Purdom