Chronograph confusion

[Whitespider]

Just one more illustration.
We humans have gotten pretty good at building things within tight tolerances, but there ain’t nothin’ perfect. Sky screen sensor mount systems leave a lot to be desired in the precision department; which means expecting both sensors to be mounted absolutely perfectly is unrealistic. There is a “margin-of-error” that increases as the distance away from the sensors increases; keeping the bullet path as close (within reason) to the sensors as possible will minimize that error margin.

[lwknight]

OK, I took for granted that no one in their right mind would shoot a chronograph at a steep cockeyed angle. My bad. I tend to forget how ignorant some people can be.

As for the second illustration: You would have to bust the plastic apart to see if the sensors look to be aligned. Even how can anyone know if they are exactly just right.

There may be some chronographs out here that can get out of whack and be adjusted back but I'd bet that its a very small percentage.

[mike in co]

am i correct in the above diagram that you have one sensor tilted back, torwards the bullet ?


with the single mounting bar of the ohler35 the sensors are all on the same 2 planes.

yes you can tilt your shot, but only a gross angle will change the distance.......


and what you dont point out, is that if all the shots are taken from the same bent position, it aint a big deal, unless comparing to prior shooting data with a diff layout.


even the lowly pact that i hate has a single bar to mount the sensors on with alignment plates.


mike in co

[mike in co]

and ...

my "eyes"(sensors) are set down in a holder that has a rectangular window.....

do your drawings take this into account ?


what is the angle of misalaignment in your last diagram ?

[Whitespider]

Good lord gentlemen. My illustrations are purposely exaggerated for visual clarity.

C'mon, we're talking about a device that can measures speeds in excess of 4000 FPS (and does it in tenths-of-a-foot-per-second) by timing the bullet between two points as close as 18-inches with some units. How much error-of-angle do you think it takes?

Let's say we use the common 2-Ft screen spacing and a bullet traveling 2000-FPS, that bullet passes between the screens in .001 of-a-second. Now if we step the bullet up to 2100, the elapsed time becomes .000952380954... of-a-second (actually that number has been rounded off to 12 digits because it spans several dozen). That was for a bullet velocity change of 100-FPS at the 2000-FPS level, imagine how minuscule the time change would be at velocities over 3000-FPS. Now imagine a velocity change of 20-FPS. And now imagine just a tiny misalignment of the sky screens (whether by user setup or manufacturing) that results in an error of just .10 of-an-inch (.24 would be 1%) at the point where the bullet passes.

The sensors for my PACT are mounted in molded plastic housings, and than screwed to a welded steel bar; is that a picture of precision to you? So if we have a tiny bit of error in sensor mounting, and the user sets up so the bullet passes just a tiny bit closer to one sensor, AND the user sets up so the bullet isn't centered over both sensors....... Well now, just how much error do we have?

Granted, we'll never get our chrono set up perfectly, 'cause it ain't a perfect world. But if we take that little extra care to align the sensors with bullet path the best we can, and keep the bullet as close to the sensors as is reasonably possible, the margin of error can be reduced dramatically. After all, isn't everything about shooting also about precision, consistency and repeatability? We measure groups to .1, or even .01 of an inch; we measure bullet run out; measure powder charges to tenths of a grain; size our bullets to .001 inch; etc., etc., etc...... Why would we just haphazardly slap our sky screens out with a "that's close enough" attitude?

[mike in co]

my point is that in the two setups i have used, the spacing is fixed...firmly bolted in place.
my bench and target height are fairly fixed....i place the shooting window in line and at the same height frt and back........
again in most situations it will be one set of numbers in one session...so while slightly skewed so is the temp and the humidity...who do you know that records the humidity ?

nice pics, nice reminder to shooters.....but in a single session...i don,t see the issue. i dont see people intentionally shooting at skewed angles.

on the other hand if they do shoot that poorly, set up that poorly , then likely all is is done the same (poorly)and they will never notice the difference.

the lack of consistant muzzle to first screen distance is a bigger issue. with in an inch or so of the muzzle velocity drops can be measured. the ohler 3 screen with 2 foot spacing for a total of 4 feet shows large drops. so measure to the muzzle for consistant data. i use 10 feet for most on mine, but ocassionally have to move to 12 for belted magnums.

mike in co

[Larry Gibson]

mike in co

my point is that in the two setups i have used, the spacing is fixed...firmly bolted in place. my bench and target height are fairly fixed....i place the shooting window in line and at the same height frt and back........ again in most situations it will be one set of numbers in one session...so while slightly skewed so is the temp and the humidity

That is certain a very good methodology to follow. Consistency is the key, especially setting the screens close to the same distance from the muzzle (within a few inches at least).

...who do you know that records the humidity ?

I do and have for many years. I also record the temperature, and altitude.

nice pics, nice reminder to shooters.....but in a single session...i don,t see the issue. i dont see people intentionally shooting at skewed angles.

You're correct about it not being much of an issue in a single session. I have seen a lot of haphazzard chronograph set ups. In all my years chronographing and watching others do it I've seen very, very few who take the time to set the screens correctly at the same distance from the muzzle. The problem comes when they begin to compare data from day to day with different set ups. Worse is when they compare their chronograph readings with those of someone else's who is across the country without any knowledge of the setup and conditions the data was obtained under. This is when we get somewhat obscured reports of fast barrels or slow barrels, 200+ fps variations of the "same load", etc. Different set ups with different conditions using different guns are going to give different velocity readings. However, with the quality of most chronographs and consistent set ups the reading will be consistent within a normal variation range.

on the other hand if they do shoot that poorly, set up that poorly , then likely all is is done the same (poorly)and they will never notice the difference.

Ahmen.

the lack of consistant muzzle to first screen distance is a bigger issue. with in an inch or so of the muzzle velocity drops can be measured. the ohler 3 screen with 2 foot spacing for a total of 4 feet shows large drops. so measure to the muzzle for consistant data. i use 10 feet for most on mine, but ocassionally have to move to 12 for belted magnums.

I use 15 feet to the first screen with either my Oehler M35P or with the M43. I've also found that a difference of +/- 3" gives a low enough variation that the MA falls within the ES of several tests of the same load out of the same rifle.

Many do not realise what a large difference in fps a small difference of screen spacing can make either. I use a "reference rifle" and one specific lot of ammunition to test the set up every time I set the chronograph and screens up. The AV must fall within the expected variation range. If not the I recheck the set up and have always found something not set correctly when this happens. A decent .22LR rifle with a brick or two of .22LR match ammo will do fine for this and will last a long time with simple 10 shot test strings.

Lots of good points in this thread. Too bad the instructions for most chronographs do not emphasize a correct and consistent set up enough.

Larry Gibson

[lwknight]

Luckily, its not all that complicated to figure out if your 30-06 is shooting 2600 fps or 2900 fps and shooting from a solid rest at a target through the chrono is common practice because we like to test accuracy while speed testing the load as well.

Personally , I don't care about a 10 fps error. The last rifle load that I worked on had some spreads of 300 or so fps when I started and when I got enough powder in the case they (according to my non professional chrono) leveled out at 12 fps spread with less than 1/2" groups @ 100 yds.

So maybe I had imperfect sensor alignment from the factory but , who cares? It worked out great and the velocity was what it was expected to be at 2990 fps with a 150 grain bullet 10' from the barrel. I was using 58 grains W760 and standard LR primers. I later learned that magnum primers probably would have stabilized with 54 grains and given the same velocity.
Thats my next project.

For most of us that just want to know whats going on with our deer rifle loads , no special instructions are required to operate the chronograph.
If you are going pro and want to worry about micro minute details , you probably are bored with this thread anyway.

I just would hate to discourage anyone from getting a chronograph because they read how complex its operation is.

[Doby45]

I am confused.

[StarMetal]

Luckily, its not all that complicated to figure out if your 30-06 is shooting 2600 fps or 2900 fps and shooting from a solid rest at a target through the chrono is common practice because we like to test accuracy while speed testing the load as well.

Personally , I don't care about a 10 fps error. The last rifle load that I worked on had some spreads of 300 or so fps when I started and when I got enough powder in the case they (according to my non professional chrono) leveled out at 12 fps spread with less than 1/2" groups @ 100 yds.

So maybe I had imperfect sensor alignment from the factory but , who cares? It worked out great and the velocity was what it was expected to be at 2990 fps with a 150 grain bullet 10' from the barrel. I was using 58 grains W760 and standard LR primers. I later learned that magnum primers probably would have stabilized with 54 grains and given the same velocity.
Thats my next project.

For most of us that just want to know whats going on with our deer rifle loads , no special instructions are required to operate the chronograph.
If you are going pro and want to worry about micro minute details , you probably are bored with this thread anyway.

I just would hate to discourage anyone from getting a chronograph because they read how complex its operation is.

You hit the nail dead on the head. How many of us are working for some big company, NASA, the government....and our chronographs have to be accurate right down to the 1/16th of a foot per second?

If you have any sense about reloading, have the manuals, etc...you're know if you have a really bad reading. For one thing you're not going to beat the max loads in the books by 100's of feet per second and your rifle stay together. With technology today I think they all have a decent set out. I have been using the Pro Electronics for years and have never had a problem with it.

[mpmarty]

A man with one watch knows what time it is. A man with two watches is never sure. Same with Chronographs. I use an F1 master chrony and am quite happy with it.

[Larry Gibson]

That's true but the man with one watch that doesn't know how to set it correctly or tell time doesn't do to well with it though he may be as happy with it. It's all just a matter of how much accuracy or data one wants out of your equipment is all.

Some are happy with a simple chrony that will tell them within reason how fast the bullet is going regardless of the accuracy of the set up or use. That's all well and fine. Same those who are happy with a simple Lee TL'd bullet over "the load" of Red Dot. That makes them happy and all is well. Many are not satisfied with just a TL'd lee bullet over "the load" of Red Dot and want more out of their loads. No difference and nothing wrong with either approach or one in the middle as long as all understand the limitations of what they are doing and the data they are collecting. Going out and setting up a chronograph, regardless of make, without any consistency gives satisfactory data that is pertimant to that load at that time with that set up. Conversly going out and setting up the chronograph consistently every time and noting the other conditions that effect the ammo's performance then gives data that is viable in comparison with other data gotten at other times. That's all that's being said here. Not much need to criticise either method or those in between. Some are just stating and explaing how and why they do things with chronographs is all. That's the nice thing about our free country, isn't it?

Larry Gibson

[montana_charlie]

It's possible to get variations that are the result of 'angles' even if the testing occurs on the same day with the same chronograph setup.

Say a guy gets all setup and fires his hunting load for group, with iron sights, while also recording velocity.
Then, he mounts his scope with it's quick release, see-thru mount, and fires another string.

Now, assume his rear sensor is glued in at a slight angle as depicted in the second diagram.

Because of the scope height, the second string of shots passes two inches closer to the sensors than the first string did. A rear sensor angled forward will record a slower velocity for the second string.
It will continue to happen as long as the shooter continues to try both configurations and has ammunition to keep testing.

He will go home convinced that mounting a scope on his rifle reduces muzzle velocity...and he'll have the numbers to prove it.

When he posts here to ask how that can be possible, which one of us will even consider the possibility that his chronograph setup may be the problem?

CM

[Larry Gibson]

Wow Charlie, that's an aweful good chrony that can make that kind of distinction , given the probable randon variation of the AV in back to back strings of the same ammo out of the same rifle. It always amazes me of the infinate amount of potential tests of theories promulgated on this board!

Larry Gibson

[AbitNutz]

I'm going to get the CED M2 with the infrared sky screens. The whole kit will run less than $300.00 and then all I need is a decent tripod to mount it on.

This thread has really driven home how important setting it up is.

After really reading, understanding, setting it up...all I have to do is figure which part I'm going to shoot first.

[lwknight]

Shoot the guide rods first , save the display for last.

[mike in co]

I'm going to get the CED M2 with the infrared sky screens. The whole kit will run less than $300.00 and then all I need is a decent tripod to mount it on.

This thread has really driven home how important setting it up is.

After really reading, understanding, setting it up...all I have to do is figure which part I'm going to shoot first.

a good choice sir....

[mike in co]

It's possible to get variations that are the result of 'angles' even if the testing occurs on the same day with the same chronograph setup.

Say a guy gets all setup and fires his hunting load for group, with iron sights, while also recording velocity.
Then, he mounts his scope with it's quick release, see-thru mount, and fires another string.

Now, assume his rear sensor is glued in at a slight angle as depicted in the second diagram.

Because of the scope height, the second string of shots passes two inches closer to the sensors than the first string did. A rear sensor angled forward will record a slower velocity for the second string.
It will continue to happen as long as the shooter continues to try both configurations and has ammunition to keep testing.

He will go home convinced that mounting a scope on his rifle reduces muzzle velocity...and he'll have the numbers to prove it.

When he posts here to ask how that can be possible, which one of us will even consider the possibility that his chronograph setup may be the problem?

CM

simple question:
can you tell me the brand name of the chron that allows you to position the sensor tilted ?
the bor cronoy's are can monted, the pact is beam mounted, the ohler is beam monted ???
what one allows one to do as you two have indicated...mount one tilted ??

mike in co
( my next statement will be: then dont buy that one)

[Whitespider]

...all I have to do is figure which part I'm going to shoot first.

Years ago I bought one of the first PACT model 1 chronographs. At the time, affordable chronographs were just becoming available, not a lot of people had them. But even the $199.oo was a lot of money for me at the time, and I spent another $50.oo or so for the tripod.

The bar, or beam, that the plastic housings mounted on was made from two lengths of square tube steel with three pieces of flat steel welded across them, keeping the tubes evenly separated. The tops of sensors housings mounted flush on top of the bars, with the body of the housings hanging between and below them. The original sky screens didn't have light defusers, or any provisions for them, which made getting the sensors lined up a royal pain-in-the-....

The "brain box" had an on/off switch on the back and two buttons on the front, a "review" and an "edit". To operate the unit you first turned it on and the display would scroll some information about PACT, software, etc. Next you needed to push both of the front buttons simultaneously to put the unit in "ready" mode.

I spent the first two weeks shooting every gun and load I had over the thing, and so did a lot of my shooting pals. One problem was that, without light defusers, and under certain light conditions, it was difficult to get consistent readings; you would have to keep moving the sensors closer and closer to the bullet path until it started working acceptably.

Well, about two weeks after I got the thing, one of my pals came out with a new .22-250 and asked if he could shoot it over my chrono, and I readily agreed because it was sill new and fun, and I liked showing-it-off. I get him all set up on the bench, making a big production of aligning the sensors, and he touches off the first round. He looks at the display and says, "didn't work."
"No problem", says I, and I step up and raise the sensors a bit.
He shoots again... "nothing" he says.
Well, we keep going through this, him shooting and telling me it ain't working and me raising the sensors, until...... his bullet smashes through the first sensor housing, slamming into the center post of the tripod, cutting it cleanly in half and than exploding the second sensor housing.

I wanted to cry... I surveyed the damage... the photo-eye in the first sensor had survived but the second was destroyed, and the tripod was history. As I'm picking up the pieces and putting stuff away I notice that I had turned the "box" on but had never bothered to push the font buttons and put the unit in "ready" mode.


I don't recall what the replacement parts cost me, but it wasn't near as bad as I thought it would be, most of the cost is in the "brain", I even ordered extra parts.

Oh, and that wasn't the last time I called PACT for sky screen parts, I believe I've called them 4 times now... maybe 5 times.

[EMC45]

I just got a PACT PC2 over the weekend. It is an older model, it looks to be in excellent shape! My buddy MGD45 (member here) has a Chrony brand chrono and he is pleased with it. It is the little green one that folds up and it was about 75 bucks with dealer discount from Midway I believe. I got mine for a trade of cast and Jbullets!!!! Thanks Wallace!! It has the tripod, spacer bar, difusers, and of course the box, which gives Velocity, SD, ES, and a bunch of other functions that I am not sure of due to no manual.