Interesting question.
I have brought up the issue of chrono accuracy several times on various boards and got no interest - most people think they are dead on accurate, and never question them. If the drop of their bullets aren't what they are supposed to get, they blame the manufacturer's BC, the software, and the phase of the moon, but they NEVER question the chrono - which is the most likely source of error.
The problem with these chrono's (all of them) are several.
The first is that all of them use a 89 cent, 4 Mhz timing chip that comes from China, or the Philippines, Somalia, or Papua New Guinea.
There is no way reasonable to check them for accuracy. And they are not temperature compensated, so the warmer it is, the faster the chip runs, so the SLOWER the velocity the chrono reads, for a given fixed trap time.
But the second is more complicated, and is where most of the error comes from.
These chronos work by catching the shadow of the bullet as it passes over a light sensitive port. This light port "sees" an area that is a trangle about 12"x12"x12" above it (if the top light defuser is in place).
The light falls onto a photo cell, and as the bullet passes over, the change in current through the cell causes a detector circuit to trip, and start the clock timing circuit to start "marking". The next cell causes the clock to stop "marking", and you have a number of "ticks" that are 4/10,000,000 of a second long - then the widdle computer takes over and posts your speed. As slick as they sound, they are really simple circuits.
The change in current is not like an off/on like a switch, it is a gradual change, and when looked at with an oscilloscope, it looks like the bottom half of a sine wave... kinda... actually, the leading ramp (the nose of the bullet) is gradual, and the trailing ramp (the tail) is sharp(er).
Now, here's the bad part. The circuits detect the change in light with what is generically called a "delta detector" circuit, which means it only detects change, but not any other part of the change - so it will trigger on a sharp on/off, or a gentile slope change... and the part of that slope that it triggers on is NOT fixed, cuz the amount of light it is set for, is NOT fixed... it floats.
Here's some more bad part... the percentage of change (the amount that the current dips) changes depending on where in the triangle, the bullet goes through.
Here's MORE bad part. The shape of the leading edge of the slope of the current change, CHANGES SHAPE, depending on how thick a section of "light" the cell sees.
The section of light the cell sees just over the little window is only a few mm's thick, but the section of light seen by the cell near the top of the triangle, is ~1 inch thick.
So the higher in the triangle you shoot, the worse EVERYTHING is.
But (more bad part) the closer to the cell's window you shoot, the better chance you have of shooting the **** cell (been there, have the teeshirts IN SPADES).
OK... the last of the bad stuff (I promise
)
If the bullet is not exactly parallel to the bar, that is to say, if the bullet does not pass over both windows at the exactly the same hight, then the velocities will read slower than they should.
My first Oehler (#33) came with a real technical book explaining most of this stuff, but it was so complicated that they had a full time person on the phone answering questions. They finely realized that the more you told the customer, the worse it got... and 99.9% of the customers just wanted to know "How fast is my bullet going?", and more importantly, WHICH LOAD IS GOING FASTER!!!
They didn't want to hear about slopes and clock speeds.
So they simplified the #33 manual, and then further simplified the P-35 manual... and everybody was happy.
Some 10 years ago, it painfully came to my attention, that my #33, and my P-35 were not telling me the same things. So I set them up together, so the bullet ran through both of them, and they didn't tell me the same things
A few years ago, I spoke at great length with Oehler's head en-ga-neer about being able to calibrate a P-35 to "traceable standards" (you knerds out there know what that means)... I figured it should be a simple matter of adjusting the spacing of the cells to make up for any inaccuracy of the clock, once I could accurately know the errors.
After several days and many phone calls, they could NOT come up with a single way to check a chronograph for accuracy.
In fact, he said that mine was the first call to inquire about it (that's not a good sign for us shooters
)
But here's the kinda good news.
Lookie - most of the time, shooters do want to know which is faster, and that's easy.
If you want more consistency, be careful where the bullet flies over the cell, the closer the better, but don't shoot the frame.
And a BIG source of error is the thickness of the light area the cell sees. Use the IR illuminator - the beam is thinner, so the ramp is sharper, and the triggering is more repeatable.
I like the CED a lot, and I plan to get another... and I still have the Oehler #33, and I will NEVER sell it.
I'm glad I sold the P-35... it was klugie!
I hope this helps - if you have more questions... just ask.
.
