colinb
Super member
I want to assess interest in a project and product. There have been many (partly uninformed) threads bemoaning the fact that a chrono cannot be calibrated. Now I was a calibration design engineer. That is one of the very top levels of Electronics. So trust me.
One type of chrono CAN be easily calibrated. An example is the cheap chinese chrono. Any chrono using LEDs and matched optical sensors can also be calibrated.
So that's what I'm talking about. Not radar chronos. I am suggesting the manufacture and availability of your personal chrono calibrator, simple, cheap, no processors, plug in and go with several fixed spot speeds set by one rotary switch or equivalent over a limited range, maybe 250- 1000 fps. Good enough for most of us.
Be aware of the limits. Initially, Calibration is the determination of error at one operating point. That determination is subject to an uncertainty. So a calibrated instrument does not necessarily read the correct answer. You have to know the error and compensate for that. Sometimes the equipment is adjusted to tweak out the error and so read correctly, but the uncertainty remains.
So my proposed equipment would only allow the user to find the error of his chronograph. It would be up to you to do the simple sums. eg. The calibrator is set to 700 fps. Your chrono measures 702 fps. So you know it's reading 2 fps high at that point, and you don't know the uncertainty- which will actually depend on just one thing.....
Unfortunately it's not quite as simple as that....an optical chrono measures TIME for the pellet to traverse the DISTANCE between two sensors, and we want :-
SPEED = DISTANCE/TIME.
Your chrono has to do sums inside to get the SPEED. Our brains are wired to understand speed. 700fps is understandable but 325usec is complete gobbledgook to most folk.
Let's break that down...
To do our calibration, we have to know that DISTANCE. Easy.
How far apart are the sensors of your chrono? That factor alone actually pretty well determines the chrono parameters. You have to know that to do the calibration. No option but to get your ruler or calipers out and carefully measure the DISTANCE between 2 long slits in the plastic, across the path of the pellet. Not hard, but typically the distance can be between about 50mm and 300mm. The better chronos have longer distances. So you now have DISTANCE, but you also have an uncertainty. How good was the measurement? And are the sensors just underneath the same distance apart? You have to live with that unless you strip the chrono and measure the sensor spacing.
TIME is the easy bit. Errors on TIME, (unless there is a serious fault) are negligible in comparison with DISTANCE. The proposed calibrator is simply 2 light sources with a variable distance apart and a shield. All you do is to put the calibrator light sources near to your two sensor slits, hopefully directly above them. These sources are normally ON and are simply blinked OFF very fast in turn. The chrono thinks that the OFF times are a pellet passing over them. So by setting the TIME apart of the two blinks on the calibrator control we have told the chrono what to do and when to do it. It will display a SPEED(Chrono). Success!
So we now know DISTANCE, because we measured it.
And we know TIME because we set that on the calibrator control. We put those two items into our phone calculator. DISTANCE/TIME = SPEED(Cal) that was generated by the calibrator. So now we know what went in and what came out. We have calibrated our chrono. No pellets. No danger. We know it's working and giving a sensible answer. And within the uncertainty of DISTANCE, we know its error. For any chrono, you would simply do this at several speed points.
So my proposed product just consists of 2 LEDS, and an adjustable electronic timer to create two brief pulses. The Chinese, bless them, would do it for a fiver. Yes, it won't be pretty, needs a battery and you'll need to do a tiny bit of work and a sum. But how long do you spend with your chrono taking measurements that you cannot trust? How can you prove to PC Plod that your chrono is good? If interested, just show this with a LIKE.
One type of chrono CAN be easily calibrated. An example is the cheap chinese chrono. Any chrono using LEDs and matched optical sensors can also be calibrated.
So that's what I'm talking about. Not radar chronos. I am suggesting the manufacture and availability of your personal chrono calibrator, simple, cheap, no processors, plug in and go with several fixed spot speeds set by one rotary switch or equivalent over a limited range, maybe 250- 1000 fps. Good enough for most of us.
Be aware of the limits. Initially, Calibration is the determination of error at one operating point. That determination is subject to an uncertainty. So a calibrated instrument does not necessarily read the correct answer. You have to know the error and compensate for that. Sometimes the equipment is adjusted to tweak out the error and so read correctly, but the uncertainty remains.
So my proposed equipment would only allow the user to find the error of his chronograph. It would be up to you to do the simple sums. eg. The calibrator is set to 700 fps. Your chrono measures 702 fps. So you know it's reading 2 fps high at that point, and you don't know the uncertainty- which will actually depend on just one thing.....
Unfortunately it's not quite as simple as that....an optical chrono measures TIME for the pellet to traverse the DISTANCE between two sensors, and we want :-
SPEED = DISTANCE/TIME.
Your chrono has to do sums inside to get the SPEED. Our brains are wired to understand speed. 700fps is understandable but 325usec is complete gobbledgook to most folk.
Let's break that down...
To do our calibration, we have to know that DISTANCE. Easy.
How far apart are the sensors of your chrono? That factor alone actually pretty well determines the chrono parameters. You have to know that to do the calibration. No option but to get your ruler or calipers out and carefully measure the DISTANCE between 2 long slits in the plastic, across the path of the pellet. Not hard, but typically the distance can be between about 50mm and 300mm. The better chronos have longer distances. So you now have DISTANCE, but you also have an uncertainty. How good was the measurement? And are the sensors just underneath the same distance apart? You have to live with that unless you strip the chrono and measure the sensor spacing.
TIME is the easy bit. Errors on TIME, (unless there is a serious fault) are negligible in comparison with DISTANCE. The proposed calibrator is simply 2 light sources with a variable distance apart and a shield. All you do is to put the calibrator light sources near to your two sensor slits, hopefully directly above them. These sources are normally ON and are simply blinked OFF very fast in turn. The chrono thinks that the OFF times are a pellet passing over them. So by setting the TIME apart of the two blinks on the calibrator control we have told the chrono what to do and when to do it. It will display a SPEED(Chrono). Success!
So we now know DISTANCE, because we measured it.
And we know TIME because we set that on the calibrator control. We put those two items into our phone calculator. DISTANCE/TIME = SPEED(Cal) that was generated by the calibrator. So now we know what went in and what came out. We have calibrated our chrono. No pellets. No danger. We know it's working and giving a sensible answer. And within the uncertainty of DISTANCE, we know its error. For any chrono, you would simply do this at several speed points.
So my proposed product just consists of 2 LEDS, and an adjustable electronic timer to create two brief pulses. The Chinese, bless them, would do it for a fiver. Yes, it won't be pretty, needs a battery and you'll need to do a tiny bit of work and a sum. But how long do you spend with your chrono taking measurements that you cannot trust? How can you prove to PC Plod that your chrono is good? If interested, just show this with a LIKE.
