# How accurate is chrony?



## Jim59 (Feb 7, 2014)

Would it be possible using two of them to find the drop in velocity between them, if they were spaced a certain distance apart?


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## Rathunter (Feb 14, 2014)

well... you would need to account for the drop of the projectile.

personally, I have very little experiance with Chronographs.

I do know that some are more accurate than others.

If you have the accuracy, it is a cool idea.

you might want to guard the electronics with some steel plate.


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## Henry the Hermit (Jun 2, 2010)

Jim59 said:


> Would it be possible using two of them to find the drop in velocity between them, if they were spaced a certain distance apart?


Of course you could do that, but this is an area that has been thoroughly explored by firearms shooters, and there are computer programs that can compute the velocity, drop, and wind drift at varying ranges. If you decide to run some tests using two Chronys, please let us all know your results.


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## Rathunter (Feb 14, 2014)

a slingshot projectile is of different shape and size, weight, and velocity than a firearms bullet.

It follows different ballistics.

Not actually different, but the FPS difference is measured in magnitude. Which really makes it a bad idea to use firearms ballistics formulas for a slingshot.

also, most firearms are rifled.

This throws an additional wrench into using firarms formulas for slingshots.

LoL

Just run the tests


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## Henry the Hermit (Jun 2, 2010)

Not all ballistics programs are written for bullets. This one works just fine for round ball and at slingshot velocities.

http://www.ctmuzzleloaders.com/ctml_experiments/rbballistics/rbballistics.html


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## Henry the Hermit (Jun 2, 2010)

Here is the direct link to the on-line version of the round ball ballistics program.

http://www.ctmuzzleloaders.com/ctml_experiments/rbballistics/web_apps/rb_ballistics.html


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## treefork (Feb 1, 2010)

Use the one chrony. Take five shots close. Take five shot far. Take an average. Were talking slingshots. Should be accurate enough. Of course you will need to possess the skill to place the far shots exactly through the chrony's sweet spot in either case.


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## Tex-Shooter (Dec 17, 2009)

In actual practice I found that the drop in velocity was less than shown on the round ball ballistic calculator. I think that the reason for this is that the calculator is working on a formula that was meant for velocities higher than the speed of sound. -- Tex


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## libel (Jul 1, 2013)

The calculator is based on data gathered in 1904 and while the data may have been correct the model it uses was purposely oversimplified. There were no computers or calculators available and they were trying to come up with something simple.

A chrony is accurate enough because we are assuming you'd be ignoring a lot of other variables than come into play. I don't think you're trying to come up with anything better than the calculator so by all means give it a try and let us know what you find out.


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## Jim59 (Feb 7, 2014)

treefork said:


> Use the one chrony. Take five shots close. Take five shot far. Take an average. Were talking slingshots. Should be accurate enough. Of course you will need to possess the skill to place the far shots exactly through the chrony's sweet spot in either case.


Marksmanship is the problem.

Who can shoot directly through two chrony's spaced 5 metres (16.4 feet) apart, and hit a very small target that has been placed directly inline with them?

Each time you miss, the readings would have to be ignored.


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## Jim59 (Feb 7, 2014)

Tex-Shooter said:


> In actual practice I found that the drop in velocity was less than shown on the round ball ballistic calculator. I think that the reason for this is that the calculator is working on a formula that was meant for velocities higher than the speed of sound. -- Tex


That's true. They say at high velocities air gets sticky.


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## Jim59 (Feb 7, 2014)

Rathunter said:


> well... you would need to account for the drop of the projectile.
> 
> personally, I have very little experiance with Chronographs.
> 
> ...


You've seen me shooting.


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## Jim59 (Feb 7, 2014)

libel said:


> The calculator is based on data gathered in 1904 and while the data may have been correct the model it uses was purposely oversimplified. There were no computers or calculators available and they were trying to come up with something simple.
> 
> A chrony is accurate enough because we are assuming you'd be ignoring a lot of other variables than come into play. I don't think you're trying to come up with anything better than the calculator so by all means give it a try and let us know what you find out.


You mean variables like.

Altitude

Temperature

Air pressure

Cross winds

Type of equipment

Surly even using a chrony these factors should be taken into account.


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## Rathunter (Feb 14, 2014)

Jim59 said:


> Rathunter said:
> 
> 
> > well... you would need to account for the drop of the projectile.
> ...


I have?

I am not trying to insult your accuracy.

I am trying to protect your chronograph. The unpredictable does happen.

I haven't watched any videos on here yet.

Just Treefork's match light with a blowgun.


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## libel (Jul 1, 2013)

Jim59 said:


> libel said:
> 
> 
> > The calculator is based on data gathered in 1904 and while the data may have been correct the model it uses was purposely oversimplified. There were no computers or calculators available and they were trying to come up with something simple.
> ...


I'm not sure what you're trying to get to.

You will end up with numbers like:

201.5 180.3

198.8 177.9

and so on

What are you going to do with them?


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## Rathunter (Feb 14, 2014)

What he is saying is that there are many variables that make it very diffucult to get consistant measurements.

Expecially across days.

Where I live, the temp has been going nutso this winter.

If I shoot in -10 deg f weather, I might get 150 FPS at the most- I get cold, I hold the bands too long, the air is slightly denser(not a big factor), and my technique drops... and response time of the bands spikes... and elasticity drops.... If I shoot inside or when it is above 50(right now), I can get well over 200 FPS fairly easily with small ammo.

If I shoot when it is super dry(humidity wise), it raises FPS slightly.

Obviously shooting different slingshots will give different speeds.

I have actually chrono'd one of my slingshots using a rather interesting device called a ballistic pendulum.

It is useful, but requires pretty precise math. It can, however, calculate the speed of a projectile of known weight, within roughly +10%.

It is a PITA to use, and stops your projectile however, It also requires modification to suit the weight of the projectile.

With very light projectiles moving very fast, it is pretty inaccurate due to projectile disentegration.

Actually it measures the Foot-Pounds of a projectile, but if you know the weight...

It is quite a bit of work, but they are very cheap.


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## Jim59 (Feb 7, 2014)

libel said:


> Jim59 said:
> 
> 
> > libel said:
> ...


If I held my slingshot parallel to the ground at a height of 1.5 metres, and took a shot.

If the first chrony read 61.3 m/s, the second chrony should read 60.9 m/s.

This means that the ammo would have dropped in height to 1.46 metres in just over 5 metres.

The total range would be about 33 metres.

Have a go yourself.

I could have done it wrong.


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## Jim59 (Feb 7, 2014)

Rathunter said:


> What he is saying is that there are many variables that make it very diffucult to get consistant measurements.
> 
> Expecially across days.
> 
> ...


I forgot about humidity.

Where I live it hovers around 80%.

A ballistic pendulum sounds like my next project, any tips?


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## libel (Jul 1, 2013)

Are you trying to demonstrate ballistics using slingshots for a science fair or something? Initially I suggested that a chrony might be accurate enough because I did not think you were going to make any other measurements. After reading the rest of your comments I now think a chrony is not accurate enough to produce anything useful. In your example you calculate a 4 cm drop in height. This could be the result of a wide range of chrony-measured velocities. Have a look at numerical analysis.


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## Jim59 (Feb 7, 2014)

I just wanted to know how accurate or inaccurate a chrony was, because I saw a YouTube video of someone recording a velocity of 272.7 m/s (894.45 ft/s) with heated bands.

Henry from Panama doubted this speed.

Have you seen the video?

What do think?


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## libel (Jul 1, 2013)

I don't recall the specific video but every 100 shots or so I get one that registers in the 500fps+ range. The sensors are fooled by changes in light, reflections or what have you. When your bands are wet the water droplets increase the likelihood of this happening.


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## Jim59 (Feb 7, 2014)

Sorry I should have posted the video.

Here it is.






First shot with green thera-band.

141m/s (462.4ft/s)

Second shot with black thera-band.

141.7m/s (462.4ft/s)

First shot with heated thera-band black.

272.7m/s (894.4ft/s)

Second shot with heated thera-band black.

217m/s (713ft/s)

The bands must have cooled down somewhat, but pretty impressive don't you think?


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## libel (Jul 1, 2013)

I think what I mentioned above holds. The conditions in the video appear ideal for producing false readings. Without having the whole session available and with direct sunlight at an angle like that my guess is those were just false readings.

Because you asked if we find this impressive I have to say I'm more interested in the chrony accuracy and ballistics program discussion than discussing this video. Had you posted that video at the start of this topic I would have never replied.


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## Jim59 (Feb 7, 2014)

When I said "pretty impressive "I was being sarcastic, thought you would have realised that.

Sorry again. Thought it would be helpful in proving my point by posting the video.

What do you do to discard false readings with a chrony?


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## libel (Jul 1, 2013)

Back atcha on the sarcasm.

Statistics. Same as you would with data from any other instrument. Your chrony is 99.5% correct not taking in account false readings.


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## Rathunter (Feb 14, 2014)

A ballistic pendulum never reads wrong.

it isn't possible for it to do so.

lol

but the amount of work required is quite a bit.


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## Lacumo (Aug 18, 2013)

Rathunter said:


> A ballistic pendulum never reads wrong. it isn't possible for it to do so.


I don't believe there is such a thing as a perfect machine in this world. Every man-made machine has a margin of error. The margin may not be much, but it's there.


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