Measuring speed of the shaft

[tromic]

I made a design of this small equipment for measuring the speed of the spear gun shaft underwater. It is supposed to measure speed on any distance from the gun, along path of about 10 cm for which time should be calculated from the voltage shown on DVM before and after shoot. 220 uF capacitor discharges though 10 Ohm resistor. This might be OK for shaft speed from 35 m/s to 5 m/s.

[ame="http://www.youtube.com/watch?v=q-nprjzlS4A"]YouTube - Brzinomjer[/ame]

DVM will be in PVC bag connected to the housing of equipment.

From the above demonstration the time is t1-t0 = ln(810/646)*0,000220*10 = 0,498 ms and the speed is 31,14 m/s because the length is 0,016 m.

 

[foxfish]

That would be fantastic if you can get it to work, at last some real figures to work from & judge against :friday

 

[tromic]

That would be fantastic if you can get it to work, at last some real figures to work from & judge against :friday

I am sure it will work. The only problem to be solved is how to fix it to the gun. :friday

 

[tromic]

I am sure it will work. The only problem to be solved is how to fix it to the gun. :friday

It will work, but some modifications shoud be done. The switch must be more reliable and with lower friction. I suppose next time will be some results.

 

[tromic]

This is a modification to switch activation:
[ame=http://www.youtube.com/watch?v=xgfYGs_JkiY]YouTube - ‪TriggerTest.MPG‬‏[/ame]

 

[tromic]

I slightly altered the version of speed measuring device on the first picture (chart on the box).

Capacitor C = 100 uF, R = 10 ohms. Before the digital multimeter I put a total of 20 MOhm resistor.
Instead of switches that I made​​, it would be better to use the switches as shown on the picture.
I stayed at the old switch that I improved. The spacing of activating a switch could be 15 - 20 cm. Resolution measurements of the speed of shaft could be better than 0.5 m/s, from about 40 m/s to 10 m/s.

 

[tromic]

If the battery voltage would be 9V and a length to measure speed 20 cm, with a 20 MOhm resistor before DVM, the voltage to speed relationship would be as shown on diagram. DVM that I use for this measurement cost only 8 EUR and has input resistance of 1 MOhm.

 

[tromic]

To be more clear how it works. The switch (SW1 and SW2) should be sliding switch as on the picture above. Of course everything must be isolated from water.

 

[tromic]

How to precisely measure switches activation distance. In this case distance was 172 mm.
On the picture billow, red is a charging and blue a discharging circuit of capacitor. Green is a measuring circuit.
I expect total measuring error of about 5 %.

 

[tromic]

Another possibility. DVM and battery could be out of water all the time. Make a conection before measurement, charge the capacitor and measure voltage on it. After shooting measure the voltage on the capacitor. In 10 sec voltage on the capacitor changes for 1 - 2 mV because of self discharge.

 

[tromic]

Now everything is ready for the speed measurement. I just need water (lake, sea...)

The gun is on 25 bar;
I will take for CR = 1.14 for Cyrano 850. I calculated it from loading effort on beginning and finishing the loading.
The medium pressure would be (25 + 25*1.14)/2 = 26,75 bar;
Friction of the piston is about 1,2 kg;
Because I will measure with vacuum barrel I will add 1 bar to medium pressure.

So the overall effective pressure would be 26,55 bar.
Shaft weight is 334 g.
Acceleration distance is 66.5 cm (piston travel length).

From above data the theoretical speed of the shaft in air should be 31,7 m/s.

After measuring speed in water I should get speed less than this result.
I am curious about what will be the actual speed.

 

[foxfish]

Lets hope it all comes together & works

 

[zmajmr]

This were some of the first tests in sea.

[ame=http://www.youtube.com/watch?v=VmJCLx1BNUU]YouTube - ‪mjerenje brzine‬‏[/ame]

 

[tromic]

Most problems, unexpected, I had with activation of switches. Either only one had been activated, or none. When I managed to activate the switches, contacts of the switch failed - the switch became broken.

Now I fixed the problem with the old switch, I hope so. I was thinking about to use factory made "sliding switch" as on the picture. That would be better, but I should make new housing for the switches. Not a big problem, but I will rather try with what I already have. I just need a few measurements. Peter, if I would make measurement in a lake and get a result, say v0 = 30 m/s, what would be a speed in salt water? If density of tap water is "1" than density of salt water is about "1.01". Is it v0/1.01 or v0/(sqrt(1.01)) or something other? Well, the difference will be less than the error of speed measurement but ... just to know.

 

[Don Paul]

How about a sealed reed type switch and a magnet trip ?

I should have a Go Pro by next month to post some Arbi images of your kit at work...I have been out with a broken rib for a month.

Cheers, Don

 

[tromic]

How about a sealed reed type switch and a magnet trip ?

I should have a Go Pro my next month to post some Arbi images of your kit at work...I have been out with a broken rib for a month.

Cheers, Don

Reed type switch is not fast enough. Sorry for your broken rib! A thought you were on vacation somewhere without internet.
Cheers, Tomislav

 

[popgun pete]

Most problems, unexpected, I had with activation of switches. Either only one had been activated, or none. When I managed to activate the switches, contacts of the switch failed - the switch became broken.

Now I fixed the problem with the old switch, I hope so. I was thinking about to use factory made "sliding switch" as on the picture. That would be better, but I should make new housing for the switches. Not a big problem, but I will rather try with what I already have. I just need a few measurements. Peter, if I would make measurement in a lake and get a result, say v0 = 30 m/s, what would be a speed in salt water? If density of tap water is "1" than density of salt water is about "1.01". Is it v0/1.01 or v0/(sqrt(1.01)) or something other? Well, the difference will be less than the error of speed measurement but ... just to know.

My commercial diving manual states that freshwater density is 1000 kg/cubic metre and for all practical purposes saltwater density is 1025 kg/cubic metre, so that is a difference of 2.5%. The density of saltwater depends on where you are in the World's oceans as it varies slightly from place to place, so the figure of 1025 is a practical estimate. I don't know what the effect on shaft speed would be in terms of an adjustment, but I suspect that the shaft and spear tip diameters present such a small cross-sectional area that the denser medium does not give a much greater resistance to their progress through the water. The spear tip pushes water aside which then regroups behind the shaft tail, that is where increased density can play a part, but the effect would be minimal unless the object was much larger in the cross-sectional area facing the direction of travel. I have never speared in freshwater (it is illegal to do so here and testing spearguns in swimming pools has never been an option) so have had no chance to compare speargun performance in other mediums. Whatever results you achieve are going to be relative measures for your particular set-up, so I would not worry too much about salt versus fresh considerations. Shooting line texture and diameter affect shaft velocity during shaft flight, so shaft speed results will be affected by choices made there as much as anything else. Spearguns give you muzzle velocity, after that the amount of speed retained depends on the spear/tip/slider and line characteristics.

There is a device called a "Chronoscope" used to measure air gun pellet velocity at the muzzle; I believe speargun muzzle velocity has been measured using such a device in a modified form for underwater use.

 

[settingsteel]

Shooting line texture and diameter affect shaft velocity during shaft flight, so shaft speed results will be affected by choices made there as much as anything else. Spearguns give you muzzle velocity, after that the amount of speed retained depends on the spear/tip/slider and line characteristics.

Thats it right there.

 

[tromic]

To be more clear how it works. The switch (SW1 and SW2) should be sliding switch as on the picture above. Of course everything must be isolated from water.

I made a mistake in previous R calcualtion. R should be 100 Ohm instead of 10 Ohm. In my excel spreadsheet I made wrong entry for measuring distance, instead of 0.17 m there was 0.017 m (on the first picture 20 cm value). The second diagram shows voltage on DVM for various speeds of the shaft. I did a measurement yesterday but because of wrong value for R, measured data were out of range of the "seed meter". I suppose next time will be better.

 

[tromic]

It was more difficult to measure the speed of the shaft than I thought it would be. This is the last setup with a sewing thread, and it works. I was using Tomba10 vacuum barrel, 1.5 mm mono and slider as on the picture. Distance to calculate the speed from measured time was 0.172 m.
Per calculation a got that the speed of the shaft should be 31.7 m/s. This speed might be even higher if the drag of piston is less than 1.2 kg. I measured the total drag of piston, shaft to cone ring during loading and unloading the gun. If the piston drag would be 0.8 kg instead of 1.2 kg than the speed in air would be near 32 m/s.

What I got as a result of measurement was:
0.2 m after the shaft separate from the piston the speed was 33.1 m/s (3.4 % more than 32 m/s).
0.5 m after the shaft separate from the piston the speed was 24.5 m/s.
Because the measurement error is less than 5 % this results might be close to actual speed of the shaft. I wanted to make some more measurements on different distances, binding the triggering line to the mono line but the knot was wiped from its position so I left it for some other time, in a sea. This was in a lake.

Of course I will repeat the measurement to be more sure about the results but the "speedmeter" was working pretty fine. What was very interesting to me is that the max speed in water (Vo) was very close to the speed in air. After only 0.5 m the speed become much lower, V= 0.73 * Vo. This was mainly because of mono line and slider drag.