Results from some tests...

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Maybe you should try to find out first what the cause for the shaking is, you could be mixing up cause and effect.
perhaps it is just sympathetic resonance picked up by the tubes from the maximum deflecting [=resonant effect] water particles.

But then again, positive tube attracts the other tube, maybe you can rule this out by pulsing without water?

think of a reason for the cause, and try to prove yourself wrong by experiment 

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You may be right but how do I identify the two options? Maybe trying the same experiment in moist air? Then how do I measure the percentage HHO? It is really hard to id the cause and I am inclined to evolving the experiments into some radio system copy with long thin pipes and matched pipe resonances and gap width experiments by coating etc. I am 100% sure we can easily reach COP 10 without exotic tech(ceramics/crystals etc) and that is more than enough if we then ionize the water fuel before ignition.

Look at the Kelvin water dropper and Leo Umila, more is not needed I believe..

Back to the experiments.

Maybe you should try to find out first what the cause for the shaking is, you could be mixing up cause and effect.
perhaps it is just sympathetic resonance picked up by the tubes from the maximum deflecting [=resonant effect] water particles.

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No vibration means no HHO
But, no HHO because of no shaking, which is your current theory, or,
no shaking because there is no resonance?
[or it is shaking because of partial standing waves  ]

Maybe trying the same experiment in moist air?

yes, or just air, see if it vibrates.
and what you could try is playing with the dutycycle, with water added, I expect that shaking will cease but HHO wont drop - because a partia standing wave becomes a whole or half wave. but am absolutely not sure.

I'm not behind the bench, so for me it is easy talking.

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For now longer and thinner pipes and then duty cycle modulation will be the easiest progress to make. Then twosided metal spacers with foam between them to minimize vibration dampening area. Finally burst frequency may be interesting(1 pulse, wait 5 periods, 1 pulse etc) if we achieve a high enough Q value.

Then coating and maybe John´s variety of mounting the cylinders, more like radio antennas. We´ll see, I am not the least worried, results will come and I hope more people go to their bench soon, this really isn´t rocket science...

No vibration means no HHO
But, no HHO because of no shaking, which is your current theory, or,
no shaking because there is no resonance?
[or it is shaking because of partial standing waves  ]

Maybe trying the same experiment in moist air?

yes, or just air, see if it vibrates.
and what you could try is playing with the dutycycle, with water added, I expect that shaking will cease but HHO wont drop - because a partia standing wave becomes a whole or half wave. but am absolutely not sure.

I'm not behind the bench, so for me it is easy talking.

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Hi,

Based upon some formulas/calculators I have found on the Internet a .75" diameter tube that
is 6.0625" long will have a primnary resonant frequency of 4978 Hz.

The mounting point to prevent damping of the resonance will be 1.3125" from the end. 

This calculation assumes the the speed of sound in AIR which is 343 m/s.  The Speed of sound
inwater is 1500 m/s. So air calculations will probably not work with a tube submerged in water.
 
My point here is that with some modifications of the formulas for AIR we should be able to
calculate for resonance in water.
 
Another significant point is that: the mounting point on the tube is critical to
maintaining optimal resonance.
 
References:

//mysite.verizon.net/cllsj/windchimes/home.htm
//home.fuse.net/engineering/Chimes.htm
//hyperphysics.phy-astr.gsu.edu/hbase/Class/p7120lab.html
//hypertextbook.com/facts/2000/NickyDu.shtml

@Steve.
 
FYI, anytime a link is entered into a message when that message also contains bold or colored text,  the entire message will appear blank when posted.   This is why I had to remove the "http :"  in front of these links.
 
Goey
 

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@Goeytex, thanks! Great job, we will note this and start pinging our pipes in air and then try to submerge them and ping again. This is the start we needed, thanks again for your effort.

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Gauss, 
 
You can download these two programs that will be of the utmost help in determining resonance. Both are freeware and both work very well. 
 
You can even run them at the same time.  Visual Analyzer will show the precise resonant frequency and harmonics of a tube via the spectrum analyzer /FFT function as well as with the frequency meter.  You will need a mic  and a soundcard.  Thats it.  Works great.
 
You can find the exact resonant frequency with Visual analyzer.  You can then program that frequency into the Chime program.  Then "ping" the tube with chime and hear it resonate sympathetically.  You can keep VA running a see all that is happening in the spectrum analyzer.  The attached inage file is a snapshot of a one of my crystal wine glasses resonating after being "pinged"
 

http://www.hitsquad.com/smm/programs/VA/
 
http://www.gdp-research.com.au/soft_1.htm
 
 
 

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very good stuff .

The tubes are critical .

This is like a keely music instrument . So the materials dont matter , great to read this .

Perhaps we have to cross the electrical resonance with mechanical oscillation .