497
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Simple measurement of blast overpressures
I'm looking for a simple, and hopefully reasonably accurate way to measure explosive overpressure (and/or impulse) at a distance.
The way the pros do it involves lots of fancy electronics, piezoelectric transducers, accelerometers, various outrageously expensive computer
programs, etc. I'm hoping to avoid as much of that as I can, while still retaining as much accuracy as possible. Obviously there are many ways to
qualitatively measure blast overpressure/impulse, but I'm hoping for something more quantifiable.
I'm not looking to try to measure very high pressures. I'd say in the range of 10 to 1 psi is a reasonable area.
I'm currently most interested in comparing conventional high explosives with thermobaric and fuel/air systems.
My first idea was some kind of spring loaded mechanism that can somehow record its maximum deflection from the shock wave. It would also be possible
to film the deflection at 1200 fps (which I fortunately have access to). Unfortunately that camera may be better used by actually filming the
detonation... and I don't have two  . Either way, it seems that it would be hard to
achieve decent accuracy with such a rig... I'll have to think about it more..
Any ideas? There must be unconventional ways of doing it that could work?
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hinz
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Perhaps to use a loudspeaker and a (digital) storage oszilloscope and measure the voltage inducted into the loudspeaaker coil. The faster the coil
moves, the higher the pressure.
With a proper DSO, this should be easy to measure, the problem could be the linearity and the inertia of the membrane.
An option could be to use a piezoelectric speaker, but they are very small ans its probably difficult to measure a proper signal from them without
amplification.
[Edited on 21-4-2009 by hinz]
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vulture
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USP 6925887
http://dx.doi.org/DOI:10.1063/1.1683886
A factor to consider is that HE's will have a high peak blast pressure which wears of rapidly, opposed to FAE's which exhibit longer overpressure
duration. You might thus want to consider adding pressure vs time measurement.
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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grndpndr
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More modern methods would surely require some pricey electronics, pressure transducers and others I dont understand.
I would look to the past when std pendulums were used with a known quantity of HE then compared to the same amount of the unknown.It would seem a
combination of the old technology with your camera and perhaps a timing device if an appropriate one could be found inexpensively
all caught by the camera if possible,det,pendulum.timer.Just a thought
but perhaps possible with research into the old methods/math and some fabrication?Almost a hobby in itself.
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497
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Hmm thanks vulture. That patent was interesting, but even more useful was that it led me to the below patent that cited it.
USP 7162919.
Looks ideal, totally simple and no electronics involved. What do you think?
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grndpndr
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USP 7162919 seems essentially a ballistic pendulum with a spring loaded plate substituting for the simple pendulum wieght.Personally Id rather
fabricate a ballistic pendulum from wood or steel assuming you have a welder and cutting tools besides the math needed is already there and
waiting.And more important as far as Im concerned Is availability/simplicity math involved as well as fabricating the pendulum from steel or
wood.Some imagination and the pendulum will indicate its maximum deflection via a fixed marker on the pendulum and surface for the marker to
inscribe.Simplicity of fabrication ,equations already done with the addition of an appropriate timer an equation for overpressue should be able to be
found.
After all it would just be unused technology a few searchs should reveal.A 5 minute google search revealed the formula for velocity @ the pendulum,
including pendulum wieght,movement etc.Charts/formula must still exist for
use of the ballistic pendulum as it must have been in use
until the early 20th century @ minimum for both projectiles/explosives revealing velocity and energy of both.
The other patent mentioned seems far from simple..." a pressure sensor located inside the tube...also includes a data processor and computer
connected by wires to the accelerometer... "The Title was simple methods wasnt it?
Regards,
Don't mind me Im still searching for the formula for the layout for a simple cone.
[Edited on 23-4-2009 by grndpndr]
[Edited on 23-4-2009 by grndpndr]
[Edited on 23-4-2009 by grndpndr]
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497
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Yes a pendulum should work nicely, except it would have to be filmed by the high frame rate camera, which will surely be busy filming the actual
detonation...
The above mentioned patent contains an equation:
Minimum Overpressure to Tip the Target = Target Weight * Target Base Width / Target Height / Target Frontal Area
This assumes the target pivots perfectly on its rear corner, and it has a central center of gravity. It seems strangely sensitive though.. For example
a standard size brick weighing 5 pounds (standing on its smallest face, with largest face toward the blast) is calculated to tip over from a mere
0.044 psi... which is really low.. So am I doing something wrong here, or is this equation BS? Or it is actually an accurate result and I'm just
underestimating the power of shockwaves?
It seems that 0.044 psi would be a reasonable number if the pressure was constant, but with a quick jolt like an explosive shockwave, how does it
account for the force required to accelerate the target (brick) in addition to that needed to surpass gravity? I don't know, I'm not very up on my
physics..
I'm hoping to figure out something to rig up soon enough for a trial run this weekend..
[Edited on 23-4-2009 by 497]
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grndpndr
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My dementia ravaged brain cells without a calculator within reach and guessing a std brick to be 8-9 long x3-4in wide?
That would be looking at approximately1.056 lbs-1.584 lbs
over what period of time.Sounds plausible, but if correct is that the max overpressure your safisfied with learning .044psi?I suppose you could
fabricate a universal stand for using various dimension/wieght for different PSI but that would require multiple detonations !Again with the pendulum
though I havent yet found the correct dimensions, pendulum wieght or mathematical formula to find velocity/psi im sure there not uber difficult to
find and would require only 2-3 detonations, baseline and actual device to be tested,another advantage is you dont have to focus the camera on the
pendulum a simple marker can be fixed to the pendulum to graph its movement saving your camera for detonation and ideally an adequate timing device in
the frame.Calculate time /distance =velocity?PSI shouldnt be hard to find after that particularly with pendulum data. Of course my medications havent
yet
kicked in so its possible this is all BS.
Would you mind my asking what design is your theoretical device? proposed wieght?Very anxious to hear of the results.
And if you would kindly share the design and calculations decided on for finding the theoretical blast wave PSI? regards
[Edited on 23-4-2009 by grndpndr] Multiple edits due to ravaged though processes.
[Edited on 23-4-2009 by grndpndr]
[Edited on 23-4-2009 by grndpndr]
[Edited on 23-4-2009 by grndpndr]
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Bikemaster
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I just find an other way to mesure over pressure, it is very easy to make and price less.
It consist of a alluminium foil stock between to metal of wood sheet. The wood/metal sheet are pre holeded of different size hole and the alluminium
foil is put between there.
If you have see the mythbuster episode of (blowing up a shark with some flare)(finish with a lot of high explosive), it is the same principe of the thing that they put in the water to know if the
hero will be alive or not.
The only difference, is that we have différence size of hole to know more exactly what is the pressure.
some literature on it: page 14
http://dspace.dsto.defence.gov.au/dspace/bitstream/1947/3630...
The only thing that i am not sure is the way to calibrate the gauge. I think of calibrate it with a compresser but i am not really sure if it can be
compare to an explosion.... My idea is to make a pressure box and close one end with one of the hole of the gauge and slowly put more pressure. went
the hole blow, it is the pressure that it will blow if the explosion reach this point. (bigger hole, more easy to blow and smaller hole harder to
blow).
the only probleme with this way of mesurement is the proble of the srapenal.... because if some piece of the explosion go in somme hole, it will
change the result...
i think that the best way to know if it work is to try it
i find somethink else...
http://www.mne.psu.edu/psgdl/HargatherAviationMtg2006.pdf
it is call (shock hole)
[Edited on 23-4-2009 by Bikemaster]
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497
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Yes I like the diaphragm method!
I'm thinking it would be easiest to simply cover the opening of some kind of metal can tightly with Al foil. Then attach it securely facing toward the
detonation. A series of them at different distances should give an accurate reading, at least for comparative purposes. The exact overpressure
required to burst it can always be figured out after the fact.
Beautiful. Something like this is exactly what I was hoping for.
The diaphragm type targets should be well suited for measuring the maximum overpressure, since they have so little inertia. I'd still like to figure
out something that will determine the actual *impulse*, which is also an important factor for unconventional explosives.
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grndpndr
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I still lean to the pendulum method, well used and understood with a myriad of supporting math . Plus instructions as to exact dimensions and
wieght of the pendulum.The ballistic pendulums made from simple cardboard boxes of a given diameter/wieght and dimensions along with the mathematical
equations needed.The same could just as easily be made for a det/vel-energy pendulum with the same support already easy to find and fabricate from at
most wood components with the pendulum a box so the exact wieght needed could be loaded.
Theres no doubt the diaghragm Idea isnt elegant in its simplicity I only question will the tensile strenghth of a sq in of Al foil be easily found and
its varying degrees of failure.What will constitute failure a tear?Puncture?, a complete puncture of the entire sq in?Are mathematical equations
available to plagarize LOL like the ancient pendulum method? the simplicity of a box type pendulum made of wood also means you could make a good # so
many shots wouldnt be required OT the important Enhanced Blast experiments and hopefully 1 baseline HE only test shot.
In any event as the experiment progresses a progress report complete with success and failures as well as( if youll share) not just PSI overpressure
reports but actual construction and math equations youve used would be of great interest at least to myself as well as Id bet many others.The Idea of
the Enhanced blast warhead whatever form it may take is becoming perhaps the new frontier for explosives and HE patents.What with increased precision
bombing and minimal collateral damage it seems the EBX warhead is one of the waves of the future. respectfully, grndpndr.
And if a brief description of the EBX Warhead youve devised wouldnt reveal any info youd rather secret, that would be greatly appreciated as well.
Regards.
[Edited on 28-4-2009 by grndpndr]
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497
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It's true the pendulum setup would be much easier to calculate..
It doesn't really matter either way, the blast I was hoping to measure is over and done with. It ended up that we didn't have time to set up any sort
of overpressure measurement at all.. Very unfortunate. The only measure I have is that I felt a decent thump on my chest from at least 600-800 feet
away (estimate). And another person who was probably 100 feet closer to it than me said his chest was in a little pain for a while afterword.
Here it is. http://www.youtube.com/watch?v=h7YJxsKD5jw&feature=chann... Enjoy 
I was surprised at how little was left, and how far it threw the pieces. A few pieces of mangled sheet metal were landing at least 1000 feet from it,
although I suspect most fell within about 250 feet.. No idea where the engine went, must be off in the woods somewhere.
The charge consisted four seperate containers of NH4NO3/(NH4)2SO4 fertilizer, ~15% -300mesh Al, and a touch of methanol/nitromethane RC fuel. Boosted
with cast ETN, connected with ETN detcord.
One charge, consisting of roughly 50kg of the above mixture was placed directly on top of the engine. A second smaller one (maybe 30kg) was placed
where the drivers feet would go. A third one (also about 30kg) was set one the right rear seat. A fourth one of similar size was in the trunk.
[Edited on 28-4-2009 by 497]
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grndpndr
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I hope any onlookers had overhead cover as well as LOS cover cause there used to be a car there! I had thought maybe measuring a few Kg device not
shredding cast Iron engine blocks. The block may be in the frozen ground as shrapnel but I doubt even its in one piece somewhere.WOW
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Bikemaster
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 Dude
this is huge!!!! How can you find that much ammonium nitrate.
Are you a professionnal of an amature pyrothecnitian
With all this unlimited ressourses, it should be easy to calculate the
overpressure because more it has explosive, more it is easy to get precise result.
For the next explosion, just go read about overpressure possible hurt. It can happen bad thing if you receive a lot of overpressure...an other thing,
chech the humidity before make those test because, humidity help to make travel the shockwave true you.
But...wath a hell of a bang
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NameWithheld
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Quote: Originally posted by Bikemaster  | How can you find that much ammonium nitrate. Are you a professionnal of an amature pyrothecnitian
With all this unlimited ressourses, it should be easy to calculate the
overpressure because more it has explosive, more it is easy to get precise result. |
I got the ammonium nitrate as fertilizer, its roughly $1/kg. Pretty easy to buy a few hundred kilos for not that much
I'm an amateur, I prefer the term hobbyist. Sounds more harmless ^_^
While there is some logic to large charges being easy to measure, the fact that they're large charges make them difficult to repeat. There's quite a
lot that goes into a blast over 50kg, so I can't just go blow off a hundred pounds every weekend for overpressure readings
And I'm leaving for my job all summer at the beginning of May, so I won't be able to do anything at all until fall, unfortunately.
Whats the worst that could happen...
I'm amused that my rank is "harmless" of all things |
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hissingnoise
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Yeah, utterly excessive describes it!
Was it a joint effort with you guys?
IMO, sulphate-free AN (available as C.A.N fertiliser) would have given an even more powerful blast.
You could gather up all the fragments, weld them together after a fashion and put the "finished" vehicle in an art exhibition with the title
"Afghanistan".
Destiny awaits. . .
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NameWithheld
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All of our projects are something of a joint effort to one degree or another
"IMO, sulphate-free AN (available as C.A.N fertiliser) would have given an even more powerful blast."
Sulfate free would be great, I'm sure it would increase power. Feel like finding a supplier in Alaska that anyone can purchase from, over the
counter, with cash? ^_^ I've looked pretty extensively.
"You could gather up all the fragments, weld them together after a fashion and put the "finished" vehicle in an art exhibition with the title
"Afghanistan"."
No offense, I seriously hope that's a joke
1. It's spread out over a massive area, say 300m in all directions, some pieces found even farther than that.
2. Most of the pieces are tiny, many of them only a couple cm across.
3. The whole idea is to stay AWAY from the scene as much as possible- this would require literally hundreds of hours of scouring the woods right at
the blast site.
Admittedly, it would be a pretty epic art display... sadly I would rather not display it for security reasons even if I COULD do it...
Whats the worst that could happen...
I'm amused that my rank is "harmless" of all things |
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hissingnoise
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Great Art requires great sacrifice. . .
No, it wasn't a serious suggestion---and not funny enough to be a joke.
But think of the fame and the notoriety!
[Edited on 29-4-2009 by hissingnoise]
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grndpndr
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Im sure No disrespect was intended but perhaps infamy might be a better word particularly to those who served there and worse lost fiends.
On a somewhat lighter note scouring the blast site from a LEO perspective might appear suspicious.Just say your a UFO researcher heard there may have
been an impact and are searching for evidence of ET.LOL I see stranger as serious?TV programs.actually in my limited experience unless you know a
farmer (very few out here,or a private HR miner) chances of aquiring any kind of AN is difficult CAN included.In fact
nitrate of soda fert. is becoming difficult to buy!I dislike the e-records of online/mail order purchases if just for the privacy principle.Finally I
admit to not really watching closely but it appeared to be a simultaneous det.Do you credit this to the homemade det cord and det(Iassume Imp det) and
again I assume electric command det?Or is it possibly a sympathetic det?
[Edited on 1-5-2009 by grndpndr]
[Edited on 1-5-2009 by grndpndr]
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franklyn
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Actual measure of resultant blast is only indicated when the amount of explosive
is unknown. The distance moved horizontally by a piece of paper fluttering to the
ground will indicate this with reasonable certainty. Knowing the amount of explosive
and it's heat value , the pressure it will generate at a given distance is another
straight calculation.
Covered here for an arbitrary value of " 1 " P.S.I. overpressure.
http://www.sciencemadness.org/talk/viewthread.php?tid=11195#...
Hinz is right , but a microphone is the proper transducer to obtain sound magnitude
in decibels and calculate the corresponding peak pressure at some chosen distance
from the explosive since the force decreases at a known rate with distance. For audio
frequency capture a computer's sound card will serve as an improvised digital storage
oscilloscope. http://www.geocities.com/lptscope
http://www.zeitnitz.de/Christian/index.php?sel=scope_en
A measure of the pressure impulse duration might be made
with an aircraft air speed indicator pitot tube , maybe.
A digital storage oscilloscope is necessary for recording and measurement.
While Computer USB devices such as this _
http://www.elandigitalsystems.com/measurement/usbscope50.php
may seem tempting at the price they are much inferior to 15 year old scopes of
similar value sold on EBAY that have 10 times as much performance, but of course
these are full size bench instruments, auctioned as surplus or from facility closure
without warrantee or guarantee beyond turning on.
- Tektronix 2232 - 100 Mhz analog amplifiers with single shot digital storage capture
up to 10 Mhz - sold for $ 5,500 new , available now for $ 119
- Hewlett Packard 54111D - All digital 500 MHz bandwidth , single shot storage capture
up to 250 MHz sold for $ 28,000 new , I just got mine for $ 199
__
This is cute allowing you to get a sense for scope settings
and operations on a Hameg flash simulation
http://www.virtual-oscilloscope.com/simulation.html#
more orientation
http://www.doctronics.co.uk/scope.htm
a useful primer
https://www.cs.tcd.ie/courses/baict/bac/jf/labs/scope/oscill...
the last word - Tektronix
www.tek.com/Measurement/App_Notes/XYZs/03W_8605_2.pdf
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franklyn
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Self contained executable
Found it, it's old from Windows 95 era, but it works
2 channel audio band oscilloscope.
http://www.tech-systems-labs.com/osc251.zip
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