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HellstormOP
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I know that I can't get all the effects, especially not the explosive power. But there are 3 challenges:
- Solve this with a semi-solid mass without the usage of an encasement, because otherwise this would just be like an ordinary grenade (idea:
thickeners like silica may form a "crust" while the ball burns during flight, that way you have a self-encasement)
- Make it explode, not necessarily very powerful (idea: when a ball thickened with a thixotropic thickener, with dissolved acetylene under
overpressure, strikes a solid surface, it splashes around and divides into small drops. That way, the acetylene can decompose/burn in the entire mass)
- Make it burn blue during flight (idea: acetone generally burns blue when it moves fast. A small mistake during a previous experiment proved that.
Now I know, that small drops made of acetone, thickened with silica and containing 25% finely ground KNO3 will burn blue when they fall to the ground.
Now I have to add acetylene, but this will take a few days so that I can find an appropriate place to run a gas generator. It may explode, so I have
to seek a well-hidden place)
All the other properties (strong explosive power etc...) are not important, at least not now. I know realizing all that is gonna be hard, but I'm sure
I will manage that.
[Edited on 24-8-2011 by HellstormOP]
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quicksilver
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The chemistry/physics involved in self containment explosions of low explosives can be extremely complex. Speed of deflagration vs air pressure in
generally considered to be the starting point and thus the need for a rate of burn that boarders on detonation becomes pivotal.
Flammable gases (as liquid) at sub-freezing often become examples as well. Gellfication of solvents actually has some research behind it. One example
in gaseous format is adulterated hydrogen, where a smaller unit is more "workable" than larger (Berger, Gentry 1953?).
Commercial adaptation was managed in the USA by Yaley & Co. through a product known as "Gel-Wax". If the firm still exists it's in Calif. and the
methods are most likely patent secret but the patent element may have run it's course by now. I do remember that this firm has found a method of
gelling paraffin and a variety of petroleum distillates. Polar solvents may require more work. However, it may have been done commercially already as
a bushing cleaner.
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HellstormOP
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Well, jellying of that stuff shouldn't be a big deal. I will simply use silica again, and I think I'm gonna be fine with it. Also, the explosion isn't
like that normal explosions from low explosives. Acetylene has completely other properties than, for example, nitrocellulose. And actually, I meant
with self-encasement that a "crust" of silica may be the key to keep the fireball from exploding prematurely.
Fortunately, I found an article from the Encyclopedia Britannica about acetylene at Wikisource. Now, I know a few very important facts about it, which
you practically cannot find anywhere else on the Internet. First fact: Acetylene will not decompose as soon as you compress it above 15 psi. As
acetylene was discovered for lighting applications, it was even stored in liquid form. There is only one problem: Above 15 psi a triggered
decomposition of the acetylene will propagate through the entire container, and may even become a detonation. Second fact: Acetylene is not very shock
sensitive. The only danger are the formed sparks when a flask with acetylene (even in liquid form!) is dropped and ruptures. That sparks may ignite
the acetylene and/or trigger the decomposition.
I suppose that it might work this way: When the fireball is ignited and fired, the semi-solid properties of the flaming mass prevent it from
dispersing too much. Also some kind of crust forms, so that the ball doesn't burst mid-flight. If silica is used as a thickener, the three-dimensional
network of silica crystals prevents a quick decomposition in the ball, just like the porous mass in acetylene flasks. When the ball strikes a solid
surface, it breaks up and splashes around. Now, a decomposition can occur in the entire ball, so that it explodes. Burning acetone is splashed around
in the form of blue rings.
OK, the KNO3 isn't mentioned there, but the function of it is obvious: Provide sufficient oxygen for an even greater blast and a whitish acetylene
flame durnig flight.
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phlogiston
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I totally understand your passion for reproducing an interesting pyrotechnic effect, but I think you should first fully appreciate that the movie or
the description is not based on fact and as such does not 'prove' or 'confirm' anything. The designers most likely thought up that description without
performing any experiment at all and I fear that if you continue to rely on it to this extent you will never reach the goal of reproducing the effect.
It may sound plausible but it is (convincing) fiction written by non-experts.
With that said, I suggest you look at what is called in pyrotechnic circles a 'comet'. Briefly, it is a chunk of pyrotechnic material, held together
by a binder. A common use is to attach them to shells so that they produce an interesting tail effect while ascending. You could prepare one that has
an outer layer that burns away producing the bright white/blue light and a dripping effect. There are numerous composiitons that will give this color.
For it to be that bright, it should be metal-powder based, most likely magnesium powder, given the color. For the dripping effect, I suggest you look
at so-called 'glitter' compositions. They tend to produce droplets of material that drip of the comet and produce delayed flashes or glowing sparks.
Chunks of glitter composition may be incorporated into the white/blue color composition, or you could glue two comets together, each with different
effect.
The fireball upon impact part of the effect will be quite difficult to produce. A layered comet, such that it breaks up when it hits a solid surface
and produces the fireball could be an option. For fireballs, the most common technique is to disperse a flammable powder using a black powder charge.
naphtalene and powdered milk are commonly used. How to make sure that the comet ignites the fireball effect upon impact, however, without resorting to
impact sensitive mixtures?? (which would be -extremely- unwise).
If you -really- want to go for this, be prepared for a lot of failed experiments.
-----
"If a rocket goes up, who cares where it comes down, that's not my concern said Wernher von Braun" - Tom Lehrer |
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HellstormOP
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Well, there are a few problems regarding the reproduction of that effect. When you look at the screenshots of that movie, you see that the effect MUST
include some kind of a liquid or semi-solid, when it should work in reality. Here are a few examples:
Uploaded with ImageShack.us
Uploaded with ImageShack.us
Have fun reproducing that with a comet or a ghostmine...
Of course, acetylene may be the wrong way to go for that white-blue flame, and some kind of metal powder should be actually used.. But recently I've
found out that acetylene will burn with a brilliant white flame when mixed with enough AIR (not pure oxygen!), also it seems to be the best way for
that "mushroom cloud". I think I'm gonna perform a few more experiments (I still didn't perform even one single experiment using a blowgun, also no
experiments where C2H2 and KNO3 were combined!) and then decide to either keep going with the acetylene method, or employing something like a
pyrotechnical composition...
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497
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I'd say your best bet would likely be using flammable gelatinous binder with a high fraction of fine magnesium/aluminum/titanium powder in it. The
"explosion" effect could be achieved by the force of the impact..
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HellstormOP
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UPDATE:
After putting the project on hold for a long time, I decided to continue it again. I didn't conduct any experiments with a moving mass yet. However,
now is the best time to do it anyway, as we have winter in Germany, and you can dissolve more acetylene in acetone when it's cold. And more acetylene
means more b00m xD
I've watched some making-of videos for the movie, and concluded that the designers didn't care about making it realistic at all. They even said there
that sometimes they were happy that they could make things unrealistic - and showed the scene of a catapult tower being blasted by a Night Fury!
However, I still want to try it out, as it would have some interesting potential in real applications (e. g. "assault flamethrowers" that shoot
high-speed balls of a burning semi-solid mass with acetylene dissolved/with other energetic materials mixed in it, that explodes on impact and creates
something like a thermobaric blast, or at least a fireball like from a gasoline bomb, and are relatively easy to build - due to no need for a
complicated grenade auto-loading system, instead you just need a few pipes and hoses and flasks and valves - , and you can vary the size of their
blast) and also actually that concept makes sense due to some interesting properties of acetylene, especially its high energy content!
By the way, I agree that the actual explosion mechanism is based on a fuel-air explosion, and the acetylene just keeps the stuff burning during the
high-speed flight, and serves as a spreading charge on impact, because its solubility suddenly decreases when the liquid spreads out and starts
burning, and also I know that impacts can make gases that are dissolved in liquids bubble out. Also the acetylene decomposes and yields a nice amount
of hydrogen, which is hardly soluble in acetone.
This is also the main reason why I don't want to stick to metal powders as the high-energy component in the mixture, because they can't provide that
additional explosive effect directly on impact (not after all the fuel is spread out and burning), which you can see in the movie , and which also
comes in handy in real scenarios (shockwave effect). The second reason is because I still want to find out if that will work in reality, even if it
was never intended to do so by the movie creators. It's just a too good sounding and unusual concept. The third reason is that I still have about 0.9
kg of calcium carbide left, and no metal powder at all.
I also understood that the potassium nitrate will under no circumstances serve as a thixotropic agent, because I would need it in colloidal fineness.
I only could achieve that by using a high-energy planetary ball mill. I already work on one but it will take lots of time, as I depend on a very
unreliable friend because he has a workshop where I can make the parts. However, it would increase the explosive effect due to the additional oxygen
provided, and would also yield that purplish flame color which is visible at day.
Under no circumstances will I use any kind of encasement for the mass, as it would turn it into an ordinary grenade, and I really want it to be some
kind of jelly/mass which can achieve all that without an encasement, making it possible to dose blasts and to build incredibly simple and effective
flamethrowers. Achieving the effect of an explosive/pyrotechnic charge in an encasement (grenade/ghostmine/comet) without actually using one is the
real challenge here.
By the way, I would like to know how I can make potassium nitrate particles float in thickened acetone/which kind of thickener I need to make them
float. (There should be about 25% KNO3 by weight in the acetone) The first experiments with silica (like Aerosil, about 3-5%) and potassium nitrate
(very thoroughly ground with a powerfull 300 watts coffee grinder) failed, the KNO3 sunk to the bottom even after thorough stirring of the thickened
acetone with the KNO3 in it.
I would also appreciate it very much if somebody told me a few ways to thicken acetone to a semi-solid/thixotropic/shear-thinning mass. It would be
ideal, if the mass stayed quite viscous while not under stress, and turned into a nearby water-like liquid when explosed to shear. As you know in the
thread, I already tried silica and nitrocellulose. Silica seems quite promising, but not ideal. Nitrocellulose is too hard for me to obtain in the
required quantity and quality. Ping-pong balls are unsuitable.
I also need some information about the solubility of acetylene in various flammable organic solvents. I googled it, but found only non-free material
which I would have to pay for.
[Edited on 19-12-2011 by HellstormOP]
[Edited on 19-12-2011 by HellstormOP]
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quicksilver
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IF you can get back issues of the Journal of Explos & Pyrotechnics, there is a great many features of (relativity) safe flame spread effects.
Gases have a real drawback with wind and what is known as the "lake effect", while dual ignition of Al powder is much more controllable for film and
placement.
Another issue is the speed of effect filming in relationship to the effect. What has been deemed a safer alternative is a small ignition charge
encompassed by an float of Al within wet conditions to control flame spread. A great amount of effects material IS available if you hunt for technique
and safety in filming. Often film is used rather than digital for a sequence as the use of film in a more manipulative form of photography.
Gases are subject to a great many unpredictable forces such as wind, timing sequence and temp. Whatever you do: read a great amount of special effects
techniques, prior to experimentation. A little goes a long way. remember you don't want to move air (noise) you want contain effect for the scene
recorded. Rubber containment is subject to freezing, pooling and poor longevity. Kerosene is much more predictable and vapor containment is much more
predictable
Fire hazard demand location and fire extinguisher needs to be well through through. Vapor spread had always been a challenge and the results are much
too fast for hand-held recording devices. Always experiment with tiny examples and progress; studying the recording issues that accomplish you end
result. Search "special effects" quite a bit before you attempt to re-invent the wheel. This is where "flame" inducement rather than imitation
(explosion) is much more value and timing is generally an electric affair. Self and surrounding issues should take president that volume as you can
always scale up by use of special effect recording.
Let safety be your guide more than anything else.
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HellstormOP
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Thanks for your safety tips, but I don't fully grasp what they have to do with my experiment, besides that it's a flame/explosion effect. It won't be
like normal pyrotechnic charges.
Regarding the safety, I will wear a flame-resistant surplus US Army ACU, which is made of "Defender M". This fabric is made of 65% cotton with flame
retardants directly incorporated into the fiber (can't be washed out), 25% kevlar and 10% nylon (no clue why they used that...). This textile is
specifically tuned to withstand short exposures to flame, like fuel-air explosions or electric arcs. It won't rip even when exposed to exploding
plasma (from electricity).
Small amounts and a careful choice of the location are mandatory for me anyway, as some things aren't fully legal in Germany and hence I need to stay
undetected by unwanted observers. This will reduce the hazard to other, unprotected people (standing or walking by) to a minimum. Since it's winter
now, the weather here is very wet and cold. No hazard to ignite natural flammable materials is given.
I can't afford making large amounts anyway, and I also would need too large devices to accelerate them to appropriate speed. The limit will be around
100 ml of flaming mass.
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HellstormOP
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Another question: Can acetylene decompose when it is dissolved in acetone by 10 to 15 percent by weight and exposed to flame and impact at the same
time? If yes, can the local overheating caused by this make potassium nitrate crystals (that are dispersed in acetone and are about 10 microns small)
release their oxygen?
If no, is there an oxidizer that
- is obtainable and affordable by the average non-professional?
- can be mixed with, dissolved, dispersed or emulsified in acetone without separating after that?
- will release its oxygen fairly easily, but will neither react spontaneously with acetone nor with acetylene?
- won't decrease the solubility af acetylene in the acetone?
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HellstormOP
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Nobody has ideas about that?
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Neil
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Your last question is a little bit bonkers with the level of complexity that would be needed to answer it.
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HellstormOP
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Well, in this case leave out the thing with the decomposing acetylene. I guess that will require several experiments anyway.
But what's actually about the oxidizer? I thought the users in this board had a lot of knowledge about various oxidizing agents and were willing to
share it by answering such questions...
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Neil
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Hoho don't try the kicked puppy approach. They are but they also know you could have found that out for your self just as they did, by doing a bit of
reading.
KNO3 is used, dissolved in methanol, to make pink/purple flames. The decomposition point for KNO3 is well above the boiling points of acetone or
methanol so the likelihood of getting it to dump its oxygen while sitting in a pool of cold fuel is low.
You can bind KNO3 powder in molten sugar - something that could not be done if it did not have a high melting point and a relatively high level of
stability.
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HellstormOP
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That's why I supposed that maybe the local overheating of decomposing acetylene may cause temperatures such great that small crystals might release
their oxygen, and get the acetylene boiling and that way the mass exploding as well.
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Neil
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Boiling = cooling.
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HellstormOP
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Well, then the released (atomic?) hydrogen from the decomposition of acetylene might reduce the nitrate directly or combust with any released oxygen
from local overheating? And then release enough heat to vaporize the acetylene to a point where the sheer pressure of forming vapor would blow the
mass apart?
The other option, of course, would be the pure heating effect of decomposing acetylene, which might vaporize acetone, and the released hydrogen. Both
will increase the pressure, and that's the point. Boiling might be cooling, but it builds up pressure which could further trigger acetylene
decomposition, or turn the mass into something like a "gasoline bomb" with additional oxidizer.
Regarding oxidizers, something that releases its oxygen very easily, but not spontaneously, might be useful, as it would make combustion within the
fireball possible and greatly increase pressure and temperature. The explosions in the movie look "semi-energetic" to me, something between a fuel-air
explosion or gasoline bomb and an ordinary explosive. Maybe a kind of semi-solid, very fuel-rich TBX?
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fledarmus
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Wouldn't it be easier just to get the movie company to send you a sputum sample from the dragon? Then you could analyze it and determine exactly what
it was composed of.
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Neil
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They look like CGI to me . unless you are trying to compress and contain the
mixture and then super heat it, the cooling effects of the gas leaving the acetone and the acetone will prevent the nitrate from getting hot enough to
decompose.
Containing such a mixture is asking for some kind of detonation or at least explosive event.
I'm going to go out on a limb and say making a pressurized oxidizer/fuel slushy is harder then you'd expect. I've never lite such a mixture on fire
but I have tested butane slushy fuels and for the most part the evaporating gas cooled the fuel to the point where it would burn more tamely then
gasoline. mixing very fine Zr dust with a very volatile fuel might work but then you are approaching the level of insanity that applies to white
phosphorus solutions.
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HellstormOP
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Of course they are CGI, but if you would see the same picture happening in reality (and I want to reproduce it in reality), it would look like, I
don't know, low-order TBX?
The fact, that escaping gas actually COOLS the mixture, is actually EXCELLENT - as long as the fireball flies. It would actually stabilize the
fireball and prevent the acetylene inside from decomposing. (But don't forget that escaping acetylene would have far less of a cooling effect than
vaporizing butane.)
But as soon as it strikes on a surface, other effects should take place, I think. For example, when you gently hit an open soda bottle, gas starts to
bubble out. Now imagine the same with acetylene, 10% by weight, dissolved in a semi-solid mass where it can't escape that easily, striking a surface
with 250 km/h...
As soon as the acetylene leaves the acetone and forms these bubbles (probably under VERY high pressure), the acetone can't prevent it from decomposing
anymore. Therefore, the bubbles could decompose, producing hydrogen gas under pressure. This could produce even more heat (adiabatic compression).
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vaslop
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Im not suggesting that anyone tries this, but all the properties could be replicated by using a gell of methanol stabalised methyl nitrate, with
nitrocellulose.
This would burn blue and yellow, be inexstinguishable at any wind speed, and (with the help of an impact sensitive detonator) explode on contact.
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HellstormOP
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Hmm, I already thought about using some kind of energetic material which I mix into the acetone. Alcohol nitrates could be worth a try, however I have
concerns that they may trigger an explosion when the ball catches fire, or they also may decrease the solubility of acetylene in acetone. Another
option would be, of course, TATP. It would make the mass burn *VERY* vigorously and additionally shock sensitive.
However, what I need most now are ideas about suitable thickeners and oxidizers. I already described the criteria above. A combination of silica and
very finely ground potassium nitrate might be the right way to go, and I already did a few experiments with it (however, I conducted no experiment
where silica, acetylene, acetone and KNO3 were combined) and noticed that KNO3 doesn't release its oxygen when it's mixed into thickened acetone (with
silica), and set on fire.
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HellstormOP
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UPDATE:
After a few weeks of very slow preparation and a complete redesign of the launcher device today, I'm finally able to proudly announce that the first
real test will come very soon. I will use a mixture of 37.5 g acetone, 12.5 g finely ground KNO3 and 2.5 g silica. It's already prepared and standing
in a not completely airtight container in the freezer compartment of a fridge for more than a week, so I may need to add some acetone to restore the
original proportions.
I will charge this mixture with acetylene using calcium carbide. For that purpose, I will use a makeshift gas generator comprising a small plastic
beaker with the mass, standing in a larger glass jar with an airtight seal (the thread of the jar will be tightly wrapped with a sealing strap), where
the water is and where the calcium carbide chunks will be thrown into. The plastic beaker will be prevented from floating on the water by a steel rod
that forces it to keep some distance to the lid. I will use a total of 10 g calcium carbide pieces, in 4 batches of about 2.5 grams each. That way I
can ensure a safer charging procedure (not such a high pressure rise as with one batch) and also a more thorough saturation of the acetone-based mass
(I will thoroughly stir the mass between the reactions of the batches).
Since acetone dissolves far more acetylene when in cold state (58g/l at 0°C vs. 28g/l at 20°C), I will keep the semi-solid mass in the freezer
compartment before charging it. I also will use a mix of water, ice and salt for the reaction of the carbide, as this will decrease the temperature
and also make the process safer and more efficient (the reaction will be slowed down by the cold and the loss at the begin, where the chunks are
thrown into the jar and the lid is still open, is minimized and since brine can hardly dissolve acetylene (5g/1000l), there will be more of it left
for the mass.). Depending on the temperature outdoors, where I will perform the experiment, I may also use styrofoam around the gas generator as an
insulation.
The launcher itself will be a pretty simple pneumatic cannon, comprising a 400 mm 3/4" steel tube which serves as the barrel, a ball valve, probably
outfitted with a self-made spring mechanism to open it faster, and a plastic bottle which will hold the compressed air for the shot. A steel rod
attached to the muzzle, with a paper towel wrapped around it, which is soaked in kerosene, will (hopefully...) ignite the mass when it's launched.
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HellstormOP
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UPDATE:
I got all the necessary parts and the experiment will be performed either today or tomorrow. I will shoot a portion of the mass that will weigh 5
grams. There will be a pressure of 0.5 bar (g) in the plastic bottle, the calculated muzzle energy willl be 12 joules (friction losses not taken into
account) and the muzzle velocity will be around 90 m/s. The plastic bottle will be connected to the ball
valve using a 1/2" hose and an a 1/2" (hose) to 3/4" (ball valve) adapter. There will be no spring mechanism for fast opening.
I decided to modify the acetylene generator/charger. Now it will comprise a bigger glass jar, filled with brine, which will serve as the gas
generator, and will be connected with a PVC hose (6 mm inner diameter /1.5 mm wall thickness) to a 100 ml glass jar with a very long lid thread and a
very tight lid. All threads, both on the acetylene generator and the launcher device, will be carefully sealed with lots of Teflon sealing strap. I'm
also thinking about using a salt/ice freezing mixture in order to cool down the acetone even further, so that it can hold even more acetylene.
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HellstormOP
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UPDATE:
Unfortunately, I failed to comduct the experiment yesterday due to lack of time. But I think I will be able to do it this week, as I bought everything
I need.
Any ideas/suggestions about the experiment are still very welcome.
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