Black powder profiling device

[mike_au]

I tried last week to compare a few different batches of BP by firing dummy loads from a mortar but it was difficult to draw conclusions due to the large number of variables (Did I compact that as hard last time? Did the wind just pick up? etc). So I have decided to try to make a device to compare powders properly.

 

Has anyone seen, heard of or made something similar? My idea so far is a small chamber with a microphone stuck to the side and a vent hole.

 

You ignite 1g of powder inside the chamber, the microphone records the changes in pressure and the vent stops it exploding.

 

I have just done a couple of trial runs with a mic just sitting next to a pile of BP and my first impression is that it will work, but I just don't know enough about audio to really understand the results.

 

The top image is a relatively fast polverone, the bottom is the same powder but straight out of the mill. As you can see the top one takes less time and has a larger amplitude.

 

Why does the wave go on both sides of zero? I would have thought that the burning powder would create an increase in pressure and then it would go back to zero, but it appears to go below zero. Is this to do with the sound wave creating a rarefaction after the compression? is it a quirk of the microphone? or something else entirely?

 

Also, as you can see the top one appears to be clipping, can anyone suggest some software that will let me capture at more than 32bit? or should I just reduce the sensitivity?

[TheSidewinder]

The graph going below zero relates to the physics of any blast wave. I don't pretend to know the exact specifics but the barometric pressure increases, then there is a "rebound" after the initial propagation wave, that is lower pressure than ambient.

 

As to your idea, make sure that the "vent" you mention is adequately large. It will be a mathematical constant in your tests, no matter what size it is. (And the bigger the safer.)

[DeepOvertone]

Are you using a dynamic mic or a condenser? How far from the powder were you placing the mic? If you were close, then then wav on the other side of zero may be the air colapsing after the expansion. But since black powder contains its own oxygen, it isn't like a fuel air bomb that creates a huge vacuum after ignition. Either way, I'm sure this will take some very expensive and precise equipment that I bet you wont want to purchase. I saw a mythbusters where they used a specialized pressure wave sensor to measure this sort of thing. It was the "shooting fish in a barrel" myth. But If you can figure it out with a mic then more power to you. But I think a mechanical solution would be better suited to testing relative power of black powder. For example, a tube and piston. The tube would be 24 or so inches long, with a piston that is sealed well. There would be a rod connected to the piston that would indicate on a scale how high the bp fired the rod. Of course I would put a spring for resistance so the piston wouldnt fire out into the ceiling and I would put a resettable ratchet on the indicator so that when the piston fired and got pulled back into the tube by the spring the highest point that the piston reached would still be indicated by the indicator. Anyone with a machineshop want to make one for me? I know this is the electronics section so here is an electronic idea. Build a channel that has optical sensors on it like a chronograph. Pour a known ammount of bp down the channel and have the device calculate the time from one sensor to the other over the known distance and output the results into fps or something.

[leosedf]

Why don't you add some pressure and temperature sensors too?

[mike_au]

My first idea was actually a mechanical device but I want more than just how much gas it produces, I want to know how quickly as well. E.g. A powder that produces a lot of gas really quickly would be better for lift vs one that slowly builds up (which might be better for a soft break) even if they both produce the same amount of gas in the end. So I really want a graph output which is a little hard to do mechanically at this speed.

 

It is a condenser mic, and it was about 1cm away from the powder.

 

Proper pressure sensors are extremely expensive (I talked to one of my mates who is an instrument tech about this before I started) but fortunately I'm not interested in the absolute pressure generated (which is what the fancy machines would measure) I only care about relative pressure which I think a mic should be able to capture well enough.

 

Optical sensors might work, but would be more involved and still only gives me a single number rather than a graph. Also I would have to dig out my PIC gear and build a circuit which is a lot of work.

 

leosedf: The mic is essentially my pressure sensor, temperature is probably something I should take into account but for the moment I will be trying to get something that I can use to compare two powders against each other on the same day so temperature variations should be negligible. In the long run if you wanted to start a database to compare a new powder against then I imagine things like temperature, pressure and maybe even humidity would need to be recorded. I'll start with little steps first I think

[DeepOvertone]

allright... I didnt think about the speed at which it produces the gas vs. how much it ultimately produces. Perhaps an accelerometer could be used somehow to measure this?

[Boomer]

I once built a simple 'cannon' from a bulk steel block. A barrel was drilled and a small fuse hole at right angle. It was extremely sturdy and basically explosion proof. All I had to do was load a certain weight of composition and a ball bearing as a bullet, and shoot into a stack of witness 'plates' (read cardboard or paper, don't make it so powerful you need stacks of plywood - this could be against gun laws).

 

Green meal went through 3 carboard layers, commercial BP through maybe 8, whistle through 15 and armstrongs mix through 23. (Not to scale, all numbers from memory ca. 1982).

 

With given dimensions and precise drill bits, this could even become a standard among us. Certainly more realistic for lift testing than stopwatching a 3 foot line burn in the open. There is a thread about this somewhere (rec.pyro?), basically the flame cuts strings that start + stop the watch. About what my VoD-meter did with ionisation probes, only in 10ms not 200ns resolution. Only measures speed and not gas output though.

[Arthur]

With shock waves it can no longer be assumed that pressure and density

remain constant. Indeed, at the peak of a strong shock wave the pressure

can be many thousands of atmospheres and the density appreciably

increased. Under such conditions the velocity no longer is that of a sound

wave. In practice, as pressure or density is increased, the compressibility

decreases so that the velocity of propagation of the disturbance increases.

If we can imagine an intense sinusoidal half-wave generated in a medium,

then the velocity of propagation of the peak of the wave where the material

is at high density would be greater than the velocity of the front of the wave

where the material is almost at its original condition. Therefore, the peak

would overtake the front and the shape of the wave would alter as shown in

Fig. 2.1, until ultimately the wave form becomes a sudden and

discontinuous jump to a high pressure followed by a gradual fall. This is

the typical and inevitable profile of a shock wave in an inert medium. As

transmission continues, however, losses gradually reduce the peak

pressure until ultimately the shock wave degenerates into an ordinary

sound wave.A A

FIG. 2.1. Development of a shock wave.

Shock waves,

 

HIGH EXPLOSIVES

AND PROPELLANTS

S. FORDHAM

Formerly of Nobel's Explosive Co. Ltd.

[Swede]

Mike, I am working on a rig that will do exactly what you are talking about. What it is... a .50 caliber muzzleloading "gun" that is very stout. On the breech are two sensors, and the third sensor will simply measure the velocity of the ball. This latter is a relatively cheap device that hunters and shooters use, and is a fine indication of kinetic energy, but doesn't address the pressure curve.

 

To measure pressure, I am going to engineer a 1/4" piston, with a tip shaped somewhat like a blunt chisel. The tip will bear on plumbing solder wire. This solder wire is very uniform, and the deformation of the solder (compared to commercial 2F BP, for example) will show what sort of pressures are being generated.

 

The third instrument is a piezo sensor that measures the flex of the breech metal. Between all three of these, I should be able to get a good idea of the relative strengths of powder. I am very interested in alternate propellants, everything from KNO3/sucrose, to perchlorate-based mixtures like "Pyrodex".

 

I hope this has perhaps given you some ideas. You could always to a pendulum rig, with an ink pen to measure the displacement of the heavy pendulum. That is a traditional way to do it, and accurate to boot.

[mike_au]

I've put this project to one side at the moment to work on a wireless firing box (I tend to have a short attention span when there are a lot of things I want to work on).

 

I would be very interested to see the finished product though Swede, it sounds quite neat.

 

Once I have my firing box and a new mortar rack sorted out I will have to come back to this and have a fiddle around. It sounds like my simple microphone idea might be a little flawed.

[FrankRizzo]

I've always been a fan of "pyro baseball" to give info on the relative strengths of different preparations of BP. It consists of a simple 3" mortar (15" length), a baseball, video camera (or stopwatch), and a consistent weight of BP for every test. The longer the ball stays in the air, the better the powder.

[DontBlink]

I also think shooting stuff is best, and most fun.

 

But..As the document indicated more power equals a more rapid compression

of the gas being compressed (to maximum compression).

And a slower descent to normal youngs mode (compression of the gas "air").

 

So.. you can use the sound alone, and detect.

A: how fast did it reach maximum height.

B: How long did it stay there.

 

There is a harder part, and that`s to calculate the air pressure and the remainder.

But you should be alright if you can find log 10, and the akoustics formula.

 

(You could use a piezo element its nice and cheap, stirdy, and is ment to pick up resonance`s

from the surface it`s attached to.

It`s basicly a two plates touching eachohter.

In sound 24bits audio reaches about the maximum capability,

(there are higher bitrates but not more accurate)).

 

Maybe.. you could use like a fan or something and see how much energy it produced, yust spinning around.

This might reach a higher terminal velocity.

 

hope this helps.

 

Greets Luk.

[DontBlink]

Digging graves here.. my bad

[hst45]

Digging graves here.. my bad

Not a problem. Any time someone adds information to the discourse I appreciate his or her effort. This isn't current events we're dealing with here, if it was we wouldn't need information from Francis Bacon or Alfred Nobel. Any thought that "advances the ball" is appreciated. Sometime ideas languish in inactivity, only to be rejuvenated by another mind. There is no expiration date on creativity.

[Aimlesspayload]

someone used a similar method. http://www.vk2zay.net/article/74 Edited January 19, 2009 by Aimlesspayload