I was wondering if anyone could tell me what the equation is to figure out the expansion rate? How many Kg's of black powder produces how many cubic meters of gas it turns into. I was wondering because, well I play airsoft and figure out how much I can displace safely with the lowest amount of powder to simulate by sound and displacement of soil emulate that of a morter blast a few hundred yards away. Wanted to test it a bit without anyone around this weekend, but want to get my math right first.


Edit: Oh yeah, not looking to make a bomb, It is going to be in a trough with wax paper keeping it seperate from the dirt on top providing the pressure. The way I figure it should not drive any shrapnel except that all ready in the dirt.

fucking google it :p and just a strip of waxpaper wont cuase the bp to detonate, you need a shell, if you want to avoid shrapnel use cardboard

If you want safe, go for pneumatic

A very rough number: 1kg of blackpowder will get you approximately 350l of gas after cooling.
But then thre's pressure spikes, heating and cooling ...

I've got a bit o' info and a few ideas for ya', but I really need to go get some smokes, and the post will take a while to type, so sit tight - there's a hell of a lot more to it than one would think at first glance.

Hey GanosaLiqueur, I don't remember speaking with you before, so in case you don't know me, here's a bit about my background, it might help explain some of my viewpoints, methods, and so on.

I work with special effects for a living, mostly mechanical and H-T-H weapon replicas, but on occasion also pyrotechnics.
I've been doing firework shows with a couple of other pyro's for a few years, and am a certified Pyrotechnician myself.
I've studied a decent amount of humanitarian demining, and have learned that the truly dangerous shit out there isn't the factory produced landmines, it's the little devils the loonies build themselves, which in turn means I've learned to create a train of ignition out of just about anything, in order to know what to do in case I ever end up in demining, which isn't all that unlikely.
Well, with that out of the way...

I was wondering if anyone could tell me what the equation is to figure out the expansion rate? How many Kg's of black powder produces how many cubic meters of gas it turns into.

I don't have the formula, but as far as I know there's about as many opinions on the subject, as people discussing it.
Most likely, the truth is somewhere in between, but I've heard everything from 1kg/0.27m³ to 1kg/3m³ (personally I'd say is in the high end on average, but of course anything can change if it's made of impure chemicals and so on)...
As it has never really been possible to find out the exact compositions (ratios) of the powders discussed , I can't say anything for sure, and have always resorted to testing to get the effect I needed, before using, I suggest you do the same.

I was wondering because, well I play airsoft and figure out how much I can displace safely with the lowest amount of powder to simulate by sound and displacement of soil emulate that of a morter blast a few hundred yards away. Wanted to test it a bit without anyone around this weekend, but want to get my math right first.

It's always a good idea to do your homework first, especially when working with explosives...
But, there's a couple of things I have to point out, this might be a long one. :D

There's two ways to convincingly simulate a HE detonation, by using LE's.
One only works on camera, and is done by filming a LE explosion in slow motion, and then running the film in standard speed.
This will make the explosion seem like it's happening a lot faster, it's the same trick they're using when making high speed car chases through downtown, or close to the edge of a cliff.

The other is to compile large amounts of LE's (a van jam packed with fireworks with a high Flash Powder content will do), and then setting off the center pieces, with a bit of "luck" you'll get something that resembles a detonation because Flash Powder can "mass deflagrate" (don't know the English word, this is the closest I can get).

Neither method will work for you, but I thought I should point it out anyway.
Think about it, even seen in real-time a detonation appears instant - at first there's nothing, and then all of the sudden there's a big cloud of dust, you don't see that with LE's...
You might not have the time to react if anything happens, but at least you brain registers that something is blowing up.
Besides, even if it was possible, you'd need someone who knew by experience exactly how a mortar shell impact looks and sounds a few hundred yards away, who also knew how to duplicate it by using the slowest burning powder of the ones commonly used, not an easy task...
Oh, and mortar shells comes in several calibers...

But don't sweat it, I won't rip your idea to bits without providing a solution...

You won't get a "real" detonation, but I'm sure that won't mess up your scenario, as long as you get a kickass effect, am I right or am I right?

Now, normally when there's actors involved close to the "explosion", you'd use a "woofer", it's basically just a big fat tank with pressurized air, connected by a short tube to a "mortar" which is then buried flush with the ground.
The mortar is filled with debris to act as dirt and so on, it's common to pour in a load of bark chips, cork, saw dust, chalk, powdered clay, or cement....
When you hit the switch to the solenoid valve, all the air is dumped through the tube and into the mortar, sending everything sky high.
As I don't think you're ready to spend the money and time needed for a project like that, just forget about it, but now know how it's done.

This is how I'd suggest you solve the problem:

1. Get a hold of some electrical igniters, preferably what the professionals use (but stay away from the ones produced in China, their failure rate is up around 15% or so, American, Italian, or German produced ones is what you'll want).
If that's not possible, make some yourself, there's been tons and tons written about them here on BB, look around.

2. Acquire Black Powder, a kilo should do, but better safe than sorry, you'll also want to test, and you can't always count on nailing it the first time.

3. Get a hold of a 4" (welded, and not cast) metal pipe, or thick walled plastic pipe, about 2' long, close off one end, and do a thorough job!

4. A few hundred yards, is a damn long cable run, and with the resistance in the wires and whatnot, you might not be able to fire by wire, this presents a few problems.
Let me know if it's possible to have someone closer by firing it for you, and we'll take it from there.
Better yet, explain the circumstances, even the background behind the explosion as seen from the observer's point of view has something to say.

5. Now, setting off explosions in the ground and making them look good isn't as easy as it sounds.
Depending on what you want the cloud to look like, there's different ways to do it, please describe how you want the explosion/cloud to look.
Lemme give you a few examples, a big half-sphere "puff", a high inverted cone, a mushroom, a center "spike" w/ an out-wards rolling base, do you want a few fragments trailing smoke, and so on.

Can't really go any further before I have something to work with...

Edit: Oh yeah, not looking to make a bomb, It is going to be in a trough with wax paper keeping it seperate from the dirt on top providing the pressure. The way I figure it should not drive any shrapnel except that all ready in the dirt.

If I had the suspicion you were making a bomb I wouldn't be helping you, besides, this isn't how to build a bomb, this is how you make a special effect. ;)

Well, what I was looking for would be primarily a noise maker, for disorientation, and secondly would move some dirt to emulate a real explosion. I was picturing either a half spherical or a mushroom effect.

I wasn't going to be wiring the whole distance, I all ready have a remote setup from hobby RC parts, the range is a good 50m, and was going to have the receiver just flip a relay to set off the igniter (I was reading somewhere that they should do for igniting black powder, plus they are easy to come by and rather cheap, and I have no idea where to find squibs lol.)

Although my math is probably off I was figuring that 1m3 of hot expanding gas should move aprox the same amount of dirt, and whatever the yield would be for black powder take by 2/3 and that would be how much dirt I would use.

Note, when I say dirt I mean clay based, which is what we have here where we play out of city limits. I was going to toy around with the use of something of a finer grade if the dirt didn't work out well, like some type of flour.

Now for the trough itself I was thinking some sewer ABS with the charge packed in the bottom and fill with dirt and submerge so you couldn't see it. Thats another reason I was wanting to know the expansion rate to see if the ABS I have can take it without blowing itself to smithereens. This is all a few weeks away, just designing and making sure my numbers are in line, but I was going to start testing here in a week or so where there are only a couple people around (and not the 20-30 as when we are playing) just in case.

Well, what I was looking for would be primarily a noise maker, for disorientation, and secondly would move some dirt to emulate a real explosion. I was picturing either a half spherical or a mushroom effect.

You might want to use several parallel wired charges then, lifting charges aren't good for noise, and it's easier to get a mushroom effect if the pressure isn't blocked/deflected by something it has to push out.

Here's what I'd do:

I'd use a BP salute in a 50 or 60 liter thick-walled plastic barrel as the sound effect.
You'll have to test to see how thick the salute casing will have to be to get the desired result, if you're using Flash Powder you'll hardly need any casing at all, but the sound will be a sharper "crack" and not a deep "boom".
Either way, the plastic barrel will work as an amplifier and make the sound deeper.
I'd start out with a 25g BP salute.

For the mushroom effect, I'd use 4" plastic tube (commonly used as drain pipe), cut off a piece twice as long as the diameter, and hotglue it to a piece of plywood to close off the bottom.
Then drill a hole a few inches above the bottom and install the igniter (remember to short out the leads as the first thing you do).
Next I'd fill it about 1/3 with BP, and push in a cardboard disc to seal the powder in - not tightly, but just to make sure it won't spill out or absorb too much moisture, as an extra precaution you can fill it in a plastic bag before loading it in the tube, as long as you make sure your igniter will burn through.

For the dust cloud I'd use a metal or plastic funnel, a traffic cone should do.
Turn it upside down, drill a hole to wire through (if you wire down from the top there's a chance the wires will start flying when it goes off), and install the charge.

This part is important!

When you attempt to make something like this, you do not want even the smallest pebble airborne, and another thing is that you should always work with the smallest charge possible for the job!
Which is why using regular dirt you just dug up is a bad idea.
You don't know what it contains of potential projectiles, and it's heavy to lift if wet.
Using cork granulate allows you to use a smaller charge, and it doesn't fly very far.

I wasn't going to be wiring the whole distance, I all ready have a remote setup from hobby RC parts, the range is a good 50m, and was going to have the receiver just flip a relay to set off the igniter (I was reading somewhere that they should do for igniting black powder, plus they are easy to come by and rather cheap, and I have no idea where to find squibs lol.)

I'm having a bit o' trouble understanding what you mean.
I understand that you'll be using a RC transmitter/receiver--->relay--->igniter solution, I'm with you that far, it's not a bad idea as long as you just make sure no one else is on the same frequency, and it's the same solution I'm used to when I'm doing firework shows (only with pro gear:D).
But I don't get the "I was reading somewhere that they should do for igniting black powder" part...
The igniter doesn't care how it's ignited, as long as it gets enough juice to ignite, and that's supplied by the battery - if the igniter goes off, the BP doesn't care how the igniter was set off, so the relay has nothing to do with how hot the igniter gets, if it snaps it snaps...

Although my math is probably off I was figuring that 1m3 of hot expanding gas should move aprox the same amount of dirt, and whatever the yield would be for black powder take by 2/3 and that would be how much dirt I would use.

That would depend on the pressure versus the weight, and I can guarantee you 1m³ of BP gas won't move 1m³ of dirt.
When gas expansion is measured it's at atmospheric pressure, the instant you bury the charge and set it off it's under pressure from the get-go.
As the gas expands the pressure drops, this happens all by itself, now, if you furthermore damp it...
See where I'm going?
Naturally it will move some dirt, but if you buried an amount of BP that'd produce 1m³ of gas at the bottom of a 1x1x1m box filled with dirt and set it off, you wouldn't even get a "blop", it's not at all impossible to "suffocate" an explosion...

Note, when I say dirt I mean clay based, which is what we have here where we play out of city limits.

You can use locally available soil, as long as as you just remember to run it through a fine mesh first to get rid of rocks n' shit.

I was going to toy around with the use of something of a finer grade if the dirt didn't work out well, like some type of flour.

Don't use anything containing cellulose, or you might end up with a bigass fireball instead of what you wanted.

Now for the trough itself I was thinking some sewer ABS with the charge packed in the bottom and fill with dirt and submerge so you couldn't see it. Thats another reason I was wanting to know the expansion rate to see if the ABS I have can take it without blowing itself to smithereens. This is all a few weeks away, just designing and making sure my numbers are in line, but I was going to start testing here in a week or so where there are only a couple people around (and not the 20-30 as when we are playing) just in case.

Let me put it like this...

I've fired 2.5" aerial shells out of these, notice the wall thickness:
http://i127.photobucket.com/albums/p124/Arachnoholic/Pyro/25in.jpg

3" out of these, remember the wall thickness:
http://i127.photobucket.com/albums/p124/Arachnoholic/Pyro/3in.jpg

4" out of these, again, look at the wall thickness:
http://i127.photobucket.com/albums/p124/Arachnoholic/Pyro/4in.jpg

And I've fired 6" aerial shells out these -wall thickness:
http://i127.photobucket.com/albums/p124/Arachnoholic/Pyro/6in.jpg
I don't think you need to worry about fragmentation...
Besides, if Lady Luck decides to fuck you over, everything will happen underground, and plastic shards don't fly too well under those conditions.