The other day I was browsing around in Home Depot looking for something or other and bumped into a shelf full of framing nailers, power tools used to nail 2x4's and the like together. They come in various types and grades, but one caught my eye -- a cordless model from Paslode. It drew my interest because I know from experience about driving good-sized nails all the way into studs and joists. Doing it over and over is a lot of work. Since I could see the nailer didn't have a battery big enough to supply the needed energy, I wondered how it could work. A little in-store investigation quickly turned up the answer: it runs on fuel delivered from something the manufacturer calls a "Paslode fuel cell." This small metal cylinder holds enough energy to drive 1000+ nails! The boxes for replacement fuel cells feature enticing warnings like, "DANGER extremely flammable" and "compressed flammable liquefied gas." Energy density. How interesting.
Having built a number of spud guns and their cousins over the years, I know that the biggest trick to making them work reliably is letting just the right about of "compressed flammable liquefied gas" into the combustion chamber to get an explosive mixture of fuel and air -- not too rich, not too lean. Clearly, the nailer does something like this over and over as it drive nails, so it must have a mechanism in it that releases just the right amount of fuel for each nail. Is that mechanism in the tool or in the fuel cell? If it's in the fuel cell it ought to be really easy to use one in something spud-gun-like that goes "Bang!" over and over without having to measure the fuel.
What the heck. I bought one and took it home.
A bit of poking around on the intertubes turned up quite a bit of interesting information. Yes, indeed, each Paslode fuel cell contains a special valve that meters out the exact same amount of fuel each time its valve stem is depressed, as helpfully explained in US Patent 7,392,922. Not only that, but the design permits fuel dispensing in any orientation. This works because the fuel cell is really two containers, one inside the other. It's a bag inside a metal shell. The flexible toothpaste-tube-like bag contains the liquefied gas fuel. Its open end is connected to the valve. The space between the shell and the fuel bag contains a propellant, another compressed liquefied gas. As the propellant tries to expand, it squeezes hard on the fuel bag, hard enough that the fuel inside remains in liquid form and pushes against the valve. When the valve stem is depressed, it opens a tiny passage from the fuel bag to a chamber inside the valve body. The propellant squeezes the liquid fuel through the passage into the chamber, filling it up. Then, when the valve stem is released, the passage between the fuel bag and the chamber is closed and a different passage, leading to the opening on the valve stem, is opened. The fuel in the chamber then squirts out the end of the valve stem as it vaporizes. Presto! Out comes a measured amount of fuel in the form of an "extremely flammable" gas.
Sounds like fun, but how much comes out with each squirt and how much air do I need to mix it with to make it not just extremely flammable but explosive?
Working on the "How much comes out?" question, I put a cylindrical, flat-bottomed bottle in a container of water and inverted it. I connected a plastic tube to the adapter for the PASLODE CF325 framing nailer and attached the adapter to my newly acquired "Paslode fuel cell." Running the tube under the inverted bottle, I released one puff of fuel. The released fuel formed a cylindrical bubble in the bottle 8 mm high and 36 mm in diameter. That makes the volume of the bubble pi * (3.6/2)**2 * 0.8 = 8.1 ml.
The amount of air 8 ml of gas needs to be mixed with to make an explosive mixture depends on what gas it is. Further poking around turns up the MSDS (PDF) for the "PASLODE fuel cell." It says that there are two gasses in a fuel cell, 1-butene (0% - 80%) and propene (20% - 100%). So, it looks like a new fuel cell has 80% 1-butene and 20% propene, while a spent one has 0% 1-butene and 100% propene. The 1-butene is the fuel and the propene is the propellant.
From Glover's Pocket Ref, 1-butene explosive limits are 1.6% - 10.0% by volume in dry air. That means the a container charged with one puff of fuel must be smaller than about 8.1 ml / 1.6% = 506ml. Similarly, the container would need to be bigger than about 8.1 ml / 10.0% = 81ml. So, something like 150 (5.4%) to 250 (3.2%) ml would be good.
To see if my measurements and calculations were correct, I needed a test setup. To build it I used a ~200ml transparent orange PET medication bottle with a screw lid for the combustion chamber, a short length of 25-mm transparent plastic tube for a handle and a hollow golf-tee shaped piece from the innards of an old hand-lotion pump for a fueling port. I added the sparker from a dollar store grill-lighter and some bits of wire to make sparks on demand and stuck the whole thing together with zip ties and strategic dabs of hot glue.
I adjusted the spark gap to about 3 mm, closed the mouth of the bottle with a piece of paper clamped in place by a screw-ring made from the bottle's lid, took whole contraption outside, pushed the fuel cell against the fueling port to release a shot of gas and clicked the sparker. Bang! Serious BANG. It works every time. No volume measuring, no one one-thousand, two one-thousand...
Now, what would this medium-sized, reliable bang be fun to do with? Hmmmm. An obvious thing is to attach a barrel and use the contraption to launch something, spud gun style.
It turns out that with a few tenths of a millimeter shaved off the diameter of its end ring, and its pointy end chopped off, the tube that forms the body of a dollar-store pool toy fits perfectly in the mouth of the combustion chamber. Better yet, it locks in place with the screw ring I already made!
What would be fun to use for "ammo:? Something solid, but a little squishy and about 25mm across. I know! Giant gumdrops. The local supermarket sells big ones that are an inch in diameter, more or less. By themselves, they're too loose, but with a piece of paper towel for wadding, they're perfect. Snug, but not so tight that I'd be worried about the pressure inside bottle when I fire it.
Yep, it works great. With a nice Boom! and muzzle flash — heck, the whole thing lights up 'cause it's transparent — it easily chucks the gumdrop 150-200 meters, just what you'd expect for a measured dose of fuel just right to drive a good-sized nail all the way into a board.