Gas Dynamics 101

[CantoXII]

Gas Dynamics 101

So what's really going on inside your gas airsoft replica? Why does Green Gas not work as well in the cold? What is that sloshing in the can? Is "Red Gas" really just CO2 with a little lubricant in it? The answers to these questions and more came as a result of a fair amount of tinkering and research I've been doing lately.

First off, I'd like to thank Bill from 21st Century Airsoft, who dedicated a Saturday afternoon and his custom machining skills and equipment to helping me build a custom nozzle for my prototype fill tool. While the end goal was not fully successful, it gave me a lot of insight and let me confirm my suspicions as to what's actually going on in a gas replica.

Note that throughout this post, I refer to Green Gas. This is just shorthand for any commonly used airsoft propellant gas (be it actual Green Gas, TOP gas, propane, or whatever).

So how do gas guns work? Pretty much everybody and their brother understands that the gas is under pressure, and that as you fire the gun, a valve is temporarily opened and high-pressure gas is released. this high pressure gas shoots the pellet and recocks the slide in the case of a GBB. But that's really just the beginning (or rather the end) of a slightly more complicated process.

The most important operational concern for gas pistols using propellant gases is actually state change. As I'm sure everyone is aware, materials can be in different states depending on the temperature. As the easiest example to grasp, take water: at standard atmospheric pressure (~14 PSI) water is a solid (ice) at temperatures under 32° F. At temperatures between 32° F and 212° F, water is a liquid, and at temperatures above 212° F, water is a gas (steam). All materials can change states, although for some the temperature required might be very high or very low.

In addition to being affected by temperature, material state can also be affected by pressure. As the pressure on a material goes up, the temperature needed to change its state also goes up. As the pressure goes down, the temperature needed to change state goes down. This is why it takes longer to boil an egg at high altitudes - the atmospheric pressure is lower, so the water boils at a lower temperature. Less heat means it takes longer to cook the egg.

If you raise the pressure in a closed container high enough, you can actually cause a gas to turn to liquid (a process known as "condensation"). This pressure is known as a material's "vapor pressure," and it is a function of temperature. For Green Gas, the vapor pressure is approximately 150 PSI at room temperature (70° F).

But here's the catch: in a closed container, you can have both liquid and gas. Furthermore, the pressure in that container will always be that material's vapor pressure for whatever temperature the material inside is. Think of it this way: with just liquid in the container, there will be some "empty space" in the container. That empty space is at 0 PSI. This low pressure will cause some of the material to change state to fill the volume. The liquid will essentially "boil" until it forms enough gas to raise the pressure in the container to its vapor pressure. At this point, the boiling will stop, because any further boiling would raise the pressure above the vapor pressure, at which point some of the gas would condense back into liquid. Thus, for any given temperature, the material will reach "equilibrium" at its vapor pressure for that temperature. Further, if you temporarily open the container and let some of the gas out, the liquid will once again boil until the pressure of the gas reaches the vapor pressure.

What does this mean for a can of Green Gas? It means that so long as the can is able to withstand 150 PSI at room temperature, it can contain liquid. And this is exactly what's going on. When you hear sloshing in a can of Green Gas, it's not silicone lubricant (although there's some of that in there as well), it's actually liquified Green Gas (or "Green Liquid" if you will).

When you fill a gas magazine from your can of Green Gas, you aren't actualy filling it with gas - you're filling it with liquid too. That's why you turn the can upside-down to fill the magazine. As you push down on the nozzle, a valve opens. The pressure in the can forces the liquid down into your mag until the pressure of the liquid/gas mix in your mag is equal to that in the can. Once these pressures are equal, the gas doesn't have anywhere to go and will try to escape any way it can, usually out around the fill nozzle. This is the source of the hissing that tells you the magazine is full. So now your magazine contains both liquid and gas, both of which are at the vapor pressure for whatever the ambient temperature happens to be.

Here's where the magic of gas replicas begins. When you pull the trigger on your GBB, a striker hits a spring-valve on your magazine. This releases some of the gas, which propels the pellet and recocks the slide. A soon as the pressure in the magazine drops, the "Green Liquid" will begin to boil to try to equalize the pressure again. And once the valve is closed, it will quickly reach equilibrium pressure again. This is why a GBB shoots consistently from one shot to the next - the material in the mag is always at the equilibrium pressure.

The important thing to remember here is that liquid is much more dense than gas. As such, it takes only a tiny bit of liquid to produce a large volume of gas. Even pressurized gas will be much larger by volume than a similar mass of liquid. This is how you can get a large number of shots out of a single magazine fill - the liquid in the magazine is essentially "refilling" it with gas several times over. If there weren't any liquid in the magazine, there would be nothing to replace the gas once it was used to shoot the pellet. This is exactly what happens on your last shot before your magazine is empty - with no state-change to back it up, the pressure simply drops because there's less gas in the reservoir.

It is of note that this is exactly the behavior I saw with my fill tool. I was able to successfully fill a GBB magazine with CO2 at 150 PSI. It worked flawlessly - for one shot. After that, the pressure had dropped, because there was no state change to back it up.

This also allows me to categorically deny that "Red Gas" is simply CO2, because the vapor pressure of CO2 at room temperature is roughly 600 PSI. In all likelihood, pressure that high would blow all the seals in your mag. Even if it didn't, your gun wouldn't work correctly. You might think that pressures that high would blow your slide right off your gun (and they might). But what is more likely is that the gun wouldn't even fire at all because your hammer spring doesn't have the force to open the spring-valve (because you're pushing not only against the force of the spring but against to force of the gas pressure in the magazine. I saw exactly this kind of behavior when I filled a GBB mag up to 300 PSI with CO2 - it would fire the pellet, but not recock the slide (because the hammer wasn't striking hard enough to open the valve far enough to let enough gas out to work the slide). But since there was no state change backing it up, the pressure in the mag would drop a little bit with each shot. Once it got down to the 150 PSI range (the range at which the gun was designed to operate), it would work perfectly - for one shot, at which point the gas pressure had dropped too low.

- Courtesy of Munin; Geist Kompanie.