PAINTBALL tech talk

EXPANSION CHAMBERS

By Joseph C. Dillon
President, CMI, International LLC

As the popularity of paintball continues to make giant gains, the availability of paint gadgets keeps pace. Of the mind-numbing array of accessories designed to attract the player, it’s often difficult to determine not only how they work, but whether they improve the game at all. Expansion chambers, which are sometimes confused with regulators, do fulfill a necessary function, and the right one will definitely improve your performance. Let’s find out what an expansion chamber really is and how it works.

Take a look at these illustrations of some typical cross-sectioned expansion chambers and check the differences in how they’re made. Basically the interior and sometimes the exterior design should dictate the name of the unit. Thermal-exchange expansion chamber is the work function.

Q: What is an expansion chamber?

A: It’s a unit that attaches between your tank or gas supply and your paintball gun, to help change liquid CO₂ to a more consistent expanded gas. CO₂ (carbon dioxide), like water, can take the form of a liquid, a solid (dry ice) or, when it boils, a gas. As liquid CO₂comes from the tank, it expands inside the chamber, but should be prevented from entering the paintball gun. If CO₂ enters the paintball gun as liquid, it will tend to freeze internal parts and may stop some paintguns from working. Also, the expansion of liquid CO₂ is very erratic and can make a paintgun that was shooting safely at 280 FPS suddenly shoot up to 350 FPS or more. This is normally known as "spiking." At these higher speeds, the integrity of safety equipment can be compromised, and damage to internal gun parts may occur.

Q: How does an expansion chamber work?

A: Two functions must be incorporated into the design of an expansion chamber when CO₂is used to propel a paintball. One, sufficient surface area is needed to bring ambient temperature to the liquid CO₂ so it changes from a liquid to a more stable expanded gas. Since CO₂ is stored under pressure, the instant the pressure is reduced, as it exits the tank, the liquid CO₂ boils, like water going from a liquid to a vapor. Boiling CO₂ absorbs heat for vaporization from its surroundings. At this point, if sufficient heat is not available, the CO₂ will freeze anything around it. So by adding fins to the body of an expansion chamber, there is more ambient heat to reduce or eliminate freezing. If you take a piece of aluminum that is one inch long by one inch wide, you have the equivalent of one square inch of surface area. Now if you have the same one square inch with eight fins and eight spaces, .200 inch high, your surface will be around 300% greater. This greater surface area will provide more ambient heat to the expansion chamber to prevent it from freezing or allowing liquid CO₂ to pass. As can be seen, an expansion chamber without fins is not efficient.

Two, sufficient space must be provided for the CO₂ gas to expand and supply the volume of gas needed to maintain the rate of fire and speed you want. For example, a pump gun will use about .033 cubic inches of gas per shot, depending upon weather conditions. Some semi-automatics can use .881 cubic inches or more of gas per shot. When the semi-automatic gun is designed as a high-volume low-pressure gun, the consumption can be 26 times more gas than the amount utilized by a pump gun. As the CO₂ gas has been aided to change to a more constant expansion of volume, shooting will use less gas.

Q: When do you need to use an expansion chamber?

A: If you use CO₂ as a source to shoot paintballs, you should use an expansion chamber. You will have more speed control and accuracy by eliminating "spikes." In cold weather, you really should use an expansion chamber, especially if your paintgun’s performance suffers in cold weather. Nitrogen and high-pressure air normally don’t need an expansion chamber.

Q: Does the size of an expansion chamber matter?

A: Yes, the size and capacity to provide ambient heat does matter. The colder the ambient temperature, the more heat exchange area is needed to vaporize the CO₂. An expansion chamber that provides large surface area in the form of fins can be very compact and still do a good job.

Q: How is the speed of a paintball affected by using an expansion chamber?

A: When the CO₂has expanded in the expansion chamber to a more uniform volume and temperature, the pressure that pushes the paintball will be more stable. Therefore, the speed of the paintball will be more constant. Don’t forget, CO₂ is affected by temperature, humidity, altitude, etc. So as conditions change, so will the behavior of the gas.

Q: Is an expansion chamber a regulator?

A: No. The regulator controls the pressure that pushes the paintball. As the name implies, the regulator is adjustable to keep that pressure. So you have much more control on the speed of the paintball. An expansion chamber allows a more uniform expansion from liquid to gas, but it does not control the pressure.