Siphons and Anti-Siphons

[Junkie]

Dealing With Liquid CO2
By Bill Mills - March 2006

Ever since Lou “Gramps” Grubb and Mike “Grizzly” Grubb started putting refillable carbon dioxide tanks on paintguns in the early 1980s, paintball players have had to face liquid CO2.

CO2 exists in three states – gas liquid and solid. CO2 has a very unusual quality. At atmospheric pressures its boiling point is lower than its freezing point. This gives rise to the name “dry ice” for solid CO2. Dry ice doesn’t melt into liquid, it sublimates straight to gas.

Under pressure however, CO2 can be condensed into liquid form, and this is what makes it desireable as a paintball propellant. A filled paintball tank has both liquid and gaseous CO2 in it. Just like a half full bottle of soda, the liquid CO2 is held in the bottom of the tank by the inescapeable force of gravity.

When a paintgun fires it uses some of the gas from the CO2 tank which is supplying it. That causes the gas pressure in the tank to drop. If only gas were stored in the tank, less and less pressure would be available to power the paintgun with every shot. For paintguns without regulators, this would mean a drop in paintgun velocity shot after shot. This is where the liquid CO2 kicks in. As soon as the pressure of the gas in the tank drops below the vapor pressure for CO2 at its given temperature, the CO2 will boil converting into gas. It is a mistake to think that boiling of CO2, water, or any other liquid requires high temperatures. The boiling point of a liquid is affected by temperature and pressure. Even at cold temperatures, liquid CO2 will boil into gas as soon as the pressure around it gets low enough.

As the boiling liquid CO2 converts to gas, the pressure level in the CO2 tank is restored, acting as an automatic method of pressure regulation. There are a host of other issues related to temperature and this phase change, but for now as we are focusing simply on liquid CO2 we’ll bypass them before this article becomes an intensive physical chemistry lesson.

Liquid CO2 can be troublesome for paintguns, especially those designed for low-pressure operation, or with low pressure pneumatics. If the liquid CO2 into a paintgun’s valve, it can cause a sharp spike in velocity as it expands into gas while propelling the paintball. In an even worse scenario, some paintguns with low-pressure pneumatics hoses or components can be damaged if liquid CO2 gets past their regulator and expands into gas there, raising pressure beyond what the components can handle.

Fortunately, liquid CO2 is much easier to control than most paintball players realize. Visualizing the model of a partially full soda bottle can help understand what is going on in the CO2 tank. If the soda bottle is held vertically and squeezed, only the air and CO2 from the soda carbonation will blow out of the top. On the other hand, if it is tipped on its side, it is the soda that will come squirting out.

n the late 1980s, paintgun designers made use of the backbottle arrangement first used by Tippmann in the SMG-60. The CO2 tank screwed into the back of the paintgun, tipped at a slightly downward angle. When held perfectly level, this arrangement fed gas to the paintgun. If shooting downward, or moving around much, bits of liquid CO2 could splash into the tank’s valve and reach the paintgun.

In the early 1990s a number of paintgun designs, including the Autococker and Minimag took this into account, utilizing vertical CO2 tank mounts. Another alternative popular during that time period was the remote hose. It was widely thought that the coiled hose of the remote helped warm the CO2. Whether that was true or not, remotes were typically used with a pack that held the CO2 tank in a vertical position on the player’s back – a fantastic defense against liquid CO2.

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warpig.com

[drewcif]

very cool. takes me back to chemistry class... scary

Drew