Bottled gas refers to gaseous substances that have been placed into pressurized containers for ease of handling and use. These substances are typically gaseous at regular atmospheric pressure, and may remain in gas form or become liquid once inside a pressurized vessel. Other bottled gases are highly explosive when compressed, so they are dissolved in a substance such as acetone rather than simply pressurized. Gas bottles typically use color to distinguish type and other factors such as toxicity, though the colors can vary between countries. In some countries, such as the United States, the term gas cylinder is used more commonly than bottled gas.
Under standard temperature and pressure (STP), the substances held in bottled gas will take the form of gaseous matter. The process of bottling gas involves inserting a greater volume of these substances in a vessel than would otherwise fit at STP. This creates a pressure within the bottle that is higher than normal atmosphere. Some substances remain gaseous under this increased pressure, and they would need to be cooled before becoming liquid. Common welding gases that fit this mold are oxygen and argon.
Other bottled gases become liquid under increased pressure at standard temperatures. Some welding gases that fit this profile include butane and propane. These gases are not as commonly used in oxygen-fuel welding as other substances, though injector style propane torches can be used to create fast, clean cuts. This is because the inner cone of a propane or butane torch does not typically burn as hot as other gases, but the outer cone is very hot.
Some substances that are gaseous at STP become incredibly volatile when subjected to pressure. Acetylene is one example of a common welding gas that is highly flammable in gaseous form but becomes explosive in liquid or solid form. In order to bottle acetylene or similar substances, they can be dissolved into a solvent solution. Solvents such as dimethylformamide (DMF) and acetone are often used to store acetylene as a bottled gas. In this case the bottled gas exists in solution until a valve is opened, at which point the acetylene or other material can become gaseous once more.
Bottled gas and gas cylinders are typically color coded according the substance they contain, though these procedures can vary between countries. In Europe, acetylene can be identified by a maroon color, while oxygen cylinders use a white colored shoulder. This type of color coding is not required by federal law in the United States, so different systems exist in various jurisdictions. The type of gas in a cylinder can sometimes be identified by the color, but other more reliable methods are recommended for safety.