A gas cylinder is a pressure vessel for storage and containment of gases at above atmospheric pressure. High-pressure gas cylinders are also called bottles. Inside the cylinder the stored contents may be in a state of compressed gas, vapor over liquid, supercritical fluid, or dissolved in a substrate material, depending on the physical characteristics of the contents. A typical gas cylinder design is elongated, standing upright on a flattened bottom end, with the valve and fitting at the top for connecting to the receiving apparatus. The term cylinder in this context is not to be confused with tank, the latter being an open-top or vented container that stores liquids under gravity, though the term scuba tank is commonly used to refer to a cylinder used for breathing gas supply to an underwater breathing apparatus. Nomenclature In the United States, "bottled gas" typically refers to liquefied petroleum gas. "Bottled gas" is sometimes used in medical supply, especially for portable oxygen tanks. Packaged industrial gases are frequently called "cylinder gas", though "bottled gas" is sometimes used. The term propane tank is also used for cylinders with propane. The United Kingdom and other parts of Europe more commonly refer to "bottled gas" when discussing any usage, whether industrial, medical, or liquefied petroleum. In contrast, what is called liquefied petroleum gas in the United States is known generically in the United Kingdom as "LPG" and it may be ordered by using one of several trade names, or specifically as butane or propane, depending on the required heat output. Materials Design codes and application standards and the cost of materials dictated the choice of steel with no welds for most gas cylinders; the steel is treated to resist corrosion. Some newly developed lightweight gas cylinders are made from stainless steel and composite materials. Due to the very high tensile strength of carbon fiber reinforced polymer, these vessels can be very light, but are more difficult to manufacture. Cylinders reinforced or built-up with a fibre material usually must be inspected more frequently than metal cylinders, e.g., every 5 instead of 10 years, and must be inspected more thoroughly than metal cylinders. They may have a limited service life. The inspection interval of steel cylinders has increased from 5 or 6 years to 10 years. Diving cylinders that are used in water must be inspected more often. When they were found to have inherent structural problems, certain