A pressure vessel is, as its name indicates, a container which holds its contents— whether it be a liquid or gas— at a specific pressure. Most have cylindrical shapes and hold gases like oxygen and nitrogen. Because gas and liquids are able to be maintained at pressures different from those of the atmosphere, pressure vessels are valuable for many different contexts and environments. They are typically made out of steel— specifically stainless steel— because of its tensile strength and resistance to corrosion and outside forces. In addition to steel, the container is also often lined with other metals, ceramics or polymers, providing it with extra protection against cracks and leaks.
In addition to stainless steel, many pressure vessels are made of other composite materials, such as filament wound composite and carbon fiber via a polymer; these kinds of vessels can be very light but are very hard to make. This can, if needed, be included with steel as a top layer, providing extra strength and protection from leaking and contamination. Other common materials used in pressure vessels include polymers like PET, such as in soda cans and copper in plumbing.