Bellows are elastic and can produce displacement under pressure, axial force, transverse force or bending moment. Bellows are widely used in instruments and meters, and their main purpose is to be used as the measuring element of pressure measuring instrument to convert pressure into displacement or force. The bellows have thinner wall and higher sensitivity, and the measuring range is from tens of pa to tens of mpa. Alternatively, the bellows may be used as a sealed isolation element to separate the two media or to prevent harmful fluids from entering the measured portion of the device. It can also be used as a compensating element to compensate the temperature error of the instrument by the variability of its volume. Sometimes used as an elastic joint between two parts. Single - layer bellows are used more. Multilayer bellows have high strength, good durability and low stress. They are used in important measurements. Metal bellows are mainly used to compensate thermal deformation of pipelines, shock absorption, settlement deformation of pipelines and so on. They are widely used in petrochemical, instrumentation, aerospace, chemical industry, electric power, cement, metallurgy and other industries. Plastic and other materials bellows in the medium transport, power threading, machine tools, home appliances and other fields has an irreplaceable role. Bellows are usually made of bronze, brass, stainless steel, Monel and Inconel.