(UKAS), who make recommendations to the Vehicle Certification Agency (VCA) for approval of individual bodies. There are a variety of tests that may be performed on various cylinders. Some of the most common types of tests are hydrostatic test, burst test, ultimate tensile strength, Charpy impact test and pressure cycling. During the manufacturing process, vital information is usually stamped or permanently marked on the cylinder. This information usually includes the type of cylinder, the working or service pressure, the serial number, date of manufacture, the manufacture's registered code and sometimes the test pressure. Other information may also be stamped, depending on the regulation requirements. High-pressure cylinders that are used multiple times — as most are — can be hydrostatically or ultrasonically tested and visually examined every few years. In the United States, hydrostatic/ultrasonic testing is required either every five years or every ten years, depending on cylinder and its service. Valve connections Valve Gas cylinders usually have a stop angle valve at one end, and the cylinder is usually oriented so the valve is on top. During storage, transportation, and handling when the gas is not in use, a cap may be screwed over the protruding valve to protect it from damage or breaking off in case the cylinder were to fall over. Instead of a cap, cylinders sometimes have a protective collar or neck ring around the valve assembly. Connection The valves on industrial, medical and diving cylinders usually have threads of different handedness, sizes and types that depend on the category of gas, making it more difficult to mistakenly misuse a gas. For example, a hydrogen cylinder does not fit an oxygen regulator and supply line, which could result in catastrophe. Some fittings use a right-hand thread, while others use a left-hand thread; left-hand thread fittings are usually identifiable by notches or grooves cut into them. In the United States, valve connections are sometimes referred to as CGA connections, since the Compressed Gas Association (CGA) publishes guidelines on what connections to use for what gasses. For example, an argon cylinder has a "CGA 580" connection on the valve. High purity gases sometimes use CGA-DISS ("Diameter Index Safety System") connections. Medical gases may use the pin index safety system to prevent incorrect connection of gases to services. In the European Union, DIN connections are more common than in the United States. In the UK, the British Standards Institution sets