What must always be used on a cylinder when in use?

Using a regulator on a cylinder when in use is essential because it controls the pressure and flow of gas being released from the cylinder. Regulators ensure that the gas is delivered at a safe and consistent pressure, preventing fluctuations that could lead to potential hazards, such as leaks or excessive pressure that could damage equipment or create dangerous situations.

The function of a regulator is critical in welding and cutting applications, where precise control of the gas supply is necessary for effective operation. This safety feature helps to prevent accidents and is a fundamental part of the equipment setup.

While items like wrenches, gauges, and safety caps serve important roles in handling and storing gas cylinders, they do not fulfill the same crucial function during operation as a regulator does. Wrenches might be used to tighten or loosen fittings, gauges indicate pressure levels, and safety caps protect the cylinder's outlet during transport or storage, but none of these can replace the need for a regulator while the cylinder is actively in use.