Which method is used for marking cylinders that have undergone hydrostatic testing?

The volumetric expansion method is the correct choice for marking cylinders that have undergone hydrostatic testing. This method involves measuring the change in volume of a gas or liquid within the cylinder when pressure is applied. When a cylinder is hydrostatically tested, it is filled with liquid (usually water) and pressurized to a specified level to check for leaks and structural integrity. The volumetric expansion is measured to ensure that the cylinder can safely withstand its rated pressure without deforming.

Marking the cylinder based on volumetric expansion provides a reliable indication of its performance during the test and indicates that it has successfully passed the required safety evaluations. This method is essential because it verifies that the container can not only handle pressure but also maintains its integrity under those conditions, which is crucial for safety.

Other methods, while useful in different contexts, do not specifically pertain to the marking of cylinders after hydrostatic testing. The air leakage test method focuses on detecting leaks in the cylinder, the proof pressure method is generally used to test the maximum pressure the cylinder can handle but does not provide the same detailed volumetric analysis, and the external visual inspection method assesses the cylinder's physical condition but does not involve testing under pressure.