Which of the following types of re-qualification methods involves filling the cylinder with water?

The re-qualification method that involves filling the cylinder with water is hydrostatic testing. This method is widely used to ensure the structural integrity and safety of pressure vessels, such as gas cylinders. During hydrostatic testing, the cylinder is filled with water, and then pressure is applied. The primary purpose of using water is that it is incompressible, so in the event of a failure, it does not produce the dangerous explosive effects that a gas would.

The pressure applied during the test must exceed the service pressure of the cylinder in order to identify any potential leaks or weaknesses in the material. Observing the cylinder under these conditions helps to ensure that it can hold pressure safely for its intended use. In contrast, other methods mentioned, such as visual inspections or pressure testing with gas, do not involve the actual filling of the cylinder with water and serve different purposes in evaluating cylinder safety and performance.