Which of the following conditions requires hydrostatic testing?

Hydrostatic testing is a critical procedure performed on pressure vessels or tanks to ensure their structural integrity and safety. This testing is particularly essential after any repair, modification, or alteration to the pressure boundary of a vessel. The reason for this requirement is that any change made to the tank can potentially affect its ability to withstand high pressures. Hydrostatic testing involves filling the vessel with water and pressurizing it to a specific level, allowing inspectors to check for leaks or weaknesses in the material.

Changes or repairs to a tank could introduce weaknesses, such as weld defects or material fatigue, which might not be visible through simple visual inspection. Therefore, conducting a hydrostatic test after such modifications is vital to confirm that the vessel can safely operate at its intended pressure without the risk of failure.

In contrast, while regular maintenance and monitoring are crucial, the frequency tied to specific conditions—like after each dive or regularly scheduled inspections every five years—may not directly necessitate hydrostatic testing unless significant repairs or alterations have occurred. Similarly, changing the water in the tank does not impact the tank's structural integrity, thus not requiring hydrostatic testing.