PADI Divemaster Certification Practice Exam

How does a hydrostatic test check for metal fatigue in a cylinder?

1. By observing color changes

2. By testing the weight of the cylinder

3. By comparing displacement before and after filling

4. By inspecting the exterior for rust

The correct answer is: By comparing displacement before and after filling

The hydrostatic test is designed to assess the structural integrity of a cylinder, particularly to identify issues like metal fatigue that may not be visible from external inspections. During this test, water is used to fill the cylinder to a specific pressure. By measuring the displacement of the water before and after the cylinder is filled, any changes can indicate issues. If there is a significant difference in the volume of water displaced, it suggests that the cylinder may have developed defects or weaknesses, such as microscopic cracks or deformation caused by metal fatigue. This is critical because even small structural weaknesses can lead to significant failures in a high-pressure environment. Other methods, such as inspection for rust or weighing the cylinder, do not effectively measure the integrity under pressure or identify internal faults. Observing color changes is not applicable in the context of a hydrostatic test as there are no relevant color indicators associated with fatigue. Thus, comparing the displacement of water before and after filling the cylinder directly correlates to evaluating the presence of weaknesses or metal fatigue.