Which of the following is NOT a method used for retesting DOT cylinders?

The correct answer is hydraulic compression test because this method is not utilized for retesting Department of Transportation (DOT) cylinders. In the context of DOT cylinder testing, there are specific methods outlined to ensure the safety and integrity of pressurized containers used for transporting gases.

The proof pressure test, external visual inspection, and volumetric expansion test are all recognized procedures within the DOT regulations for assessing cylinder safety. The proof pressure test involves applying pressure higher than the normal service pressure to check for leaks and material integrity. External visual inspection is critical to identify any visible signs of damage, corrosion, or imperfections on the cylinder's surface, while the volumetric expansion test measures how much a cylinder expands under pressure, which indicates its capacity to withstand operational stresses.

In contrast, a hydraulic compression test is not a standard procedure for checking the structural integrity or safety of gas cylinders, making it the outlier in this context. Understanding these testing methods and their applications is essential for ensuring compliance with safety regulations and maintaining the proper functioning of DOT cylinders.