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

The correct answer is that magnetic pressure assessment is not a method for retesting DOT cylinders. When it comes to ensuring the safety and integrity of DOT cylinders, several established testing methods are commonly used. These methods, like volumetric expansion, externally visual inspection, and proof pressure tests, are designed to evaluate the cylinder's structural integrity and ability to hold gas under pressure.

Volumetric expansion involves measuring the amount a cylinder expands when filled with a liquid to check for any weaknesses. Externally visual inspection allows inspectors to look for obvious signs of wear, damage, or corrosion on the outside of the cylinder. Proof pressure testing involves filling the cylinder with a liquid and applying pressure to a level higher than the standard operating pressure to ensure that it can withstand operational conditions.

In contrast, magnetic pressure assessment is not recognized as a valid or standard method for retesting DOT cylinders. This method does not effectively evaluate the structural integrity of the cylinder in the same rigorous manner as the standard techniques established for cylinder safety.