Final answer:
Hydrostatic testing for steel and aluminum cylinders used in breathing apparatus is required every five years to ensure their safety and integrity.
Step-by-step explanation:
The question pertains to the periodic hydrostatic testing required for steel and aluminum cylinders used in breathing apparatus, typically for safety purposes like in scuba diving or firefighting equipment. Hydrostatic testing ensures the integrity of the cylinder to hold pressure without failure. For these types of cylinders, hydrostatic tests should be conducted every five years, as per industry standards and government regulations. This ensures that the material has not degraded and that the cylinder can still safely hold the high-pressure gases needed for breathing in various environments.