Final answer:
Refillable tanks must undergo hydrostatic testing at different intervals depending on the type of tank and its contents.
Step-by-step explanation:
A refillable tank must undergo hydrostatic testing at different intervals depending on the type of tank and its contents. For example, the Department of Transportation (DOT) requires hydrostatic testing of certain types of compressed gas cylinders every 5 years. This involves filling the tank with water and applying pressure to check for any leaks or deformations. If the tank passes the test, it can continue to be used safely. However, if any issues are found, the tank may need to be repaired or retired.