Final answer:
Refillable refrigerant cylinders must be re-tested every 5 years to ensure safety and integrity, in compliance with DOT and environmental safety regulations.
Step-by-step explanation:
The question pertains to the testing frequency for refillable refrigerant cylinders. According to regulations established by agencies such as the Department of Transportation (DOT) and environmental protocols, refillable refrigerant cylinders must be tested every 5 years to ensure they maintain their integrity and safety standards. This is to prevent potential hazards related to overpressure and material degradation, which could lead to leaks or bursts. Regular testing helps in maintaining a safe environment for those handling the cylinders as well as the ozone layer which can be affected by refrigerant leaks. These cylinders are used to store and transport refrigerants used in various cooling systems such as air conditioners and refrigerators.