Which type of extinguishers must undergo hydrostatic testing every five years?

The correct answer is the Aqueous Film Forming Foam (AFFF) extinguishers, as they are subject to hydrostatic testing every five years to ensure their integrity and performance under pressure. Hydrostatic testing is a critical procedure that helps identify any potential weaknesses or failures in the extinguisher's container or components. This testing is especially important for AFFF extinguishers because they are designed for use on flammable liquid fires and must be reliable to effectively suppress such hazards.

Regular hydrostatic testing helps maintain safety standards and compliance with regulations. It ensures that the foam extinguishing agent is capable of forming a stable film over the combustible liquid, which is essential for smothering vapors and preventing re-ignition. Keeping these extinguishers in optimal condition by adhering to the five-year testing requirement is crucial for effective firefighting capabilities in environments where flammable liquids are present.