Which of the following is not a type of mechanical device for backflow protection?

The correct answer is the check expansion valve, as it is not recognized as a mechanical device specifically designed for backflow protection. Backflow protection devices are utilized to prevent the reverse flow of water in plumbing systems, ensuring that contaminated water does not enter clean water supplies.

An air gap is a physical separation between the water supply outlet and the flood level of a fixture, which effectively prevents backflow due to gravity. Similarly, a double check valve consists of two independent check valves that work together to prevent backflow, particularly in non-potable systems.

A reduced-pressure backflow device is designed to protect against backflow by maintaining water pressure below the supply pressure. It utilizes a relief valve to ensure that if system pressure drops, water does not flow back into the clean water supply.

In contrast, a check expansion valve is typically associated with regulating pressure in mechanical systems (like refrigeration) rather than backflow prevention in plumbing. Thus, it does not serve the same function as the other listed devices, making it the correct answer to the question regarding backflow protection.