What device on a boiler burner makes it easy to detect a main flame?

The UV fire eye is a critical device on a boiler burner that is specifically designed to detect the presence of a main flame. It operates using ultraviolet radiation to sense the flame; when the flame is established, it emits UV rays that the fire eye can detect. This capability allows the system to confirm that the ignition has successfully occurred and that the burner is operating safely. If the flame were to go out for any reason, the UV fire eye would not detect the UV radiation, triggering safety mechanisms to shut down the burner and prevent hazardous conditions.

In comparison, a thermocouple measures temperature rather than directly detecting flame presence. It is often used to monitor temperatures within various parts of the boiler system but does not confirm flame status. A pressure switch measures pressure levels, typically to ensure safe operational ranges for the boiler, but again does not indicate if the flame is present. Meanwhile, a flame separator is a device used to prevent flame propagation in certain applications, but it does not serve the function of detecting the flame itself. Therefore, the UV fire eye's specific use in flame detection sets it apart as the correct answer in this context.