Two types of flame sensors generally are used on industrial burners: flame rods and ultraviolet scanners. While each can perform the essential role of safeguarding industrial combustion systems, this article will focus on ultraviolet scanners.
First, a review of flame rods allows comparison of the technologies. Flame rods are simply stainless steel rods that intersect the burner flame. A voltage potential from the combustion safeguard is applied to the flame rod. When a flame is present, an electrical current — measured in millionths of an amp — flows from the flame rod through the ionized gases of the flame to the burner, which is grounded. This current is amplified in the combustion safeguard and energizes a relay output to power the fuel valves (figure 1).