A flame sensor for high-temperature ambients

The flame sensor consists of a photoelectric counter tube which is triggered by ultraviolet photons emitted by flames in the spectral region beyond the solar cutoff. Tubes of this type have been made previously, but this new tube differs in being able to operate in ambient temperatures up to and in excess of 1000°F. The new tube has high sensitivity and a lower operating voltage than older tubes. The flame sensor is encapsulated in a titanium and forsterite ceramic envelope and uses a sapphire window to admit the ultraviolet photons. The photocathode consists of a molybdenum rod which has been cleaned by heating and sputtering. The anode is a molybdenum hemisphere, which also serves as a mirror to concentrate the light from the flame on the photocathode. The gas-fill is 100 torr of a helium-argon mixture, where the small amount of argon is used to help remove helium metastables following a count. The sensor operating voltage is about 150 V, and a dead time of 1 ms or more is used to avoid multiple counting. Typical counting rates obtained with a small methane flame at 12 ft are 60 per second. A sensor was life-tested at 1000°F in air for longer than 2000 hours.