They generally operate when the temperature of either the operating element or the surrounding air reaches a predetermined value. Fixed-temperature devices may be more reliable in detecting slowly developing fires. Smoke detectors respond to particles of combustion. Approved smoke detectors for room or area protection can operate more quickly than an approved heat-sensitive device exposed to a slowly developing fire. Smoke detectors can monitor HVAC systems and supervise and control fans and fire dampers, especially where positive control of smoke is essential.
Flame detectors sense infrared, visible or ultraviolet radiation from a fire.
Typical applications are conveyor housing, yard storage and chemical plant yards. An ionization detector is cheaper to manufacture and has been installed in many homes during the past 20 years. It responds better to a flaming fire, one without much visible smoke. It also responds well to a slow burning, smoldering fire condition. Where ionization is weak, the photo detector is well suited.