Advancing The Art Of Optical Path Verification In Infrared Flame DetectorsSource: Emerson Process Management, Rosemount Analytical
Principle of Operation - VI Testing
The primary intention of the Visual Integrity test feature on optical flame detectors has historically been to detect and report the presence of accumulated material on the lens of a flame detector that would prevent it from detecting flame.
Secondarily, the VI test verifies functionality of the various sensors and circuitry of the detector.
The most prevalent self test schemes involve an internal light source that projects a beam through the lens and onto a metal reflector from where the beam is reflected back through the lens and on to the primary sensor where it is detected to verify integrity of the optical path.
Drawbacks of Conventional VI Tests
Reflector-based VI testing has the drawback of requiring the maintenance of the refl ectivity of the metal reflector, which is subject to fouling by accumulations of airborne materials and corrosion. These degradations of the reflector result in false VI fault alarms and subsequent costly procedures to clean or replace the reflector. Another drawback is that the lens can have distributed deposits of material that block flame signals, but allow the test beam to pass normally.
The function of this system is easily tested by interrupting the beam of light with any opaque material like a piece of paper, metal or a finger.