In many detectors smoke is detected in a black chamber located inside the unit. The unit is designed to prevent light from getting into the chamber while allowing air and smoke to pass through freely. Inside the chamber there is a light sensor and a light source. The sensor and the source are separated by a black barrier. Under normal conditions the air in the chamber is clear and the light emitted from the source is absorbed into the black barrier and sides of the chamber, and no light reaches the sensor. When smoke or other airborn particles enter the chamber, they reflect light from the source. When reflected light reaches the sensor, the alarm is sounded. It is like driving on a clear night and entering a patch of fog; the fog reflects the light from your headlights that would otherwise disappear into the distance.
Unfortunately, the sensor cannot tell the difference between smoke and a small insect. Therefore, to help prevent false alarms ORION fabricates screens to keep the bugs out. The screen can be made from metal, nylon, or fiberglass and must meet two important criteria: 1) the opening in the mesh must be small enough to keep bugs from crawling in, and 2) the screen must have enough open area to allow the free passage of air and smoke. The percentage of open area is a function of the mesh wire diameter and the spacing between wires. Back to Screening