What principle do air duct smoke detectors operate upon?

Air duct smoke detectors primarily operate on the photoelectric principle. This method involves using a light source and a light sensor. In a typical setup, a beam of light is emitted and is directed either at a photoelectric sensor or within the detector's housing. When smoke particles enter the duct and disrupt this light beam, the change in light intensity is detected by the sensor, triggering the alarm.

Photoelectric smoke detectors are particularly effective in air ducts because the presence of smoke can be detected early as it moves through the airflow, allowing these detectors to respond promptly to the presence of fire. This principle is adept at sensing smoldering fires, which often produce larger particles that scatter light more effectively than more rapid-flame fires.

This understanding of the detection principle underscores why photoelectric detection is favored in air ducts, as it provides a reliable method for smoke detection in environments where airflow could hinder the performance of other types of detectors.