Older type duct detectors used the _.
a) Light scattering principle
b) Light obscuration principle
c) Smoke rate compensation principle
d) Ionization principle
The Correct Answer and Explanation is :
The correct answer is b) Light obscuration principle.
Explanation:
Older types of duct detectors used the light obscuration principle to detect the presence of smoke in air ducts. The principle behind this method is simple: it involves shining a light source, typically a light-emitting diode (LED) or a lamp, through a sensing chamber. The light travels through the air stream in the duct, and if there is smoke present, the particles in the smoke scatter or absorb the light, reducing the amount of light that reaches the sensor.
The detector’s sensor, typically a photodetector (like a photodiode or a phototransistor), measures the reduction in light intensity. This reduction in light is proportional to the amount of smoke in the air. As the smoke concentration increases, the more light is obscured, and the sensor sends a signal indicating the presence of smoke. The detector is designed to trigger an alarm when the light obscuration reaches a predefined threshold, signaling a potential fire or other hazardous situation.
This method is highly effective in detecting visible smoke, and since it measures the reduction of light, it is well-suited for environments where smoke particles can accumulate, such as in air ducts where airflow and smoke conditions may change. However, the light obscuration principle can be more sensitive to dust and dirt that may accumulate in ducts, potentially leading to false alarms.
In contrast, newer detectors might use the light scattering principle (choice a), which involves measuring the scattering of light at different angles and is often found in advanced smoke detectors. Ionization principles (choice d) are used in ionization-based smoke detectors but are more common in residential and commercial fire alarms, not typically in duct smoke detection. The smoke rate compensation principle (choice c) is not a common method in duct detectors.
Thus, older duct smoke detectors typically operated on the light obscuration principle.