CHIEF ENGINEER Unit 18 Fire Q5 Answer

  1. Describe, with the aid of sketches, the difference between an obscuration type smoke detector and a light scatter type smoke detector.(10)

Obscuration Type Smoke Detector

  • Principle: Operates on the reduction of light intensity when smoke passes between a light source (lamp/LED) and a photoelectric cell in a straight line.
  • Arrangement:
    • Light beam projects across a chamber to a photocell.
    • In clear air, full light reaches the cell → no alarm.
    • When smoke particles enter, they obscure or block the beam, reducing light intensity on the sensor.
  • Trigger: Alarm activates when the received light intensity falls below a set threshold.
  • Best suited for: Open areas where dense smoke is expected (e.g. cargo holds, machinery spaces).

Sketch idea:

  • Rectangular chamber.
  • Light source (LED) on one side.
  • Photodiode/photocell directly opposite.
  • Smoke cloud shown between them, blocking light beam.

Light Scatter Type Smoke Detector

  • Principle: Uses the Tyndall effect (light scattering). Smoke particles entering the chamber scatter light from the source in multiple directions.
  • Arrangement:
    • Light source (LED) projects across the chamber.
    • A photodiode sensor is placed at an angle (not in direct line).
    • In clear air, sensor receives no light.
    • When smoke enters, particles scatter the beam, and light is deflected onto the sensor.
  • Trigger: Alarm sounds when scattered light exceeds a threshold.
  • Best suited for: Early detection of light/incipient smoke (e.g. electronic spaces, control rooms).

Sketch idea:

  • Circular/rectangular chamber.
  • LED on one side, beam passing across chamber.
  • Sensor mounted at ~90° angle to beam.
  • Smoke cloud drawn in, deflecting light onto the sensor.

Key Differences

  1. Detection principle:
    • Obscuration: Reduction in transmitted light.
    • Scatter: Increase in scattered light.
  2. Sensor placement:
    • Obscuration: Directly in line with source.
    • Scatter: At an angle to the source.
  3. Smoke type sensitivity:
    • Obscuration: More effective with dense smoke.
    • Scatter: More effective with light/incipient smoke.