Five types of Smoke Detector!

Recently I updated our internal training module titled "Smoke Detectors 101". These training modules are available only to Maintenance Essentials employees and official Partners.

Each of these training modules are bite sized and designed to be easily read in about 30 minutes.

This article is an extract from this training module...

What is smoke?

Smoke is the collection of airborne solid and liquid particulates and gases emitted when a material undergoes combustion, mixed with the quantity of air that is mixed into the particulate mass.

Smoke particulates are generally grouped in three particle sizes. Depending on particle size, smoke can be visible or invisible to the naked eye.

The five types of smoke detector...

1. Ionisation smoke detector...

An Ionization smoke detector consists of the following main components;

  • a radioactive source, usually Americium-241 that emits alpha particles
  • a ionization chamber, an air-filled space between two electrodes

The alpha particles pass through the ionization chamber, and permit a small, constant electric current between the electrodes.

Smoke particles that enter the chamber absorbs the alpha particles, which interrupts the electric current, activating an alarm.

2. Photo-electric smoke detector...

An photo-electric smoke detector is a scattered light sensor or nephelometer, that consists of the following main components;

  • a light source
  • a lens to focus light into a projected beam
  • a sensor at an angle to the beam as a light sensor

In the absence of smoke, light passes in front of the sensor in a straight line.

When smoke enters the optical chamber across the path of the light beam, some light is scattered by the smoke particles, directing it at the sensor and thus triggering the alarm.

3. Projected beam smoke detector...

An projected beam optical smoke detector operates on the principle of light obscuration and comprises;

  • A projected beam transmitter & lens
  • A light receiver
  • A light reflector (not in all cases)

The light transmitter emits an invisible beam of light that is received by a receiver in a normal condition.

The receiver is calibrated to a preset sensitivity level based on a percentage of total obscuration. When smoke obscures the beam, an alarm signal is activated.

4. Aspirating smoke detector...

An aspirating smoke detector is a very sensitive light sensor or nephelometer, that works by actively drawing a sample of air and other contaminates through a pipe network into a sensing chamber and consists of the following main components;

  • a network of small-bore pipes
  • a particulate filter
  • a sensing chamber
  • a focused light source
  • a sensitive light receiver

When smoke enters the sensing chamber across the path of the light beam, some light is scattered or obscured by the smoke particles, which is detected by the sensor. The output is analogue and can trigger multiple alarms.

5. Video smoke detection...

Video Smoke Detection (VSD) is based on the computer analysis of video images provided by standard video (CCTV) cameras.

A video smoke detection system comprises the following components;

  • one or more video cameras
  • a computer
  • software to analyse the video signal

The computer uses specialised software to identify the unique motion and pattern of smoke. This unique signal when identified triggers an alarm.

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