SMOKE DETECTORS

As the name implies, these devices are designed to identify a fire while in its smoldering or early flame stages. Smoke detectors operate on either an ionization or photoelectric principle, with each type having advantages in different applications.

The result of full-scale fire tests, using typical fires in family living units, have shown that detectable quantities of smoke precede detectable
levels of heat in nearly all cases. Thus fire alarm system designers use smoke detectors more extensively today. The common operating characteristics
of smoke detectors include the ionization spot-type smoke detector, the photoelectric spot-type smoke detector, liner beam-type
smoke detector, the air-sampling smoke detector and the duct-type smoke detector.

Photoelectric Detectors:
  • Photoelectric smoke detectors use light and how it is reflected to detect smoke. Normally light is projected into a smoke sensing chamber inside the detector assembly. The light hits a black background of the chamber and is absorbed. When enough smoke enters the chamber it reflects the light on to a sensor inside the chamber. This causes the sensor to indicate an alarm. Photoelectric detectors are suitable for most applications giving the fastest response to slow burning fires – the most common start to fire events. Use of photoelectric detectors is highly recommended to provide coverage for escape routes due to their superior ability to detect optically dense smoke that would easily obstruct the use of escape routes.
Ionization Detectors:

Ionization detectors were the first type of detector to be commercially developed and are also a popular choice. These generally contain two chambers. One is used as a reference to compensate for changes in ambient temperature, humidity or pressure. The second contains a radioactive source, usually alpha particle, which ionizes the air passing through the chamber where a current flows between two electrodes. When even invisible smoke enters the chamber and it disrupts the flow of current and generates an alarm.

An ionization smoke detector has a small amount of radioactive material that ionizes the air in the sensing chamber, thus rendering it conductive and permitting a current flow through the air between two charged electrodes. When smoke particles enter the chamber, they attach themselves to the ionized air molecules and decrease the conductivity between the electrodes. This decrease in conductivity can be measured by an electronic circuit that initiates a fire alarm signal when the reduction in conductivity reaches a pre-set threshold.

  • •Detect minute particles, aerosols produced during combustion
  • •Use a tiny amount of radioactive material to ionize air molecules as they enter chamber within detector
  • •Respond satisfactorily to most fires
  • •Respond faster to flaming fires than smoldering ones
Air Duct-type Smoke Detector:

Detects smoke for the primary purpose of controlling the propagation of smoke through the heating, ventilation and air conditioning system (HVAC). This helps prevent possible panic and damage from distribution of smoke and gaseous products. These detectors only detect smoke
when smoke is circulation in the duct. They sample a small amount of great volumes of air from large areas of coverage. Air duct smoke detectors are not a substitute for:

  • Area smoke detection
  • • Early warning
  • • A building’s regular fire detection system