Smoke detectors testing and supply

Smoke detectors testing and supply

Smoke detectors are used in confined areas where smoke can be easily detected. When used with CO2 or Halon extinguishing systems, the detectors are mixed zoned types. Usually one is an ionisation type and the other one is a photoelectric type.

Ionization smoke detectors

The detectors are sensitive to visible and invisible smoke particles produced during the incipient stages of a developing fire. They are suitable for use as early warning fire detectors in a wide range of industrial,commercial and residential applications.

Operating Principle

The detector contains an ionisation chamber consisting of a small Americium 241 radioactive source mounted between two metal electrodes to which a voltage is applied. The alpha particles emitted by the americium ionise the air into positive and negative ions which separate and travel to electrodes of opposite sign. Some ions collide and recombine. But the net result is the flow of a small electrical current, in the order of 2 X (10)-11 amps, between the electrodes. When smoke particles enter the chamber ions become attached to them. As the particles are very much larger than the ions and collide more frequently, an increase in the rate of ion recombination and a reduction in the chamber current occurs. This reduction in current can be used to monitor smoke concentration. In practice it is more practical to monitor the voltage change necessary to maintain an essentially constant current through the ionisation chamber. This is achieved in the detector by connecting in series with the smoke sensing chamber an almost sealed reference ionisation chamber containing clear air, see Figure 4.If the voltage across the two chambers is held constant, the voltage appearing across the sensing chamber increases when smoke enters the detector. A field effect transistor (MOSFET) monitors the voltage and initiates an alarm when a pre-set level is exceeded. The use of an almost sealed reference chamber provides a degree of compensation for changes in sensing chamber current due to changes in atmospheric pressure, temperature and humidity

Photo electric “optical” smoke Detectors

Descrlption

Photoelectric Smoke Detector consists of a detector head and base.

The head Is a solid-state device which has a high reliability light sensor (silicon photodiode) and a pulse signal processor (LED). The base contains an LED for alarm verification, screw terminals, and bifurcated contacts. When lit the LED Indicates the smoke detector has latched into an alarm condition.

The detector head twist locks Into the base and Is secured by a special locking screw.

The detector head can be inserted or removed for inspections and cleaning without effecting wiring connections.

 Principle of Operation

The Photoelectric Smoke Detector incorporates the light scattering principle within its solid- state sensing chamber. This allows the detector to sample optimum products of combustion (smoke) entering from any direction while rejecting ambient light levels.

In its non-activated state, the beam generated by the pulsating light source is not seen by the light sensor, as it is positioned out of the direct path of the beam.

In its pre-activated state, smoke will enter the sensing chamber causing the light source beam to deflect across the light sensor. This deflection must persist for a preset period of time to be recognised as an alarm condition.

The detector latches into an activated state when the smoke concentration exceeds the calibration point and is verified by the detector time delay circuit. The detector will then transmit an electrical signal to the control unit producing an alarm condition.