Working Principle of Smoke Detector

Basic Working Principle of Ionized and Photoelectric Smoke Detectors for Fire Alarm System

Smoke Detector is the main sensing device in Fire Alarm Equipment System. To understand the basic working principle of smoke detector, we should remember Ionization and Photoelectric structure, smoke detectors are manufactured basically based on these two principle. According to these rules there are mainly two basic types of Smoke Detector, you will find in the market.

  • Ionization Smoke Detector
  • Photoelectric Smoke Detector

How Ionization Smoke Detector Work

Working Principle of Ionized Smoke Detector
Fig: How Ionized Smoke Detector Works

In ionized smoke detector, basically two positive and negative charged plates are consisted in an ionization chamber wherein a radioactive material like americium 241 is placed, shown in figure-1.1.

Air molecules collide with radioactive particles and extricate their electrons. Since the molecules lose electrons, they become positively charged ions; the same as other molecules gain electrons, they become negatively charged ions. Equal numbers of positive and negative ions are created, shown in figure 1.2; the positively and negatively charged ions are attracted by negatively and positively charged plate respectively.  

As shown in figure 1.3, larger smoke particle colliding and combining with positively and negatively ionized air molecules become causes of reduction total charged particle. This reduction in the ionized particles decreases the chamber monitoring current what is sensed by electronic circuit and generate alarm signal.

To get better result due to affect of humidity and atmospheric pressure, a modified system is developed adding a reference chamber, shown in figure 1.4 & 1.5.


How Photoelectric Smoke Detector Work

Working Principle of Photoelectric Smoke Detector
Fig: How Photoelectric Smoke Detector Works

Smoke particle can scatter or obscure the light beam between Light Emitting and Light Sensing devices, shown in figure 2, since smoke produced by a fire affects the intensity of a light beam passing through air.

In figure 2.1, light beam from LED not reaching to photo sensing device; but in figure 2.2, when smoke presence the light beam scattered to photo sensing device causes to detector respond and fire alarm generate.

In figure 2.3, light beam reaching directly the photo sensing device without any obstruction; but in figure 2.4, when smoke particle obstructing the beam, no light beam reaching the light sensing device causes to detector respond and fire alarm generate.


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