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
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| 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
Sponsored: |
| 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.
