Light Obscuration and Light Scattering Smoke Detectors
Obscuration Smoke Detectors
A diagrammatic arrangement of an obscuration smoke detector is shown in figure above. An infrared light beam, at an operating frequency of 1000 pulses per second, is received by a photoelectric cell and analysed. In a fire, smoke rises and spreads below ceiling level and the intensity of the light falling on the receiver is reduced due to light scattering or absorption. The fire alarm sounds when the signal strength is reduced to between 40 - 90 % for a period of about 5 seconds. Using a pulsed light source saves power and allows the use of a receiver turned to the pulse frequency to reduce false alarm caused by sunlight or illumination. Such detectors are suitable for covering large areas with flat ceilings. They are not suitable for outside use.
Light Scattering Smoke Detectors
When the sizes of small particles are greater than the wave length of the incident radiation, the light is scattered in different directions. Known as the 'Tyndall Effect', named after its discoverer, this is used for smoke in several different arrangements, however, all depend on light being scattered by smoke onto a photoelectric cell that is obscured from the light beam in normal circumstances.
In the arrangement shown above, the detector is placed in line with the pulsed infrared light source. In the absence of smoke, the masking disk casts a shadow over the circular region of diameter 'AB' and prevents light from emitting diode from falling onto the detecting photocell. In the presence of smoke, light is scattered and some of it falls onto the photocell. The evaluation circuit is arranged to trigger an alarm if the threshold value is exceeded for a predetermined number of consecutive pulses.
"Fire Safety at Sea" by Dr. James Cowley