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Smoke Detector basics | Smoke Detector types

This article covers smoke detector basics and smoke detector types.It compares photoelectric smoke detector vs ionisation smoke detector and mentions difference between photoelectric smoke detector and ionisation smoke detector. It also describes laser based smoke detector,Aspiration smoke detector and Air sampling detectors.

As we know residential fires have become a burning topic now-a-days. Most of the lives are lost due to residential fires. This can be prevented by installing smoke alarm and fire alarm systems in the residential and official premises. Smoke detectors and heat detectors are used in these smoke alarm and fire alarm systems.

Definition: The type of detector which operates based on detection of smoke and consecutively generates alarm or signal is known as smoke detector. Following are the types of smoke detectors.
• Photoelectric smoke detector
• Ionisation smoke detector
• Laser based smoke detector
• Aspiration smoke detector
• Air Sampling detector

Photoelectric smoke detector

photoelectric smoke detector-light obscuration type
Figure-1: photoelectric smoke detector light obscuration type

Photoelectric smoke detector works in the fire condition as a result of long period of smoldering. There are two types of this smoke detector based on inner design viz. light obscuration and light scattering.

The figure-1 depicts inner design of photoelectric smoke detector of light obscuration type. As shown it consists of source and detector LEDs and sensing chamber or space. In the normal state, light transmitted by source LED is received and detected by detector LED. In the smoky fire state condition, light is blocked by the smoke particles. This results in reduction in the amount of light received by detector circuit. This is monitored by the circuit and when particular threshold is crossed, an alarm is generated.

photoelectric smoke detector-light scattering type
Figure-2: photoelectric smoke detector light scattering type

The figure-2 depicts inner design of photoelectric smoke detector of light scattering type. In this type of smoke detector, light is transmitted in the area which is normally not seen by the detector LED. Hence in normal condition there will be no light received by the detector circuit. In the smoky condition, light is being scattered by the smoke particles and hence light gets reflected. This light is received and detected by the detector circuit and consecutively alarm is triggered.

Ionisation smoke detector

ionisation smoke detector
Figure-3: Ionisation Smoke Detector

The figure-3 depicts the Ionisation smoke detector. As shown it is constructed by placing small radioactive source or component between reference chamber and sensing chamber. Americium-241 is used as radionuclide in this type of smoke detector. The radioactive material is placed between two electrically charged plates.

Normally, current is stable in sensing chamber. In smoky condition, smoke enters in the sensing chamber and this results into disruption in flow of ions between the plates. This results into reduction in the flow of current. This is being detected by the circuit and alarm is being triggered.

Difference between photoelectric smoke detector and ionisation smoke detector

Following table-1 summarize important difference between photoelectric smoke detector and ionisation smoke detector.

Specifications photoelectric smoke detector ionisation smoke detector
Triggering source smoldering fire flaming fire
Detection sensitivity slow smoldering fire in 0.4 to 10 micron fast flaming fire in 0.01 to 0.4 micron range
Obscuration rating 2% obs/ft to 4% obs/ft 0.8% obs/ft to 1.5% obs/ft

Laser based smoke detector

This type of smoke detector is used in the places where early warnings of fire is needed. They are designed to detect early signs of fire. They are ultra-sensitive. Laser based smoke detectors have very low sensitivity which is hundred times better than standard smoke detectors. They have obscuration range of about 0.02% obs/ft to 2% obs/ft.

Aspiration smoke detector and Air sampling Detector

This type uses pipe and fan based system in order to draw smoke particles into the detection system. They have obscuration range of about 0.0015% obs/ft to 6.25% obs/ft.

Most of the air sampling detectors are of aspiration type. In this type, small bore pipes are installed in parallel to the ceiling. This should be installed in the area to be protected. Small holes are being drilled in the pipes. This detectors work by drawing air samples into optical device which is tuned to detect tiny particles as a result os combustion. Solid state laser is also often used in this detector.

There are some smoke detectors which are based on carbon-monoxide sensor or carbon-dioxide sensor. These are used in extreme combustion conditions.

Basics and types of Heat Detectors

Also refer article on basics and types of heat detectors which covers fixed temperature and rate of rise type of heat detectors.

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