Smoke Detector types and their working principles

The safety device which detects presence of smoke in air or environment is called smoke detector. It may also indicate presence of fire which is about to spread. It is installed in home and office premises as fire safety devices. It is very useful to provide early alerts to occupants which helps them to take appropriate actions as per condition of the fire such as evacuating the place or dousing the fire. There are different types of smoke detectors based on their respective working principles such as Photoelectric, Ionization, Laser based, Aspiration, Air Sampling detector etc.

What is a Photoelectric smoke detector ?

This smoke detector has two components viz. light source and photosensitive sensor. Infrared LED is used as light source typically. When particles of smoke enter the space between light source and sensor, it will scatter the light between them. This change in light intensity is detected by the sensor and based on which it triggers the alarm. The advantages of photoelectric smoke detector are their effective to detect slow and smoldering fires and their to detect avoid alarms by differentiating cooking smoke and steam.

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

Photoelectric smoke detectors are of two types based on the inner designs. They are light obscuration and light scattering.

The diagram shows 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 either blocked by the smoke particles or its intensity is reduced to greater extent. This is detected and 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 diagram 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.

What is an Ionization smoke detector ?

These detectors use a small amount of radioactive material such as Americium-241 to ionize the air inside the detection chamber. When smoke particles enter the chamber, they disrupt the ionization process. This results into drop in the electrical current, which triggers the alarm. The advantages of Ionization smoke detector are their quick response to fast flaming fires and lower cost.

ionization smoke detector
Figure-3: Ionization Smoke Detector

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 & ionization smoke detectors

Following table summarize important comparison between photoelectric and ionization smoke detector type.

Specifications photoelectric smoke detector ionization 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. These 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.

Conclusion: As we know, Ionization smoke detectors are quick to respond to fast-burning fires while photoelectric smoke detectors are better at detecting slow smoldering fires. Hence smoke detectors involving both photoelectric and ionization sensors are more reliable, effective and most efficient as smoke detection system. Modern smoke detectors house both of these sensors to leverage benefits of both.

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