An Overview of Smoke Detectors

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It is now common practice to install fire alarms into households, as well as in commercial and industrial environments for the safety of the occupants. The type of fire detector used is dependent upon these settings.

For example, smoke alarms in a standard household indicate the presence of smoke by a flashing light or an alarm. Whereas, smoke alarms in industrial settings will connect a signal to a fire alarm system and usually the emergency services.

In Britain from 2010 to 2011, the fire and rescue service attended 624,000 fires and false alarms - a five percent decrease compared to 2009-2010. Research has also shown that the introduction of smoke alarms into households rose from 8% in 1988 to 86% in 2008; a significant increase that could explain the drop in the number of fire-rescue cases between 2009 and 2011 in Britain.

A typical household smoke detector is disc-shaped and powered by a disposable battery. There are two types of smoke detectors - photoelectric and ionizing.

Photoelectric Smoke Detectors

A photoelectric smoke detector is a light detector that uses a light beam and electrical photocells (photodiodes) to track smoke particles. The light beam (made of an incandescent bulb or infrared LED light that helps to collect the light source into a beam) in the smoke detector will reflect onto the photocells. The photocells are arranged at a right-angle to the beam of light so that light entering the chamber does not typically attract the photocells.

When smoke particles enter the optical chamber, these particles interfere with the light beam (i.e., the lights reflects off of the smoke particles) and then make contact with the electrical photocells. This contact increases the electrical charge in the detector to a threshold level, which initiates an alarm signal.

Photoelectric smoke detectors are typically installed in large commercial and social settings, including gymnasiums and auditoriums.

Advantages

• Ideal for detecting dense smoke
• Less prone to false alarms from cooking fumes or shower steam
• Do not contain radioactive materials, making them safer for use

Disadvantages

• Sensitive to dust particles and insects, meaning that regular maintenance is needed
• Expensive to maintain
• Require more current to operate (they are typically wired to a 110-volt power source)

The light beam in a typical photoelectric smoke detector can be obscured in two ways:

1. The light-obscuring method
   Dense smoke tends to only obscure part of the light beam, whereas less-dense smoke particles can move around more freely, obscuring more of the light beam and therefore, impacting the alarm system. In some instances, a mirror is used to position the light beam to measure the area of coverage of the smoke particles.
    
2. The light-scattering principle
   A light scattering principle is based on the idea that smoke particles entering the sensing chamber strike the light beam and scatter the light onto photoelectric particles.

Ionizing Smoke Detector

Ionizing smoke detectors use radioisotopes, such as americium-241, which ionize smoke particles in the air. The functionality principle of the radioactive isotope americium-241 involves the emission of alpha particles, which ionize air particles in the sensing chamber so that they conduct electricity.

During the emission of the alpha particles into the air, a chemical reaction occurs where air molecules in the collecting chamber are ionized and, as a result, carry an electrical charge. Smoke particles surrounding the ionizing smoke detector enter the collecting chamber and attach to the ions, making it more difficult for the ions to deliver an electrical charge.

A decrease in the electrical flow is detected by a circuit that then triggers an alarm system. This type of smoke detector is ideal for detecting invisible particles produced by fires, and so will work much faster and be more sensitive compared to a photoelectric detector.

The following video demonstrates exactly how the ionizing smoke alarm works.
 

Alpha particles are used for the ionizing smoke detector as they have high ionizing potential. Although there are concerns that this type of smoke detector emits radiation, the alpha particles used have low penetrative potential and so will not pass through plastic material.

The standard amount of americium-241 used in the ionizing smoke detector is not enough to raise regulatory concerns – a standard amount of radioactive americium-241 is made up of 37KBq or 1ìCi, which is enough to detect smoke particles without contaminating the outer surface of the detector.

Prior to being sold on the market, ionizing smoke detectors go through a radiation safety analysis to meet safety requirements. Compared to photoelectric smoke detectors, the ionizing smoke sensor is less expensive and more commonly used.

Advantages

• Cheaper than photoelectric smoke sensors
• Ionizing smoke alarms outperform photoelectric smoke alarms when detecting fast flaming fires with little visible smoke

Disadvantages

• Very sensitive, which can lead to false alarms as a product of cooking
• Not as responsive to smoldering fires - they are minutes slower than photoelectric sensors in detecting smoke particles from smoldering fires
• Use of radioactive material is a concern

In summary, it must be stressed that, despite the functional differences and levels of sensitivity between photoelectric and ionizing smoke detectors, both devices are acceptable as life-saving smoke sensors.

References and Further Reading

• Kubba, S. (2010). LEED. Practices, certification, and accreditation handbook. Oxford: Elsevier Inc. Pages369–372.
• Lucht, D.A., McCormack D.M., Strother, R., Clark, J.E., Schaenman, P.S., Lyons, J.W. Center for Fire Research, National Bureau of Standards (1977). Smoke Detector Technology. U.S Department of Commerce. National Fire Prevention and Control Administration. Public Education Office. Pages 5–8.
• Jenaway, W. Association of Fire Chief International. (2012). Fire Inspector: Principles and Practices. Bristol, UK: Jones & Bartlett Learning International. Pages 153–155.
• Cote, A.E., Bugbee, P. (1988). Principles of Fire Protection. United States of American: National Fire Protection Association. Pages 248–251.
• Smoke Alarms Installation and Maintenance. Information for Retail and Industry. Queensland Government. Department of Emergency Services.
• Structure Tech. Ionization vs. Photoelectric Smoke Alarms.
• Fire Statistics Great Britain 2010-2011 – national statistics. Communities and Local Government.

This article was updated on the 26^th July, 2019.