October 18, 2012

Smoke Detectors: Which Type is Best? The Science Behind the Technology

Recently there has been a debate regarding the effectiveness of different types of household smoke alarms. Many common residential smoke alarms look similar on the outside but employ very different technology inside to detect smoke particulates and the by-products of combustion. Knowing the best choice for your home will ensure an alarm sounds, without delay.

The technology in question involves the two most commonly available smoke alarm types: Ionization and Photoelectric.

During combustion, microscopic particles and aerosols are produced. The ionization detector uses a tiny amount of radioactive material, usually americium, to ionize air molecules as they enter a chamber within the detector. The chamber contains positive and negative electrodes through which a small, constant electric current flows. When these invisible by-products of combustion enter the chamber, they become ionized and attach themselves to the electrodes altering the flow of electrical current and triggering the alarm to sound. Some would say that this detects ions not smoke; that’s true.

The photoelectric detector contains a chamber equipped with a light source and a photocell receiver. There are several types of photoelectric detectors.

One design utilized is the beam application type. In this type the beam is constantly focused on the photocell which generates a current and keeps a switch open during normal operation.

In a refractory type the light does not strike the photocell during normal operation. In each case, when visible particles of combustion (smoke) enter the chamber, the beam is disrupted.

In the beam application type, the smoke blocks the beam and the switch closes, sounding the alarm. In the refractory type the beam is refracted or scattered by the smoke within the chamber and strikes the photocell generating a current and activating the switch. Some will say that you need “visible” smoke to trigger this type of unit, and invisible particles can be more deadly; that’s also true.

The debate still continues as to which type is better, you will hear experts talk about ionization devices responding faster to “flaming” fires, such as kitchen fires, fireplace fires, and set fires, but in general an uncontrolled fire during the incipient stage will be detected quickly by an ionization type model. Some people believe that these types of fires occur when one is awake, negating the need for an alarm because the person who started the unexpected fire will be immediately aware of the emergency. There is evidence that photoelectric detectors respond faster to “smoldering” fires, someone falling asleep while smoking, soldering iron left on a combustible, or some relatively low heat producing device in an area allowing the heat to “build” over time. The logic that this can occur when a person is unaware and there is a need to alert sleeping individuals is certainly valid. There is also research that they do not have a significant delay during flaming fires.  

In my home I have dual technology models that employ both ionization and photoelectric detection. I have them located in places where combustion by-products will reach them quickly in likely scenarios, and in sleeping areas. There are some jurisdictions that mandate that a photoelectric unit is installed in all residential occupancies.

There are models today that incorporate carbon monoxide (CO) detectors and heat detectors as well, I believe these would be even better if the detector will sound the alarm when of any of these by-products of combustion are present. Until smoke alarms that employ all the technologies are commonplace the debate will continue to arise. In residential occupancies there are an infinite number of possible scenarios.

Remember that having an emergency plan for all members of your family, including several means of escape, will help people get away from danger quickly and save more lives than a complete reliance on any technology.

The author is a Fire Protection Engineer with expertise in Arson Investigation and Domestic and International Building Codes. See his full resume here. He is an experienced expert witness, also.