How Do Smoke Alarms Detect Smoke?
Ionization Smoke Detectors
The most common type of smoke detector is the ionization model, which uses a tiny ionization chamber to detect the presence of smoke. The ionization chamber is flanked on opposite ends with metal conductors through which a live electrical current flows, powered either by 120 volt home wiring or a battery. The chamber also contains a trace amount of a radioactive substance called americium-241. This substance sheds radioactive alpha particles that ionize the oxygen and nitrogen atoms present inside the chamber.
When ionization occurs, free electrons are cut loose from their oxygen and nitrogen atoms and receive negative charges. The negative free electrons become attracted to the positive conductor in the chamber, while the other positively charged particles get attracted to the negative conductor. This clears the inside of the chamber and strengthens the electrical current between the conductors.
As smoke seeps into the ionization chamber through vents, the smoke particles attach themselves to the free electrons and reverse the ionization process. This pulls particles away from the conductors, creating a sharp drop in current that the device can detect. When a large enough decrease occurs, an alarm sounds.
Photoelectric Smoke Detectors
While not as common as ionization detectors, photoelectric smoke detectors are used in many homes, businesses, factories and public facilities. They rely on photo beam detection systems, which is the same technology used in modern motion sensing equipment. Photoelectric smoke detectors usually contain a single low-wattage laser light and a photo detection lens, which reads light levels. In motion detectors, these two components are usually positioned in direct opposition to one another, but in photoelectric detectors they are positioned perpendicular to one another. This is because the affordable photo detection equipment used in these devices is not sensitive enough to detect the minuscule light beam interruptions caused by tiny smoke particles. Instead, they're used to detect the minuscule presence of light.
When the photo detection lens is pointed perpendicular to the laser beam in a smoke-free environment, no light can reach the lens. But as soon as smoke particles begin to fill the detector's cavity, the light from the laser beam starts reflecting off of them in every direction. Some of the light will be reflected off of the smoke and over to the photo detection lens, and an alarm is triggered as soon as this detection is made.
Effective Use of Smoke Detectors
In order for a smoke detector to be effective, it must be installed on the ceiling of a room or high on a wall. Smoke rises, and smoke detectors near the floor will often not detect a measurable amount of smoke until it was too late. There should be at least one smoke detector installed on every floor of every home or building, and they should be installed within earshot of every occupied bedroom.
Home safety authorities urge smoke alarm owners to test their alarms every month and to replace the batteries twice each year. Daylight savings time reminders are often accompanied with reminders to replace smoke alarm batteries.
Tags: photo detection, detection lens, free electrons, ionization chamber, photo detection lens
