Smoke Alarms FAQ
What should I know about Smoke Alarms?
The primary thing you should know about smoke alarms is that there are two basic types: Ionization and photoelectric smoke alarms.
Ionization alarms sound more quickly when a flaming, fast moving fire occurs. Think of a candle catching a curtain on fire- it happens quickly. A Photoelectric alarm is quicker at sensing a slower, smoky fire- such as an electrical fire that starts within your walls.
A dual sensor smoke alarm combines the two types, photoelectric and ionization.
Because there is no way of telling what type of fire may occur in your home, it is strongly recommended by the USFA to have both an ionization smoke alarm and photoelectric alarm- or a dual sensor smoke alarm to detect both types of fires.
For those with hearing disabilities, First Alert also has smoke alarms that utilize flashing strobe lights and vibrations to alert them to danger.
How Do Smoke Alarms Work?
Smoke alarms are one of those amazing inventions that, because of mass production, cost practically nothing. You can buy a general use smoke alarm for $10. And while they cost very little, smoke detectors save thousands of lives each year. In fact, it is recommended that every home have one smoke detector per floor.
Photoelectric Smoke Detectors
Occasionally, you will walk into a store and a bell will go off as you cross the threshold. If you look, you will often notice that a photo beam detector is being used. Near the door on one side of the store is a light (either a white light and a lens or a low-power laser), and on the other side is a photodetector that can "see" the light.
There would have to be a LOT of smoke before the alarm would go off -- the smoke would have to be thick enough to completely block out the light. It takes quite a bit of smoke to do that.
In the normal case, the light from the light source on the left shoots straight across and misses the sensor. When smoke enters the chamber, however, the smoke particles scatter the light and some amount of light hits the sensor:
The sensor then sets off the horn in the smoke detector.
Ionization Smoke Alarms: Ionizing Radiation
Ionization smoke detectors use an ionization chamber and a source of ionizing radiation to detect smoke. This type of smoke detector is more common because it is inexpensive and better at detecting the smaller amounts of smoke produced by flaming fires.
Inside an ionization detector is a small amount (perhaps 1/5000th of a gram) of americium-241. The radioactive element americium has a half-life of 432 years, and is a good source of alpha particles.
Another way to talk about the amount of americium in the detector is to say that a typical detector contains 0.9 microcurie of americium-241. A curie is a unit of measure for nuclear material. If you are holding a curie of something in your hand, you are holding an amount of material that undergoes 37,000,000,000 nuclear transformations per second. Generally, that means that 37 billion atoms in the sample are decaying and emitting a particle of nuclear radiation (such as an alpha particle) per second. One gram of the element radium generates approximately 1 curie of activity (Marie Curie, the woman after whom the curie is named, did much of her research using radium).
Ionization Detectors: Ionization Chamber
An ionization chamber is very simple. It consists of two plates with a voltage across them, along with a radioactive source of ionizing radiation, like this:
The alpha particles generated by the americium have the following property: They ionize the oxygen and nitrogen atoms of the air in the chamber. To "ionize" means to "knock an electron off of." When you knock an electron off of an atom, you end up with a free electron (with a negative charge) and an atom missing one electron (with a positive charge). The negative electron is attracted to the plate with a positive voltage, and the positive atom is attracted to the plate with a negative voltage (opposites attract, just like with magnets). The electronics in the smoke detector sense the small amount of electrical current that these electrons and ions moving toward the plates represent.
When smoke enters the ionization chamber, it disrupts this current -- the smoke particles attach to the ions and neutralize them. The smoke detector senses the drop in current between the plates and sets off the horn.
Speaking of alarms, whenever the words "nuclear radiation" are used an alarm goes off in many people's minds. The amount of radiation in a smoke detector is extremely small. It is also predominantly alpha radiation. Alpha radiation cannot penetrate a sheet of paper, and it is blocked by several centimeters of air. The americium in the smoke detector could only pose a danger if you were to inhale it. Therefore, you do not want to be playing with the americium in a smoke detector, poking at it, or disturbing it in any way, because you don't want it to become airborne.
Now, let's take a look inside an ionization smoke alarm.
Inside an Ionization Smoke Alarm
Here's what the outside of a typical smoke detector looks like.
When you take off the cover, you find that a smoke detector is pretty simple. This one consists of a printed circuit board, an ionization chamber (the silver cylinder toward the top right in the following picture) and an electronic horn (the brass cylinder toward the bottom right in the following picture):
Here is a close-up of the board:
and the underside of the board:
The ionization chamber is an aluminum can containing the ionization source. You can see that the can has slots in it to allow air flow. The can itself acts as the negative plate of the ionization chamber.
Underneath the can is a ceramic holder that contains the positive plate of the ionization chamber. Under that plate is the ionization source, WHICH YOU WANT TO BE SURE NOT TO DISTURB.
Battery operated dual ionization and photoelectric sensor smoke alarm offers maximum protection from both primary types of fires: flaming and smoldering. Remote mute silences nuisance alarms from cooking smoke or shower steam by pressing any button on a standard household remote control (remote not included). Single button also tests alarm functions. Easy access battery drawer: no need to remove alarm from ceiling to change battery. 10-year limited warranty. Smoke alarm meets UL standards.
This smoke & fire alarm is for you if:
This battery operated ionization smoke alarm comes with a long-life lithium battery, which is warranted for 10 years--no need to change the battery for the life of the alarm. This is convenient if your property is a rental and you do not trust your tenants to change the battery annually, or if you tend to be forgetful at replacing your own! A mute feature will silence the alarm- just in case you have burnt dinner or are taking a steamy shower.
10-Year Battery Smoke Alarm Features:
The 120VAC hardwired smoke alarm wires directly into your homes electrical system and protects against ionization smoke. This has a battery backup and mute button. No need to worry about alarm failure during a power outage- the battery provides assurance of coverage, even if the lights are out! This unit also has the easy access battery door and mute feature.
This is the standard smoke alarm found all over the country and features an ionization smoke sensor. Battery operated; it features a mute button, test function and is reliable.
This is the right alarm for you if:
This unit is the only smoke detector that features an ionization and photoelectric sensor! This dual alarm protects you from slow, smoldering fires as well as fast fires. The battery operated dual ionization and photoelectric sensor smoke alarm offers protection from both types of fires. This unit also features a remote control feature (not included) to mute nuisance alarms from cooking smoke or shower steam by pressing any button on a standard household remote control. This alarm also has ‘Smart technology’ to tell the difference between cooking smoke or shower steam and true fires.
This smoke alarm features a light for escape during a fire. Heavy smoke often diminishes visibility, and it is easy to become disoriented and lose your way- even if you’ve lived in the home for years.
This smoke alarm is designed for those with hearing impairments and has a powerful 177 candela strobe light. This smoke alarm meets the Americans with Disabilities Act. 120VAC hardwired alarm wires directly into your home's electrical system.
This green unit (completely recyclable) battery operated smoke alarm reduces nuisance alarms from cooking smoke and shower steam due to photoelectric smoke sensing technology. This unit also features the popular mute feature.
Similar to the photoelectric smoke alarm, this smoke alarm also features an escape light that is activated by the alarm going off. As with the original, this unit is completely recyclable.
The First Alert® tamperproof lithium powered smoke detector is the only one available proven to last at least 10 years without needing to replace power source. This is ideal for landlord/tenant uses or to place in areas of the home where you may forget to replace the battery annually. This smoke alarm features an ionization smoke sensor.