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.
Where should I place my Smoke Alarms?
You should have a smoke alarm in every bedroom, and at least one on each level of your home. 

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.
All smoke alarms consist of two basic parts: a sensor to sense the smoke and a very loud electronic horn to wake people up. Smoke detectors can run off of a 9-volt battery or 120-volt house current.
Let’s examine the two most common types of smoke alarms used today: photoelectric detectors and ionization detectors. And, we'll also take a look inside an ionization detector.
We’ll start with photoelectric detectors.

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.
When you cross the beam of light, you block it. The photodetector senses the lack of light and triggers a bell. You can imagine how this same type of sensor could act as a smoke detector. If it ever got smoky enough in the store to block the light beam sufficiently, the bell would go off. But there are two problems here:

1. It's a pretty big smoke detector.
2. It is not very sensitive.

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.
Photoelectric smoke detectors therefore use light in a different way. Inside the smoke detector there is a light and a sensor, but they are positioned at 90-degree angles to one another, like this:

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.
Photoelectric detectors are better at sensing smoky fires, such as a smoldering mattress.

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).
Let's take a look now at the ionization chamber.

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.

References
1   http://home.howstuffworks.com/smoke.htm

First Alert Store Dual Ionization & Photoelectric Smoke Alarm

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:
• You want the convenience of "point & click" to turn off nuisance alarms
• You want optimal protection for your family from both types of primary fire - flaming and smoldering

What are the different types of Smoke Alarm First Alert makes? How are they different?
First Alert makes a wide array of smoke detectors and alarms. While they all serve the same function- alerting you of a fire- they also have different features. Some features are on all models- such as muting the unit, testing the unit, and easy access battery doors (no more taking the unit off the ceiling to replace batteries)! Other features, such as lights, strobe lights for the hearing impaired or use by a remote control are on specific models only.

10-Year Battery Smoke Alarm

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:

• Mute button, mutes unnecessary alarms immediately. Also tests alarm function with same button
• EZ access battery door
• Ionization smoke sensor
• Loud 85db alarm

120VAC Hardwired Smoke Alarm with Battery Backup

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 right smoke alarm for you if:
• You want alarms wired directly into your home's electrical system with the added safety of a battery backup
• You want to easily mute unwanted alarms with the press of a button
• 120vac hardwired, easily replace existing or install new hardwired alarms into home's electrical

Basic Smoke Alarm

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:
• You want basic protection from fire
• You want to easily mute unwanted alarms with the press of a button

Dual Ionization & Photoelectric Smoke Alarm

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 is the right alarm for you if:
• You want the convenience of a remote control to turn off nuisance alarms
• You want protection from both types of fire - flaming and smoldering

Escape Light Smoke Alarm

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 is the right smoke detector for you if:
• You want a light source to assist your family in escaping during fire
• You want to easily mute unwanted alarms with the press of a button

Hearing Impaired Strobe Smoke Alarm

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 smoke & fire alarm is for you if:
• You need to install alarms for the hearing impaired that meet ADA requirements

Photoelectric Sensor Smoke Alarm

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.

This is the right smoke detector for you if:
• You want to protect your family and the environment by purchasing a smoke alarm that is recyclable.

Photoelectric Sensor Smoke Alarm with Escape Light

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.

This is the right smoke detector for you if:
• You want the added guidance and protection of a light to assist your family in escape during fire.
• You want a smoke alarm that can be recycled.

Tamperproof 10-Year Smoke Alarm

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.

This smoke & fire alarm is for you if:
• You need to install tamper-resistant alarms that meet local codes for rental properties
• You want the convenience of never changing batteries